Question

An element's most stable ion forms an ionic compound with chlorine, having the formula XCl2. If the ion of element X has a mass number of 40 and has 18 electrons, what is the identity of the element?

Answer 1

Answer: The unknown element X is calcium.

Explanation:

We are given:

An ionic compound having chemical formula = $XCl_2$

The given compound is a neutral compound and in a neutral compound, the oxidation states are exchanged.

We know that:

Oxidation state of chlorine ion = -1

So, the oxidation state of X ion will be +2

Atomic number is defined as the number of protons or electrons that are present in a neutral atom.

Atomic number = number of protons = number of electrons

Number of electrons = Number of protons - charge

We are given:

Mass number = 40

Number of electrons = 18

So, number of protons = Number of electrons + Charge

Number of protons = 18 + 2 = 20 = Atomic number

Hence, the unknown element X is calcium having atomic number '20'