Question

Classify the following atom as excited, ground state, or not possible.

1s^2 2s^2 2p^6 3p^1

I'm unsure of how to find the answers from the electron config.

Answer 1

compare the order you're given with the order from a proper filling diagram. You'll see that something is "missing" in this order...

Answer 2

First, you need to determine whether it is possible. The way to do that is make sure none of the restrictions on quantum numbers labeling a subshell are violated. For example, if you say a subshell labeled "1p" you would know that's impossible, since the n=1 shell can only have an l=0 (s) subshell. Similarly, if you saw a subshell labeled "2d" you'd know that was impossible, because l=2 (d) subshells only become possible for n=3.

The second important rule to check is the number of electrons in each subshell. You know they're limited to certain numbers, for example all s subshells can fit only 2 electrons, so if you saw a subshell like "1s^3" you'd know that was wrong. Similarly, all p subshells can only fit 6 electrons, so "2p^8" would be impossible.

These are the two most critical rules to check. Once your electron configuration passes the rules of quantum mechanics, then it's just a question of whether it's the ground state or an excited state. The ground state is that state in which EVERY electrron is in its LOWEST POSSIBLE energy subshell. There's only one of those. Every other electron configuraiton represents an excited state -- a state where one or more electrons has been promoted to a higher energy level.

For example, the ground state of the electrons in a carbon atom is: 1s^2,2s2,2p^2. That's the one that has each electron in the lowest possible energy state. You'd construct it using the aufbau method and either the n+l rules or the Periodic Table (the PT is best). Any other electron configuration for carbon that follows the rules is an excited state, for example 1s^1,2s^2,2p^3, in which an electron has been promoted from the 1s subshell to the 2p subshell, or even 3d^6, in which all six electrons have been promoted into the 3d subshell. There are always an infinite number of possible electron configurations for excited states, but only one ground state.