The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds. Atoms will react to get in the most stable state possible.

Octet rule can be stated as the requirement of elements to possess eight electrons in their valence shell to gain stability.As mentioned above, in order for an element to fulfill the octet rule it must either lose electrons or gain electrons. For example, in covalent bonds a pair of electrons are shared.

Oxygen obey the octet rule when reacting to form compounds by adding 2 electrons. Its means the charge on Oxygen is -2. It has oxidation state −1 is found in a few compounds such as peroxides.

The nucleus and the electrons in the inner shell remain unaffected when atoms come close together. The electrons in outermost shell of an atom get affected. Thus electrons in valence shell are responsible for the formation of bond between atoms.

The bond joining two hydrogen atoms in a hydrogen gas molecule is a classic covalent bond. Because the hydrogen atoms are identical, neither can take the electron from the other to complete its electron shell and form an ionic bond.

Covalent Bonds Shared electrons located in the space between the two nuclei are called bonding electrons. It forms from two hydrogen atoms, each with one electron in a 1s orbital. Both hydrogen atoms share the two electrons in the covalent bond, and each acquires a helium-like electron configuration.

Although core electrons do not take part in chemical bonding, they play a role in determining the chemical reactivity of an atom. This influence is generally due to the effect it has on valence electrons. The effect can be observed from the gradual change of chemical reactivity in a group.

The highest-numbered shell is the third shell, which has 2 electrons in the 3s subshell and 3 electrons in the 3p subshell. That gives a total of 5 electrons, so neutral phosphorus atoms have 5 valence electrons. The 10 remaining electrons, from the first and second shells, are core electrons.

You can also find the core and valence electrons by determining or looking up the electron configurations of the main group elements. The atomic number is the number of protons in the nuclei of the atoms of an element. A neutral atom has the same number of electrons as protons. We can look at period 2 as an example.

10 core electrons

Chlorine has 7 valence electrons in its outer shell and needs 1 more electron to get a complete octet. Chlorine will not donate any electrons but will take 1 electron.

The atomic number of chlorine is 17. Hence it has 2 electrons in its inner-most shell, 8 electrons in its second shell and 7 electrons in outer-most shell respectively. Electrons in the outer shells that are not filled are called valence electrons. Hence it has got 7 electrons in its outermost shell.

seven valence electrons

Unlike atoms from periods one and two that only have the s and p orbitals (total of 8 valence electrons), atoms like phosphorus, sulfur, and chlorine can have more than 8 electrons because they are not restricted to the s and p orbitals and have a d orbital for additional electrons needed for bonding.

There is another rule, called the duplet rule, that states that some elements can be stable with two electrons in their shell. Hydrogen and helium are special cases that do not follow the octet rule but the duplet rule. They are stable in a duplet state instead of an octet state.