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