Question

`1s^1 2s^1` is an electron configuration of an excited atom. What is the identity of the atom and what is its ground state configuration?

Answer 1

Helium.

Explanation:

For starters, your atom contains `2` electrons, so right from the start, you should be able to say that you're dealing with a helium atom, `"He"`.

You can say that because a neutral atom has equal numbers of protons inside the nucleus and of electrons surrounding the nucleus, regardless if the atom is in its ground state or not--keep in mind that this is true when dealing with atoms, not with ions.

`"2 electrons " -> " 2 protons " -> " atomic number"color(white)(.)(Z) = 2 " "-> " He"`

Now, for a given atom, an excited state is characterized by the fact that one or more electrons occupy higher energy levels than they do in the ground state.

You can spot which electrons have been promoted to a higher energy level by looking at the electron configuration of the atom.

In your case, you have

`color(blue)(1)s^1 color(red)(2)s^1`

Now, you should know that electron configuration of an atom in its ground state is determined by Aufbau's Principle, which states that electrons must occupy orbitals in order of increasing energy.

As you know, the second energy level, denoted by the principal quantum number `n=color(red)(2)`, is higher in energy than the first energy level, denoted by the principal quantum number `n=color(blue)(1)`.

Now, notice that your atom has `1` electron in the `color(blue)(1)s` orbital and `1` electron in the `color(red)(2)s` orbital.

As you know, the first energy level can hold `1` orbital, which, according to Pauli's Exclusion Principle, can hold a maximum of `2` electrons of opposite spins.

This implies that the `color(blue)(1)s` orbital is half-empty because it only holds `1` electron. In the ground state, the second electron should have been added to the `color(blue)(1)s` orbital as well in order to fill the orbital and have a full first energy level.

Since the second electron was added to the `color(red)(2)s` orbital instead, you can say that this electron is higher in energy than it should have been.

Consequently, you can say that the ground state configuration of a helium atom should look like this

`"He: " color(blue)(1)s^2`