1 – there is no element in group 12 and period 3
2 – X has 3 electron in its outermost shell and so has a valency of +3. Oxygen has 6 electron in its outermost shell. It can accept 2 electrons and so, it has a valency of -2. If X reacts with Oxygen, the formula would be X2O3.
3 – Valency of X = +3. Valency of Br = -1. Formula would be XBr3
4 – The atomic number is the number of protons in an atom. The number of electrons in an atom is equal to the number of protons. No. of electrons = 2+8+3 = 13. Hence, no. of protons = 13
5 – Alkali metals are only in group 1. X has 3 electron in its outermost shell. Hence, X is in group 3.
The number of outermost electrons help to determine which group the atom is in.