Electronic Structure

The electrons in an atom are not scattered at random — they are arranged in shells (energy levels) around the nucleus, and that arrangement controls how the element behaves. Knowing the electronic structure of an atom lets you work out where it sits in the periodic table and predict how reactive it is. This lesson, part of Topic C1 of OCR Gateway Science A, shows how electrons fill the shells, how to write an electronic structure, how to draw a shell diagram, and how electronic structure links to an element's group and period.

By the end of this lesson you should be able to state the order in which electron shells fill and their capacities, write the electronic structure of the first 20 elements, draw shell diagrams, and explain the link between electronic structure and an element's position in the periodic table.

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How Electrons Fill Shells

Electrons occupy shells (also called energy levels) around the nucleus. Two simple rules tell you how they fill:

1. Lowest shells fill first. Electrons go into the shell closest to the nucleus (the lowest energy level) first, and only start a new shell when the inner ones are full.
2. Each shell has a maximum capacity. For the first 20 elements (the ones you need at GCSE):

Shell	Capacity (first 20 elements)
1st (innermost)	2 electrons
2nd	8 electrons
3rd	8 electrons
4th	begins filling after the 3rd has 8

So the first shell fills with 2 electrons, then the second with 8, then the third with 8. (Beyond GCSE the third shell can actually hold more, but for the first 20 elements you treat it as full at 8 before the fourth shell begins. Calcium, element 20, is 2,8,8,2.)

Why do the lowest shells fill first? The shells nearest the nucleus are the lowest in energy, because the electrons in them are held most strongly by the positive nucleus. Electrons, like most things in nature, settle into the lowest-energy positions available, in the same way that a ball rolls to the bottom of a slope rather than resting halfway up. So an atom is most stable when its electrons occupy the inner shells first, only beginning a new, higher shell once the ones below are full. This single rule, together with the capacities, is all you need to write the structure of any of the first 20 elements.

Exam Tip: Learn the GCSE capacities as 2, 8, 8. A very common error is to put more than 8 in the third shell — for the first 20 elements, fill the third shell to 8, then start the fourth.

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Writing Electronic Structures

The electronic structure (or configuration) of an atom lists how many electrons are in each shell, from the inner shell outwards, separated by commas. The total number of electrons equals the atomic number (for a neutral atom).

Worked Example 1 — Sodium (atomic number 11)

Write the electronic structure of sodium. Sodium has 11 electrons.

Step 1 — fill the first shell: 2 electrons (9 left).

Step 2 — fill the second shell: 8 electrons (1 left).

Step 3 — put the last electron in the third shell: 1.

Answer: sodium is 2,8,1. (Check: $2 + 8 + 1 = 11$2+8+1=11.)

Worked Example 2 — Chlorine (atomic number 17)

Write the electronic structure of chlorine. Chlorine has 17 electrons.

Step 1 — first shell: 2 (15 left).

Step 2 — second shell: 8 (7 left).

Step 3 — third shell: 7.

Answer: chlorine is 2,8,7. (Check: $2 + 8 + 7 = 17$2+8+7=17.)

Worked Example 3 — Calcium (atomic number 20)

Write the electronic structure of calcium. Calcium has 20 electrons.

Step 1 — first shell: 2 (18 left). Step 2 — second shell: 8 (10 left). Step 3 — third shell: 8 (2 left). Step 4 — fourth shell: 2.

Answer: calcium is 2,8,8,2. (Check: $2 + 8 + 8 + 2 = 20$2+8+8+2=20.)

Here are the electronic structures of the first 20 elements — worth learning the pattern:

Element	$Z$Z	Structure	Element	$Z$Z	Structure
H	1	1	Na	11	2,8,1
He	2	2	Mg	12	2,8,2
Li	3	2,1	Al	13	2,8,3
Be	4	2,2	Si	14	2,8,4
B	5	2,3	P	15	2,8,5
C	6	2,4	S	16	2,8,6
N	7	2,5	Cl	17	2,8,7
O	8	2,6	Ar	18	2,8,8
F	9	2,7	K	19	2,8,8,1
Ne	10	2,8	Ca	20	2,8,8,2

Exam Tip: Always check that your numbers add up to the atomic number. If they do not, you have put too many or too few in a shell.

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Drawing Shell Diagrams

An electronic structure can be drawn as a shell diagram: the nucleus in the centre, then concentric circles for the shells, with electrons shown as dots placed around each circle (conventionally in pairs). Here is sodium, 2,8,1:

The two inner electrons sit on the first circle, eight on the second, and the single outer electron on the third — that lone outer electron is what makes sodium so reactive.

Exam Tip: When you draw a shell diagram, place the electrons in pairs spread around each shell, and double-check the total equals the atomic number. The outer shell is the one that matters most for chemistry.

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Linking Electronic Structure to the Periodic Table

The electronic structure is directly connected to an element's position in the periodic table, which is what makes it so useful:

• the number of occupied shells = the period (row) the element is in;
• the number of electrons in the outer shell = the group number (for the main groups);
• an element with a full outer shell is in Group 0 (the noble gases), which are very unreactive.

For example, sodium (2,8,1) has 3 occupied shells → Period 3, and 1 outer-shell electron → Group 1. Chlorine (2,8,7) has 3 shells → Period 3, and 7 outer electrons → Group 7. Neon (2,8) has a full outer shell → Group 0.

Worked Example 4 — From structure to position

An atom has the electronic structure 2,8,6. Which group and period is it in, and which element is it?

Step 1 — count the occupied shells: there are 3 shells → Period 3.