The elements whose electron configurations end with f electrons are in the Lanthanide and Actinide series: those long series at the bottom of the Periodic Table (see image).
The atoms are adding electrons into f orbitals, but the f electrons are not always the last electrons in the electron configuration.
For example, the electron configuration of Eu is [Xe]6s²4f⁷, but that of Gd
is [Xe] 6s²4f⁷5d.
I have blocked off in <em>red</em> all the <em>exceptions</em> like Gd.
I gave the opportunity to help you for now on
Hydrogen, helium, lithium, berylium, boron, carbon, nitrogen, oxygen, florine, neon, sodium, magnesium, alumini, silicon, phosphuros, sulfure, chlorine, argon, postassium, calcium, scandium, titanium, vanadium, chromium magenese
1N2 + 3F2 --->2NF3
the number of atoms in reactants= that in product