Write down :

(i) the mass number of the atom having 20 neutrons and 15 protons.

(ii) the number of neutrons in the nucleus of an atom having atomic number 17 and mass number 37.

State what is a proton. What is the significance of the number of protons found in the atoms of each of the different elements.

The electronic structure [configuration] of fluorine can be written as 2, 7. In a similar way give the electronic configuration of :

(i) aluminium

(ii) phosphorus

What would be the reason for an element to have atoms with differing mass numbers.