Question

if an element has the atomic number 20, what sublevels are present? it has a p sublevel. it has an s sublevel. the first energy level has an s; the second energy level has an s and a p; the third energy level has an s, p, and d; the fourth energy level has an s. the first energy level has an s; the second and third energy levels have an s and a p; the fourth energy level has an s.

Explanation:

Step1: Recall electron - filling rules

Electrons fill energy levels and sublevels according to the Aufbau principle, Pauli - exclusion principle, and Hund's rule. The order of filling is \(1s<2s < 2p<3s<3p<4s<3d\) etc.

Step2: Determine electron number

An element with atomic number 20 has 20 electrons.

Step3: Fill electrons into sublevels

The first energy level (\(n = 1\)) has only an \(s\) sublevel and can hold 2 electrons (\(1s^{2}\)). The second energy level (\(n=2\)) has an \(s\) and a \(p\) sublevel, with \(2s^{2}2p^{6}\) (2 in \(s\) and 6 in \(p\)). The third energy level (\(n = 3\)) has an \(s\), \(p\), and \(d\) sublevel, filled as \(3s^{2}3p^{6}\) (before filling \(3d\), \(4s\) is filled first according to the Aufbau principle). The fourth energy level (\(n=4\)) starts to be filled, and since we have 20 electrons in total, after filling the above sublevels (\(2 + 2+6+2 + 6\)), the remaining 2 electrons go into \(4s\), giving the electron - configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}\).

Answer:

The first energy level has an s; the second and third energy levels have an s and a p; the fourth energy level has an s.