Question

Use the helium-4 isotope to define atomic number and mass number. Why does knowledge of the atomic number enable us to deduce the number of electrons present in an atom?

Answer 1

Answer: The atomic number of helium atom is 2 and mass number of helium atom is 4.

Explanation:

Atomic number is defined as the number of protons or electrons that are present in a neutral atom.  It is represented by the symbol 'Z'

Atomic number = number of protons = number of electrons

Mass number is defined as the sum of number of protons and neutrons that are present in an atom.  It is represented by the symbol 'A'

Mass number = Number of protons + Number of neutrons

The general representation of an isotope is given as $_Z^A\textrm{X}$

where,

A = mass number of element

Z = atomic number of element

X = symbol of the element

We are given:

An isotope of helium having formula of $_2^4\textrm{He}$

where,

Atomic number of helium = 2 = Number of protons = Number of electrons

Mass number of helium = Number of protons + Number of neutrons

4 = 2 + Number of neutrons

Number of neutrons = 4 - 2 = 2

Hence, the atomic number of helium atom is 2 and mass number of helium atom is 4.