An element
has 12 neutrons calculate its atomic number if its nucleon number is 42

Atomic number is 30

Explanation:

An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.

All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons. The sum of neutrons and protons is the mass number of an atom it is also called nucleon number,

while the number of protons are number of electrons is the atomic number of an atom.

For example

if an element have 12 neutron and nucleon number is 42 it means the number of protons will be = nucleon number - number of neutrons

42 - 12 = 30

Thus atomic number is 30.

30.

Explanation:

The nucleon or mass number is the total of protons and neutrons in the  nucleus.

Atomic number = nucleon number - number of neutrons in the nucleus

= 42 - 12

= 30.