Which particle is formed when an atom donates electron? a) anionb) atomc) cationd) molecule

D)Cation is the answer. A cation is a positive ion fromed by the removal of electrons from the outermost shell of an atom.

Let's take Na ( Sodium ) whose At. No is 11 as an example.
When we arrange them in electronic configuration Sodium has 3 shells. 2 in its innermost shell (K), 8 electrons in its 2nd shell which is named as 'L' & lastly 1 electron in its outermost shell (M).

We know that in an atom the no. of protons, neutrons and electrons are equal.

In case of sodium, when only electrons are reduced while protons remained same. Previously, the No. of protons and electrons were equal. But this time, There is an extra proton(+). This is the reason when an electron is removed we get a cation.

There are two factors that causes atoms to loose electrons -

1) Atomic Size

2) Nuclear Charge