Science, Chemistry & Atoms

Introduction to Science:

Have you ever wondered: "How did The Universe form?", "Why diamonds are so hard?", "Why do we cough?" or anything like that? Well, you can get the answers to all those questions through science!

Definition:

"The knowledge that provides an understanding of this Universe and how it works is called "Science".                                                                                OR 

"The knowledge gained through observations and experiments is called "Science".

Simplified Definition:

The knowledge that tells us how this Universe works is called "Science".

Etymology:

The word "Science" is derived from the Latin word "Scientia" which means "knowledge" and it is taken from the world "Scire" (pronounced as "Shee-eray") which means "to know".

Brief History:

At first, the properties of materials were studied under a single branch called "Natural Philosophy". But, as the knowledge increased it was divided into branches. In the eighteenth century, natural philosophy was divided into "Physical sciences" and "Biological sciences" or "Life sciences".

Branches:

The five main branches of science are:

1.) "Physical sciences" (the study of non-living things)

2.) "Biological sciences" or "Life sciences" (the study of living things)

3.) "Social sciences" (the study of society)

4.) "Formal sciences" (the study of logic and numbers etc)

5.) "Applied sciences" (the study of using and applying science)

These branches are divided into more and more branches.

This blog is created to help you in understanding "Chemistry", which is a branch of Physical sciences. 

Introduction to Chemistry:

When we look at different things, many questions arise in our mind e.g. "What is this?", "What is it made up of?" and "How was this thing made?" etc. Well, Chemistry helps us in understanding what something is made up of.

Definition:

The branch of science that deals with the study of composition, properties, changes, and structure of matter (anything that takes up space) is called "Chemistry" or "Chemical Science".

Simplified Definition:

The branch of science in which we learn: "What is matter made up of?", "What things make up matter?", "How those things are arranged?", "What are their properties?", "How do they change?" etc is called "Chemistry" or "Chemical Science".

Atoms:

The main focus of chemistry is on atoms and how they react to form and bring changes in matter. Atoms are sometimes called the "basic building blocks" of matter.

Definition:

"An atom is the smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule."

Simplified Definition:

"An atom is the smallest piece of matter that takes up space, joins with other atoms to form molecules which in turn combine us and everything that is around us."

Explanation:

  In chemistry, we learn that anything that takes up space is made up of them or forms them, whether the thing is small or big, soft or hard, solid or gas, round or irregular in shape or whatever! In a building (whether small or tall, strong or weak, old or new), bricks are the smallest units and similarly, in matter, atoms are the smallest units. Or just like how all puzzles are made up of small individual pieces of puzzles and all kinds of LEGO shapes are made up of small LEGO pieces, atoms are the smallest individual units that make up all matter. But, don't forget that they are WAY smaller than puzzle pieces and these are just examples to get an idea. Atoms don't have a definite shape but usually seem spherical under microscopes and are depicted as spheres in books etc.  They cannot be broken down into simpler parts by chemical means.  Solids, liquids, gases, and plasma are all made up of atoms. Almost EVERYTHING is made up of atoms. 

Size:

Typically, an atom's size is 100pm (pico meter - a very small mass measuring unit). That's about tenth-billionth of a meter. They are very very small and can only be seen with very powerful microscopes such as Electron Tunneling Microscopes (ETM) and Atomic Force Microscopes (AFM).

There are about a million atoms in the width of a human hair.

Sub-Particles:

Sub-particles are further and smaller particles of an atom. They form the structure of an atom. They form the "Nucleus". Any and all particles inside the nucleus are called "Nucleons". It contains particles called protons and neutrons together. Protons have a positive (+) charge and Neutrons have 0 or no charge (same thing).

Mass of proton = 1.0072766 a.m.u. or 1.6726 x 10 to the power -29 kg
and
 Mass of neutron = 1.0086654 a.m.u. or 1.6749 x 10 to the power of -27 kg

Over 99.94% of the atom's mass is in the nucleus. Around the nucleus, negatively charged (-) very very small particles called "electrons" whiz (not orbit in "perfectly circular" paths, It is said to make it easier for the beginners) around the nucleus in "Shells" (sort of paths also called orbits or energy levels)  which are divided into "Sub-shells". Sub-shells are further divided into "Orbitals". In an orbital, every two electrons spin (rotate like a top on their "axis") in opposite directions. Electrons have 3 parts. Spinon gives an electron it's spin, Holon gives it it's charge and Orbiton helps it to orbit around the nucleus. These are the quasiparticles of the electrons.
Mass of electron = 0.000548597 a.m.u. or 9.1 x 10-31 kg

 A proton is about 1837 times larger than an electron's and a neutron is about 1842 times larger than an electron.

Symbols:

Electron: "e"
Protons: "p"
Neutron: "n".

Note: The following information in this post is related to Physics and not Chemistry. You may skip it if you want.

Quarks:

Inside the protons and neutrons, there are smaller particles called "Quarks". Their symbol is "q". Quarks are held together by neutral particles called "Gluons". They are are named that because they kind of "glue" the quarks very strongly (in easy words). "On" is the usual particle suffix i.e. a word added after their name.
There are 6 types of quarks. These "types" are called the "flavors" of quarks. There is a group of particles called "Hadrons" which are particles made up of quarks. Protons consist of 2 up quarks and 1 down quark held together by the gluon and Neutrons consist of 2 down quarks and 1 up quark held together by the gluon. This is what gives them their properties and identity. No other particles have these combinations of quarks.

Masses:

.The masses of quarks are not confirmed. Their names and masses are:

1.) Up quark: 1 attometer OR 1x10 to the power of -18

2.) Down quark: 1 attometer OR 1x10 to the power of -18

3.) Strange quark: 400 zeptometers or 4x10 to the power of -19 meters

4.) Charm quark: 100 zeptometers or 1x10 to the power of -19

5.) Bottom quark: 30 zeptometers or 3x10 to the power of -20 meters

6.) Top quark: 100 yoctometers or 1x10 to the power of -22

Charges:

1.) Up, Top and Charm quarks, all have a charge of  +2/3

2.) Down, Bottom and Strange, all have a charge of -1/3

Why do protons have a positive charge and neutrons have no charge?

Protons are made up of two up quarks and one down quark held together by the gluon (neutral). These particles make up a proton. That means if we add the sum of their charges, we get the charge of the proton!
We can easily use simple math to do this:

Charge of a proton = Charge of 2 Up quarks + Charge of 1 down quark + Charge of Gluon
                            = (2/3 + 2/3) + (-1/3) + 0
                            = 2/3 + 2/3 - 1/3 + 0
                            = (2+2-1+0)/3
                            = 3/3
                            = 1 Answer
Hence, the charge on a proton is +1!

Now, here's why neutrons have 0 charge:

Neutrons are made up of 1 up quark and 2 down quarks held together by the gluon.

Charge of a neutron = Charge of 1 Up quark + Charge of 2 Down quarks + Charge of the gluon
                               = (+2/3) + (-1/3) + (1/3) + 0
                               = 2/3 -1/3 -1/3 + 0
                               = (2-1-1+0)/3
                               = 0/3 (Any number divided by 0 is equal to 0.)
                               = 0 Answer
And that's why they have 0 or no charge!

Atoms colloid

Atoms under microscope

Atoms under ATM (Atomic Force Microscope)

Elements

Elements:

An element is something made up of the same type of atoms (atoms with same no. of protons, neutrons and electrons). For example: Gold is only made up of gold atoms and there are not any other atoms in it. So, it is an element. More examples include: Hydrogen, Carbon & Iron etc.

In old ages, it was believed  that fire, water, earth and wind etc are elements. But, it has been proven wrong. Then later, only 9 elements were known which are carbon, gold, silver, mercury, tin, copper, lead, iron and sulfur. It was believed that elements can't be broken down into simpler things by simple ways. Until the 19th century, 63 elements were known. Now, 120 (not 118) elements have been found, out of which 92 are natural and 24 are made by scientists in labs. Modern definition of elements is that "Elements are things made up of same types of atoms having same number of protons and they cannot be broken down into simpler things by simple ways." Elements are found in different quantities throughout the universe. For example oceans contain 86% oxygen.

States:

Mostly, elements may be solids, liquids, gases or plasma. These are the 4 main states of matter.  They will be discussed later. There are many more states of matter. See: State of matter Most elements are solids, some are gases and a few are liquids. 103 are solids, 9 are liquids and 11 are gases. More elements have yet to be found or made.

Types:

There are 3 types of elements: Metals, Non-metals and Metalloids. 80% elements are metals. The concept of gaining and losing electrons shall be discussed later. Metals are electropositive i.e they lose electrons, non-metals are electronegative i.e they gain electrons and metalloids have some properties of metals and some of non-metals. Metals are malleable i.e if hit by a hammer, they expand. They let electricity flow through (good electricity conductors). They are not brittle (they don't break into pieces if hit by a hammer.) Metals are shiny i.e, They reflect light. (How? Discussed later.) Also, they are ductile meaning they can be made into wires. Non-metals are brittle and don't let electricity flow through (bad conductors of electricity). They are not malleable and ductile. Metalloids have properties in between metals and non-metals. 

Gold

Carbon

4 states of matter

Molecules & Compounds

Molecules:

Atoms can join to form larger things called "Molecules" just like you can join pieces of a puzzle to make larger individual pieces or you can join LEGOs to build larger things. The things around us are all actually structures formed by trillions and trillions of molecules, ion pairs, metal atoms or a mixture of these things (depending on what type of thing it is). If you are wondering how atoms bond (join together), click here.

Definition:

"A molecule is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound."

                                                                         OR

"A molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction."

Simplified

When two or more atoms bond (stick together), a "molecule" is said to be formed. The joining together of atoms is discussed in the post: Duplet, Octet, Valency and Chemical Combination/Bond. Atoms by the use of electrons join each other and become independent molecules, these molecules together form all other things like us and the objects around us. The properties of elements are lost and new properties are achieved like sometimes the color changes, it becomes harder etc. It is the smallest particle of a compound which can exist independently and shows all properties of a compound. Molecules are neutral and stable.

Types:

There are many types of molecules:

Di-Atomic & Tri-Atomic Molecules:

A molecule made up of 2 atoms bonded together is called a "Di-atomic Molecule", 3 atoms bonded together: Tri-atomic Molecule" and so on. e.g. tetra atomic (4 atoms), penta-atomic (5 atoms), hexa-atomic (6 atoms).
Poly-Atomic Molecules: 

A molecule made up of 2 or more atoms bonded together is called a "Poly-Atomic Molecule". Di-atomic & tri-atomic molecules can also be called poly-atomic molecules.

Homo-Atomic Molecules:

 A molecule made up of the same types of atoms is called a "Homo-Atomic Molecule" e.g. if 2 hydrogen atoms bond, a homo-atomic molecule is formed. It is called homo-atomic because the same type of atoms form it (hydrogen). 

Hetero-Atomic Molecules:

A molecule made up of different types of atoms is called a "Hetero-Atomic Molecule" e.g. if flourine and hydrogen stick together, a hetero-atomic molecule is formed because they are 2 different atoms.

Compounds:

A compound is formed by the chemical combination (bonding/sticking together) of atoms of different elements in a fixed ratio by mass. For example: Carbon Dioxide is formed when 2 oxygen atoms and 1 carbon atom stick together. It's ratio by mass is 12:32 (as the mass of carbon is 12 and of 2 oxygen atoms is 16 plus 16 = 32.) It can be simplified: 12:32 = 3:8

Properties:

Elements have specific properties like their own hardness, color and smell etc. When they join to form a compound, these properties change. Now, the compound formed by the combination of those elements has a different color, smell, taste, melting & boiling point etc.
In chemistry, breaking down means "breaking the bonds and thus making it smaller and simpler" like you can separate the the pieces in a puzzle. Compounds cannot be broken down (changed back to the elements that joined to form them) by simple ways, it is a bit difficult. For example, the compound water (H2O) can't be broken down to hydrogen and Oxygen gases by simple methods.

Types:

There are 4 main types of compounds:
1.) Covalent compounds (Polar and Nonpolar), Ionic compounds, Metallic Compounds and Coordinate Covalent Compounds. They shall be discussed later.

Examples of types of molecules

Ionic and Covalent compounds

Covalent and ionic Bonding

Water / H2O, a tri-atomic molecule with 2 hydrogen atoms and 1 oxygen atom. (The fourth atom is a part of another water molecule.)

Carbon Dioxide

Atomic Number, Atomic Mass/Mass Number and Isotopes

Atomic Number:

The number of protons in an atom is called Atomic Number of that atom e.g. Hydrogen contains 1 proton. So, it's atomic number is 1. Carbon contains 6 protons. So, it's atomic number is 6. The symbol of Atomic Number is Z. For example: For Helium, Z = 2 because Helium has 2 protons in it's nucleus. All atoms of an element have the same number of protons because they are same types of atoms. So, they have the same atomic number.  Different elements have different atomic numbers.

Atomic Mass and Mass Number:

Atomic Mass (Symbol: m) is the average mass of an atom of an element measured in amu (atomic mass units), whereas Mass Number (Symbol: A also called "Nucleon Number" or "Atomic Mass Number") is simply the sum of the number of protons and neutrons in an atom of an element. It has no units because it is simply a number. It is calculated as A=Z plus n, where n is the number of neutrons. It is calculated like this because Z (the number of protons) and n (the number of neutrons) is equal to A (Atomic Mass , The sum of protons and neutrons.) For example: Beryllium has 4 protons and 5 neutrons so 4 plus 5 = 9. So, it's Mass Number = A = 9. Similarly, Helium has 2 protons and 2 neutrons. 2 plus 2 = 4. So, it's Mass Number = 4.

Example: How many protons and neutrons are in an atom having 238 mass number and 92 atomic number?

Solution:

Data:

A = 238

Z = 92

As we know that Z = number of protons. So,

Number of protons = Z = 92 (Found)

Number of neutrons = ?

                                 = A - Z

                                 =238 - 92

                                 = 146 (Found)

So, such an atom contains 146 neutrons and 92 protons. Answer.

Isotopes:

There are version of some atoms. An element having same atomic number. But, different mass number is called an Isotope. For example: There are 3 isotopes of Hydrogen namely Protium, Deuterium and Tritium. Protium (normal hydrogen) has 1 proton and no neutrons, deuterium has 1 proton and 1 neutron & in tritium there are 2 neutrons and 1 proton. Similarly, there are 3 isotopes of Carbon. 2 are stable and 1 is radio-active (emits radiations). Carbon-12 (Normal carbon) has 6 protons and 6 neutrons, Carbon-13 has 6 protons and 7 neutrons & Carbon-14 has 6 protons and 8 neutrons. The Carbon-12 isotope is found 98.9% in nature. Carbon-13 and Carbon-14 are found 1.1% in nature.

Application of Isotopes:

Radiotherapy:

For the treatment of skin cancer, P-32 and Sr-90 are used because they emit less penetrating beta radiations. For the treatment of cancer, Cobalt-60 is used because it emits strongly penetrating beta radiations.

Tracer for Diagnosis & Medicine:

Iodine-131 is used to diagnose the presence of tumor and technitium is used to monitor bone growth.

Archaeological & Geological Uses:

The radioactive isotopes can be used to estimate the age of dead fossils e.g. dead plants & animals. Estimating the age of old objects is called Radio-Active Isotope Dating. If done with Carbon, it is called Carbon Dating OR Radio-Carbon Dating. Carbon-14 is used for this purpose.

Chemical Reaction & Structure Determination:

The radio-isotopes are used in a  chemical reaction to follow a radio-active element during the reaction and ultimately to determine the structure. For example: Carbon-14 is used to label Carbon Dioxide. As Carbon Di-Oxide is used by plants for photosynthesis (Photo means light and synthesis means making. So it means "making (plant food) with light). to form glucose, it's movement is detected through the various intermediate steps up to glucose.

Applications in Power Generations:

Radioactive isotopes are used to generate electricity by carrying out controlled nuclear fission reactions in nuclear reactors. For example, when Uranium-235 is bombarded  with slow moving neutrons, the Uranium nucleus breaks up to produce Barium-139, Krypton-94 and 3 neutrons.

A large amount of energy is released which is used to convert water into steam in boilers. The steam then drives the turbines to generate electricity. This is the peaceful use of atomic energy for the development of a nation.

Atomic Number And Mass Number

Hydrogen's Isotopes

Carbon's Isotopes

Production of Kr-92, Ba-141, Energy & 3 neutrons.

Basics

Matter:

Anything that has mass (i.e. occupies/takes space) is called "Matter". We and most of the things around us are matter. Matter is of mainly 4 states: Solids, Liquids, Gases and Plasma. 

Substance:

Pure matter is called "substance". It has specific properties and characteristics. Elements (homo/same atoms) are substances.

Mixture:

Impure matter is called "mixture" OR When 2 or more elements or compounds mix without the atoms sticking together, a mixture is formed. It shows the properties of the constituents. The things in a mixture can be separated by simple physical methods e.g. distillation, filtration, evaporation, crystallization & magnetization. 

Types:

It has 2 types:

Homogeneous Mixture:

If a mixture has uniform (same) composition throughout, it is called a "Homogeneous Mixture". e.g. air, gasoline and ice-cream. For example: Air is a mixture of nitrogen, oxygen, carbon dioxide, noble gases and water vapors.

Heterogeneous Mixture:

If a mixture does not have uniform composition throughout, it is called a "Heterogeneous Mixture". e.g. soil, rock & wood. For example: Soil is a mixture of sand, clay, minerals, water, salts and air.

Separation of constituents of

Mixture:

Let us consider an example. Suppose there are a few stones in some water. If the water is filtered, the stones are separated from the water. Filtration is a simple physical method. So, the constituents of mixture can be separated by simple physical methods.

Classification of Matter

Homogeneous & Heterogeneous Mixtures

Filtration

Symbols & Periodic Table

Symbols:

Elements have symbols which are short names for them. This saves time and writing space. Also, they are used to write chemical formulas/formulae (discussed later). In a symbol, the first or first 2 letters of the German, Greek, Latin or English name of the element are used. If the symbol is made up of 2 letters, then the first letter is capital/uppercase and the second letter is small/lowercase. For example: Hydrogen's symbol is H (English Name) & Lead 's or Plumbum's (Latin name for lead) symbol is Pb (Latin name's symbol). Similarly, Cl (English name's symbol) is used for Chlorine and Fe (Latin name's symbol) for Iron.

Periodic Table:

The periodic table is the list of all elements. It consists of 7 (horizontal i.e left to right) rows called "periods" and 18 (vertical i.e up to down) columns called "groups or "families". Periodic table mainly shows the atomic number and atomic mass. Some periodic tables show oxidization state, valency, electronegativity (discussed later) etc. Many scientists were trying to make a nice list of elements. Mendeleev made the periodic table in which the elements were arranged in order of atomic number. It was widely accepted and liked. It is still used. Older and unaccepted lists of elements are  Dobereiner's Triads by a German chemist named Doberiner & Newlands Octaves by John Newland.

Types of Elements:

Except for hydrogen most of the elements on the left are metals. The few elements on the right and hydrogen are Non-metals and in between are the Metalloids.

Lanthanides And Actinoids:

Elements number 57 to 71 are called "Lanthanides" and Elements number 89 to 103 are called "Actinoids". They have different properties. So, they are expressed below in different lines. (not periods even though they are horizontal).

Names of Periods & Groups:

Names of Periods:

Period Number    Name

1st                        Short Period

2nd                       Normal Period

3rd                        Normal Period

4th                        Long Period

5th                        Long Period

6th                        Very Long Period

7th                        Very Long Period

Names of Groups:

Group Number    Family/Group Name             Group Number           Family/Group Name

1st                        Alkali Metals                        14th                            Carbon Family

2nd                      Alkaline Earth Metals           15th                            Nitrogen Family

3rd to 12th           Transition Metals                  16th                           Oxygen Family

13th                      Boron Family                       17th                            Halogen Family                                                                                                      18th                            Noble Gases

Trends:

In a periodic table, a trend is the increasing or decreasing of a quantity in a period or group e.g. ionization energy, atomic radius, electronegativity & electron affinity (all to be discussed later).

Atomic Radius:

Decreases from left to right in a period and increases from up to down in a group.

Ionization Energy, Electron Affinity 

& Electronegativity:

Increase from left to right in a period and and decrease from up to down in a group.

The 4 Blocks:

The periodic table can be divided into 4 blocks/parts. In the 1st 2 groups, the s-subshell (sub-division of shell) is being "filled with electrons" (discussed later). So, it is called the "s-block". In group number 13 to 18, the p-subshell is being filled. So, it is called the "p-block". In group number 3 to 12, the d-subshell is being filled. So, it is called the "d-block". In lanthanides and actinoids, the f-subshell is being filled. So, it is called "f-block".

                                  

The Periodic Table

Thee Periodic Table with Trends

The Periodic Table with blocks

Duplet, Octet, Valency and Chemical Combination/Bond

Chemical species tend to be stable and to become stable they follow the duplet and octet rule.

Duplet Rule:

Filling the outer k-shell (the first shell/energy level/orbit which can have 2 electrons at most) with a total of 2 electrons is called "duplet rule" or "duet rule". Only Hydrogen and Helium have just a K-shell so, they are the only elements that follow duplet rule.

1.) Hydrogen atoms have 1 electron in their K-shell naturally, if they gain one more electron, they will have 2 electrons. Duplet rule will be completed as now there are a total of 2 electrons in the K-shell and Hydrogen will become stable.

2.) Helium atoms naturally have 2 electrons in their K-shell. Their duplet rule is naturally complete. So, it means Helium atoms are naturally stable

Octet Rule:

All elements other than Hydrogen and Helium have more than just the K-shell (the innermost and first shell). All of those elements follow the octet rule.

Filling the outermost shell with a total of 8 electrons is called "Octet Rule" e.g.
Flourine has 7 electrons in it's outermost shell. If it gains one more electron, it will have a total of 8 electrons in it's outermost shell. In this way, octet rule will be completed and Flourine will become stable.
When following octet rule, a chemical species already has 2 electrons in the K-shell and it needs a total of 8 electron in the outermost shell called the "Valence Shell".

Also see: 18-Electron rule

Valency:

The word valency is derived from the Latin word "valencia" which means "capacity".

Definition:

Valency is the combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.

It is the number of times an element can bond with another atom or molecule.

Simplified:

Valency means how many electrons a chemical species needs to complete duplet or octet rule

  For example: Hydrogen atoms needs one more electron. So, their valency is 1. Helium's duplet is  naturally complete. It needs no electrons so, it's valency is 0.  Flourine has 7 electrons in it's outermost shell, it needs 1 electron to complete the octet rule so, it's valency is 1.

Chemical Combination Or Bond:

It is a rule of the universe that everything wants to be stable. Atoms want be to stable as well. For this purpose, they form bonds/go through chemical combinations /stick together. The forces that hold atoms together are called "Chemical Forces", "Chemical Bond" or "Chemical Combination" or simply "Bond". Now, how that works is discussed below:

Types:

There are many types of bonding. But, here we shall discuss 4 types of bonds.

1.) Co-valent Bonding:

"Co" means "together" and "valent" is taken from "valency"  which is derived from the Latin word "valencia" which means "capacity". In a covalent bond, the mutual sharing of electrons happens. For example: Suppose there are 2 hydrogen atoms. Hydrogen's valency is 1. So, each H atom needs one electron.So, Hydrogen atoms are flying here and there. If by coincidence they get close to each then, the electrons will be attracted to the proton of of the other atom as well as their own atom's proton. So, once the electrons will orbit around  not just the first H atom but also the other H atom. This way the electrons will orbit both of the atoms. Let's consider another example. A flourine atom and hydrogen atom come close to each other. Now, flourine has 7 electrons in it's outermost shell and needs 1 more electron to complete the octet (valency: 1) and hydrogen has 1 electron in it's 1st shell and needs 1 more electron to complete the duplet. (valency:1). So, when they come close to each other the electrons of the of H and F will be attracted to both nuclei (Plural of nucleus), So, they orbit both of the atoms.

Types of Covalent Bond:

a) Polar Covalent Bond:

In our previous example, (hydrogen and flourine), the F (flourine) atom attracts electrons with more force so the electrons will spend more time near the flourine atom than the H atom. This will give flourine a slight and partial negative charge and hydrogen will automatically get a slight and partial positive charge. Partial charge is shown by the delta sign " δ". This is a polar covalent bond.

b) Non-Polar Covalent Bond:

If 2 different atoms bond, then their power to attract electrons (electronegativity) is different. So, it forms a polar covalent bond. But, if two same atoms bonds then, there is no difference between the electronegativities. So, both atoms attract the electrons with equal force. This way no partial charge is formed and electrons spend the same amount of time around both atoms.e.g. 2 hydrogen atoms.

c)  Network Covalent Bond:

Some elements form network covalent bonds. For example lets talk about diamond. It is formed with carbon atoms. The valency of Carbon (Symbol: C) is 4. So, it needs 4 more electrons to complete it's octet. If there are lots of carbon atoms then, each atom forms 4 single bonds with other carbon atoms. This forms a very very long structure as there are trillions of carbon atoms bonded like that. This is called "Network Covalent Bond".

d) Dative bond or
Co-ordinate Covalent Bond:

The word "dative" is derived from the Latin word "dataaray" which means "to give". In this bond, one atom donates it's electron to another atom/ion. For example: Suppose there is an NH3 molecule (A nitrogen atom bonded to 3 hydrogen atoms). It still needs one more electron to fill it's octet. If and when a Hydrogen cation (A hydrogen that has lost it's one electron and has become a cation due to more protons (1) than electrons (0).) Hydrogen cation needs 2 electrons to fill the K-shell. So, 2 of the electrons of NH3 will get attracted towards hydrogen cation and will be donated to it. These electron will whiz once around N and once around H. This is a dative bond.

2.) Ionic Bond:

In an ionic bond, instead of sharing electrons, one atom takes away the other atom's electron because of having way more electronegativity than it. For example: Suppose there is a Sodium atom (Symbol:Na, Valency:1) and a Chlorine atom (Symbol:Cl, Valency:1) if and when they get close to each other, since chlorine's electronegativity is way more than sodium's, so it takes away 1 electron of sodium and completes it's octet. The number of electrons and protons is same in a neutral atom. but, now chlorine has one more electron than the number of protons. So, it will gain an overall negative charge  and sodium has 1 less electron than the number of protons. So, it will have an overall positive charge. (When an atom loses or gains electrons, it is called an "Ion". An ion's two types are cation (positively charged atom which lost an electron) and anion (negatively charged atom which gained an electron.) Now chlorine is negatively charged and sodium is positively charged. So, both of them will attract each other and stick together. Remember! Atoms don't stick to each other. They only stay close to each other. Very close. This will be called "Sodium Chloride" (Symbol: NaCl).

3.) Metallic Bond:

Most of the elements on the left of the periodic table are called "Metals" which are electropositive i.e. lose electrons and form cations. In metal atom, the hold of the nucleus on the electrons is very weak. They have very low electronegativity.  They tend to lose their electrons easily. So, each atom is attracting slightly the electrons with very low electronegativity. So, what happens is that each atom keeps losing it's electron and other ones keep picking it one by one (with very low force) and this cycle continues. This forms a sea of mobile ( i.e. moving) electrons.The electrons are not particularly attached to one atom, they continuously travel from atom to another. As the atoms lose the electrons, they become cations. The electrons hold the atoms together.

4.) Vibrational Bond:

It is a newly discovered type of bond that is formed between two very large atoms (e.g. Bromine) and a very small atom (e.g. Hydrogen) at very high energy states. It exists only for a few milliseconds.

See: Vibrational Bond

Types depending upon number of bonds:

1.) Single Bond:

If one electron is shared by each atom in a bond then, the bond is called a "Single Bond" (also called Single Covalent Bond). It is shown by a line between the symbol of the 2 atoms. e.g. 2 hydrogen atoms.

2.) Double Bond:

If 2 electrons are shared by each atom in a bond then, the bond is called a "Double Bond" (also called Double Covalent Bond). It is shown by 2 lines between symbols of the 2 atoms. e.g. 2 oxygen atoms.

3.) Triple Bond:

If 3 electrons are shared by each atom in a bond then, the bond is called a "Triple Bond" (also called Triple Covalent Bond).It is shown by 3 lines between the symbols of the 2 atoms, e.g. 2 Nitrogen atoms.

Also check Quadruple bond, Quintuple bond & Sextuple bond.

Single, Double & Triple Bonds

Dative Bond

Diamond, network covalent bond

Polar & Non-Polar molecules

Ionic Bond

Metallic Bond