In the ground state, an atom of which element has two valence electrons

When we go left to right and top to bottom in a group in a period, the energy level below the calcium element contains above the 5 in a periodic table.

The 2nd group of periodic table contains two valence electrons along with calcium.

As atomic size become larger the valence electron move farther away from the neuclus and force of attraction between the valence electrons and neuclus are more than the electrons remove easily.

So when the more electrons sorround the valence shell hence the shielding effect becomes larger and larger and influence the inner electrons. The first ionization energy requires to remove one valence electron.