what is the difference between the number of electrons in an atom selenium and the number of electrons in an atom of aluminum

There are 13 electrons in atom of aluminium and 34 in atom of selenium.

Explanation:

Aluminium:

It is present in group 13.

It has three valance electrons.

It form cation.

Its electronic configuration is:

Al = [Ne] 3s² 3p¹

The electron in 3s and 3p are considered as valance electrons of aluminium.

Aluminium atom has 13 protons and 14 neutrons thus its mass number is 27 amu.

Selenium:

It is present in group 16.

It has six valance electrons.

It form anion.

Its electronic configuration is:

Se = [Ar] 3d¹⁰ 4s² 4p⁴

The electron in 4s and 4p are considered as valance electrons of selenium.

Selenium atom has 34 protons and 45 neutrons thus its mass number is 79 amu.