12 min readPathway sheetSection A MCQ (1 mark) and objective (2 marks, two parts, often match an element with its configuration); Section B very short answer (2 marks) or short answer (3 marks): write the configuration and find the valency

Filling electrons into shells (K, L, M), drawing atoms, valence electrons and valency

Electrons fill shells from the inside out: K holds 2, L holds 8, and an outer shell at most 8. The valence electrons in the outer shell set the valency.

Electronic configurationHow an atom's electrons are arranged in its shells, written K, L, M, N; sodium is 2, 8, 1

Valence electronsThe electrons in the outermost shell

ValencyThe combining capacity of an atom: equal to the valence electrons if they are 4 or fewer, otherwise 8 minus them

Noble gasesA full outer shell (8, or 2 for helium), so the valency is 0

Rules for filling electrons (Bohr and Bury)

1
Shells are named K, L, M, N (n = 1, 2, 3, 4) in order of increasing energy.
2
A shell holds at most 2n² electrons: K 2, L 8, M 18.
3
Electrons fill the lowest energy shell first: K, then L, then M.
4
A new shell starts only when the shells inside it are filled, with one exception below.
5★
The outermost shell never holds more than 8 electrons. So potassium (Z = 19) is 2, 8, 8, 1, not 2, 8, 9.
Your NIOS lesson does not give this rule, so its rules alone would give 2, 8, 9. Lesson 6 itself writes potassium as 2, 8, 8, 1.

★ key step

Electronic configuration of the first 20 elements

Element (Z)	K, L, M, N	Valence electrons	Valency
Hydrogen H (1)	1	1	1
Helium He (2)	2	2	0
Lithium Li (3)	2, 1	1	1
Beryllium Be (4)	2, 2	2	2
Boron B (5)	2, 3	3	3
Carbon C (6)	2, 4	4	4
Nitrogen N (7)	2, 5	5	3
Oxygen O (8)	2, 6	6	2
Fluorine F (9)	2, 7	7	1
Neon Ne (10)	2, 8	8	0
Sodium Na (11)	2, 8, 1	1	1
Magnesium Mg (12)	2, 8, 2	2	2
Aluminium Al (13)	2, 8, 3	3	3
Silicon Si (14)	2, 8, 4	4	4
Phosphorus P (15)	2, 8, 5	5	3 (also 5)
Sulphur S (16)	2, 8, 6	6	2
Chlorine Cl (17)	2, 8, 7	7	1
Argon Ar (18)	2, 8, 8	8	0
Potassium K (19)	2, 8, 8, 1	1	1
Calcium Ca (20)	2, 8, 8, 2	2	2

Valence electrons and valency

Valence electrons are the electrons in the outermost shell. They take part in chemical reactions.
Valency is the combining capacity of an atom: the number of hydrogen atoms one atom can join with. In H₂O, NH₃ and CH₄, the valencies of O, N and C are 2, 3 and 4.
Atoms combine to get a full outer shell of 8 electrons (an octet) by losing, gaining or sharing electrons.
If there are 4 or fewer valence electrons, valency = number of valence electrons (Na 1, Mg 2, Al 3, C 4).
If there are more than 4, valency = 8 − number of valence electrons (N 3, O 2, Cl 1).
A full outer shell (He 2, Ne 8, Ar 8) means valency 0. These noble gases hardly react.

Worked example: configuration and valency

Problem

Write the electronic configuration and find the valency of (a) sulphur, Z = 16 and (b) calcium, Z = 20.

Working

(a) Sulphur has 16 electrons. K takes 2, L takes 8, and 6 are left for M: 2, 8, 6.
It has 6 valence electrons, more than 4, so valency = 8 − 6 = 2.
(b) Calcium has 20 electrons. K takes 2 and L takes 8. M takes 8, because an outermost shell holds at most 8. N takes the last 2: 2, 8, 8, 2.
It has 2 valence electrons, so valency = 2.

Answer

Sulphur 2, 8, 6, valency 2. Calcium 2, 8, 8, 2, valency 2

How to draw the structure of an atom

Draw a small circle in the centre. Write the numbers of protons (p) and neutrons (n) in it, such as 11 p, 12 n for sodium.
Draw one circle around it for each shell in use, labelled K, L, M from the inside out.
Put dots or crosses on each circle for the electrons: 2 on K, 8 on L and 1 on M for sodium.
Write the configuration underneath: Na 2, 8, 1.
Check that the electrons add up to Z: 2 + 8 + 1 = 11.

In real life

The salt in every Indian kitchen forms because sodium (2, 8, 1) readily gives its one valence electron to chlorine (2, 8, 7). Both then have a full outer shell.

Exam angle

1 marks: “How many electrons are there in the L shell of a nitrogen atom (Z = 7)?” 5 (configuration 2, 5).

2 marks: “An atom has 13 electrons. Write its electronic configuration, its number of valence electrons and its valency.” 2, 8, 3; 3 valence electrons; valency 3, because 3 is 4 or fewer. (It is aluminium.)

3 marks: “State the rules for filling electrons into shells, and use them to write the configuration of an atom with Z = 19.” Rules: shells K, L, M, N in order of increasing energy; at most 2n² electrons in a shell; lower shells fill first; an outermost shell holds at most 8. Z = 19: 2, 8, 8, 1 (potassium).

Remember it

2, 8, 8, then start a new floor. Valency: up to 4, count them; above 4, take them from 8.