If a carbon atom can form four covalent bonds, it also has four valence electrons. For neutral atoms, the number of valence electrons the same as an atom's main group number.
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.
B.
Spring tides occur during a new/full moon. The sun, moon and earth form a straight line during these phases.