Question

Review the electron configurations of the elements in period 3, from sodium (Na) to argon (Ar). What do you observe?

Answer 1

Answer: After the 3s sublevel is filled with two electrons, each additional electron goes into the 3p sublevel.

Explanation: plato/ edmentem answer

Answer 2

Answer:

Electronic configuration represents the total number of electrons that a neutral element contains. We add all the superscripts to know the number of electrons in an atom.  The electrons are filled according to Afbau's rule in order of increasing energies.

$Na:11:1s^22s^22p^63s^1$

$Mg:12:1s^22s^22p^63s^2$

$Al:13:1s^22s^22p^63s^23p^1$

$Si:14:1s^22s^22p^63s^23p^2$

$P:15:1s^22s^22p^63s^23p^3$

$S:16:1s^22s^22p^63s^23p^4$

$Cl:17:1s^22s^22p^63s^23p^5$

$Ar:18:1s^22s^22p^63s^23p^6$

The last electron enters the third shell in all elements from Sodium to Argon.