Configurations

[Kaitlin Joya 1I]

How do we tell when atoms don't have electrons in a certain subshell?

[Sophie Vikram 1L]

This is a somewhat vague question, but if you are referring to electron configurations, then we can determine how many electrons exist in certain subshells. By determining what already exists we can determine what is missing from that subshell. For instance, for Carbon our electron configuration would be 1s2, 2s2, 2p2. Because we know that the p subshell holds 3 orbitals that can hold 6 electrons total, we can figure out that there are two electrons in the 2p subshell. From this, we can determine that we can have up to 4 more electrons in this subshell.

[Anne Mariano 3F]

When writing the electron configuration of elements, you can refer to its position on the periodic table to help you out. The periodic tables attached as images below display the element's subshells in GROUND STATE. There will be a difference in excited state electron configuration. An excited state typically looks like the configuration skipped an orbital.
Focusing on the ground state electron configuration, it follows this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f and so on. By looking at the images below, understanding the s-,p-,d-,f- blocks and that only two electrons are able to be in each subshell for its ground state, you will be able to figure out which subshell to stop writing the configuration for.