What is the electron structure of a sodium atom?
2s 225 22p 6351
1s 12s 220 6351
1s 225 22p 635 6
1s 22s22p 635

1s2 2s2 2p6 3s1

Explanation:

All the elements in the periodic table, has an atomic number. This number tell us different things, such as number of protons, position in the periodic table among others.

One of the things that tell us the atomic number is the electron structure.

First, all elements begins always with 1s. This is because the first number indicates number of energy level which is in. This tell us, the period of the periodic table is the element, the s, is the type of orbital which the electrons are located. the s orbytal can only have 2 electrons max, and with the increasement of the atomic number, this electrons also increase.

Therefore for an element like Helium, (atomic number = 2) means that we have to begin with the 1s and put 2 electrons, to complete the atomic number. In this case, it would be 1s2.

When we surpass this number, we use another energy level, in this case, 2, and begin again with orbytal s. Then, when we surpass 2s2, we use now another orbytal, which is p. This is because we have more elements in the second period of the table, so we need another kind of energy levels to carry all the electrons. orbytal p can carry up to 6 electrons max, so for an element like Florine (Atomic number = 9), it's structure would be 1s2 2s2 2p5. As you can count the electrons, we have 2 + 2 + 5 = 9, we complete the atomic numbers and the electrons would be, the numbers of electrons located in the last energy level, in this case, the 2s and 2p, and it would be 7 electrons.

Now after this explanation, in the case of sodium (Na), the atomic number of sodium is 11, therefore, we need to count til 11 so:

[Na] = 1s2 2s2 2p6 3s1

As you can see, sodium is the first element of period 3 of the table, and we begin with the energy level 3. In your case it would be the last option you put.