Question

Which of the following is the electron configuration of an atom in the ground state?

(a) 1s2 2s1
(b) 1s2 2s2 2p5 3s2
(c) 1s2 2s1 2p6
(d) 1s1 2s2

Answer 1

(a) 1s2 2s1

Step-by-step explanation:

Electron configurations of atoms are in their ground state when the electrons completely fill each orbital before starting to fill the next orbital.

Understanding the notation

It's important to know how to read and interpret the notation.

For example, the first part of option (a) says "1s2"

• The "1" means the first level or shell
• The "s" means in an s-orbital
• The "2" means there are 2 electrons in that orbital

Other things to know about electron orbitals

It important to know which orbitals are in each shell:

• In level 1, there is only an s-orbital
• In level 2, there is an s-orbital and a p-orbital
• in level 3, there is an s-orbital, a p-orbital, and a d-orbital (things get a little tricky when the d-orbitals get involved, but this problem is checking on the basic concept -- not the higher level trickery)

So, it's also important to know how many electrons can be in each orbital in order to know if they are full or not. The electrons should fill up these orbitals for each level, in this order:

• s-orbitals can hold 2
• p-orbitals can hold 6
• d-orbitals can hold 10 (but again, that's beyond the scope of this problem)

Examining how the electrons are filling the orbitals

For option (a):

• the 1s orbital is filled with 2, and
• the 2s orbital has a single electron in it with no other orbitals involved.

This is in it's ground state.

For option (b):

• the 1s orbital is filled with 2,
• the 2s orbital is filled with 2,
• the 2p orbital has 5 (short of a full 6), and
• the 3s orbital has a single electron in it.

Because the 3s orbital has an electron, but the lower 2p before it isn't full. This is NOT in it's ground state.

For option (c):

• the 1s orbital is filled with 2,
• the 2s orbital has 1 (short of a full 2), and
• the 2p orbital is filled with 6

Although the 2p orbital is full, since the 2s orbital before it was not yet full, this is NOT in it's ground state.

For option (d):

• the 1s orbital has 1 (short of a full 2), and
• the 2s orbital is filled with 2

Again, despite that the final orbital (in this case, the 2s orbital), is full, since the 1s orbital before it was not yet full, this is NOT in it's ground state.