Question

An element X has mass number 35 and the number of neutrons in its atom is 18.
(a) To which group of the periodic table element X belongs?
(b) To which period of the periodic table element X belongs?
​(c) What will be the formula of chloride of element X?

Answer 1

The mass number of element X = 35
Number of neutrons of element X = 18

The mass number of an element equals the total number of protons and neutrons present in it.
Mass number = number of protons + number of neutrons
$$\begin{gathered} 35=\mathrm{number}\mathrm{of}\mathrm{protons}+18 \\ \mathrm{Number}\mathrm{of}\mathrm{protons}=35-18 \\ \mathrm{Number}\mathrm{of}\mathrm{protons}=17 \end{gathered}$$
And we know, number of protons = number of electrons
So, the number of electrons = 17

(a) Electronic configuration of element X will be 2, 8, 7.
So it belongs to the halogen family, 17 group.

(b) It belongs to 3^rd period of the periodic table.

(c) The formula of chloride of element X will be XCl.