Question

An ion x 2+ contains 18 electrons and 20 neutrons. Calculate the atomic number and mass no. Of element x. Name the element x.

Answer 1

Answer: Atomic no. = 20, mass no. = 40, Calcium.

Atomic no. = No.of electrons - (negative charge) + (positive charge)

Thus,

At. no.= 18+(+2) = 20

Mass no. = 20 + 20 = 40

Answer 2

Answer:   Atomic number =  20

Mass number = 40

Element = Calcium

Explanation:

Number of electrons in $X^{2+}$ ion = 18

Number of electrons in X = Number of electrons in $X^{2+}$ ion + Cationic charge = 18 + 2 = 20

Since positive ion is formed by the loss of electrons from the neutral atom and the number of electrons lost is equal to the number of units of positive charge on the ion.

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

Atomic number = Number of protons = number of electrons = 20

Mass number is defined as the sum of number of protons and neutrons that are present in an atom.

Mass number = Number of protons + Number of neutrons = 20 + 20 = 40

Thus the element with atomic number of 20 is calcium.