The electronic configuration of an element is 2, 8, 7:

1. What is the group number of T?
2. What is the period number of T?
3. What is the valency of T?
4. Is T a metal or non-metal?
5. How many valence electrons are there in the atom of T?

Electron configuration:

• Electronic configuration, also called electronic structure, is the arrangement of electrons in energy levels around an atomic nucleus.
• The electron configuration of an element describes how electrons are distributed in its atomic orbitals.
• It can be written in K, L, M, and N format. Where the K, L, M, and N shells represent the energy shells.
• The total number of an electron in the shell is given by the formula 2N², where N is the shell number.
• For the K shell, the maximum number of electrons that can be acquired is 2.
• For the L shell, the maximum number of electrons that can be acquired is 8.
• For the M shell, the maximum number of electrons that can be acquired is 18 and so on.

(1) To determine the group, we need to understand some rules:

• If the element is in s block, then the group number is equal to the number of valence electrons.
• If the element is in the p block, then the number of the group can be determined by the formula: (number of valence electrons + 10).
• If the element is in the d block, then the number of the group can be determined by the formula: number of electrons in(n−1)d subshell + (number of electrons in (n)s subshell).
• If the element is in the f block, then the number of the group is always 3.

The electronic configuration of an element is 2,8,7 so the valance electrons will be 7 and hence group number will be 7+10 = 17.

(2) Period Number:

• The period number equals the number of shells in an atom.
• The electronic configuration of an element is 2,8,7 means it has 3 shells.

Hence the period number will be equal to 3.

(3) Valency of the element:

• The combining capacity of an atom is known as its valency.
• the total number of electrons lost, gained, or shared to complete its octet determines the valency of the atom.
• An atom’s valency is equal to the number of electrons in the outer shell when that number is four or fewer.
• If the valance electrons are more than four then the valency is equal to eight minus the number of electrons.

So in this case of the element having Electronic configuration 2,8,7, its valency would be -1 because it will gain one electron in order to complete its octet and acquire an inert gas configuration.

(4) Metal or non-metal:

• The metallic character is the tendency of an element to lose electrons and form positive ions or cations.
• The elements which have a tendency to gain electrons are known as non-metals.

Since the element has a valency of -1 means it will gain one electron to acquire the Nobel gas configuration therefore it is a non-metal.

(5) Valence electrons:

• “Electrons in the outer shells that are not filled are called valence electrons”.
• These are the number of electrons an atom must lose or gain to attain the nearest noble gas electronic configuration.

So in this case the element has Electronic configuration 2,8,7 hence the valence electrons in this will case are 7.

The element here is Chlorine having an atomic number equal to 17.