An atom has an atomic number of 20. Its electron configuration would be:

Answer 1

the atomic number is the number of protons and so also is the number of electrons because the positive charge of the protons and the negative charge of the electrons balance each other.

to answer this specific problem i googled "electron filling order" for the order of electron shells filled. from one of those images i settled on 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² where each superscript refers to the number of electrons in that particular shell. the 2p⁶ shell has six electrons. if you add all the superscripts you get 20, the number of electrons you specified.

calcium is element 20, has 20 electrons, and has the same electron configuration as argon but with an additional filled 4s² orbital so sometimes people say the configuration is [Ar] 4s².