Valency and the position of an element · Chemistry Class 9–10 | StemPy Lab
Course 9.2·Lesson 1 of 3
Valency and the position of an element
The valency of an element is the number of electrons that it loses, gains or shares to get a full outer shell. It follows the outer electrons: it goes 1, 2, 3, 4, 3, 2, 1, 0 across a period, and it is the same for all of the elements of a group.
about 25 min1 simulation2 practice questions
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What is valency?
In Chapters 7 and 8, we saw that an atom bonds to get a full outer shell. The valency of an element is the number of electrons that its atom loses, gains or shares to do this. It is the number of bonds that it makes.
Valency from the outer electrons
An atom with few outer electrons gives them up. An atom with many gains the rest.
If the outer electrons are 1, 2, 3 or 4: the valency is the number of outer electrons.
If they are 5, 6 or 7: the valency is 8 minus the number of outer electrons, since the atom needs that many more.
A full outer shell, 8 (or 2 for helium): the valency is 0. The atom does not need anything.
Element
Arrangement
Outer electrons
Valency
Sodium
2,8,1
1
1
Magnesium
2,8,2
2
2
Aluminium
2,8,3
3
3
Silicon
2,8,4
4
4
Phosphorus
2,8,5
5
8−5=3
Sulphur
2,8,6
6
8−6=2
Chlorine
2,8,7
7
8−7=1
Argon
2,8,8
8
0
The pattern
A bar chart of the valency of the elements from lithium to argon. Period 2: Li 1, Be 2, B 3, C 4, N 3, O 2, F 1, Ne 0. Period 3: Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0. The same pattern repeats in each period.
Across a period, the valency goes 1,2,3,4,3,2,1,0. It rises to 4 and falls to 0.
Down a group, the valency is the same. Lithium, sodium and potassium all have 1 outer electron, so all have valency 1. Fluorine and chlorine have valency 1 too.
The valency is periodic: the same pattern repeats in every period.
Which group has which valency?
Group
1
2
13
14
15
16
17
18
Valency
1
2
3
4
3
2
1
0
You should find that the valency climbs to 4 and then falls back to 0, the same shape as across every period.
A common mistake
The valency is not the same as the number of outer electrons for every element. For nitrogen, there are 5 outer electrons, but the valency is 3, and it is not 5.
Practise with new numbers
Try it in Python
This program finds the valency of the elements from lithium to argon.
Remember these
Valency is the number of electrons lost, gained or shared to get a full outer shell.
Outer electrons 1 to 4: valency equals outer electrons. Outer electrons 5 to 7: valency is 8 minus outer electrons. Full shell: 0.
Across a period: 1,2,3,4,3,2,1,0. Down a group: the same.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What is the valency of an element with the arrangement 2, 6?
Why do fluorine and chlorine have the same valency?
Nitrogen has five outer electrons. Why is its valency not 5?
Carbon is 2, 4. What is its valency, and how does it reach a full shell?
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