First, you have to find the atomic number of the atom and this number should be equal to the number of electrons. The electrons in an atom occupy the available orbitals in sequence, called the scale of energy: 1s / 2s, 2p / 3s, 3p / 4s, 3d, 4p / 5s, 4d, 5p / 6s, 4d, 5d, 6p / 7s, 5f, 6d, 7p.
The structure of the electron shells can be determined with the help of electronic graphic formulas. To write the formula using a matrix, you have to place one or two electrons with opposite spins in each shell. Electrons are represented by arrows. The matrix clearly shows that the s-orbital of the two electrons can be placed on the p-orbitals.
In general, the filling of the energy levels of electrons occurs in the following sequence: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1 and so on. Thus for s-shell – not more than two electrons (one orbital); for p-shell - not more than six electrons (three orbital); for d-sublayer - not more than 10 (five orbitals); and for f-sublayer - not more than 14 (seven orbitals).
Write down the serial number and the symbol of the element next to the matrix. In accordance with the scale of the energy fill in series 1s, 2s, 2p, 3s, 3p, 4s levels inscribing two electrons in the cell. Get 2 +2 +6 +2 +6 +2 = 20 electrons.