Week 4 · Lesson 2 The Periodic Table of Elements (B): metalloids, period and group from the electrons, valency and Lewis dots, patterns in alkali metals and halogens · 3 lessons
2
Valency and dot pictures
Science Prep 1 Term 1, Unit 1: pp. 14–17
GoalDraw dot pictures for atoms and find their valency from the unpaired dots.
New words
valence electrons · the electrons in the outer energy levelLewis dot picture · the symbol with the valence electrons drawn as dots around itvalency · the number of unpaired dots in the Lewis picture
Explanation
The electrons in the outer level are the valence electrons. We show them as dots around the symbol, on four sides. Put one dot on each side first (top, right, bottom, left). Only when all four sides have a dot do we start to pair them up. The valency of an element is the number of unpaired dots. For groups 1A to 4A the valency equals the group number: sodium 1, magnesium 2, aluminium 3, silicon 4. For groups 5A to 0 the valency is 8 minus the number of outer electrons: phosphorus 8 − 5 = 3, sulfur 8 − 6 = 2, chlorine 8 − 7 = 1, argon 8 − 8 = 0. A valency of 1 is monovalent, 2 divalent, 3 trivalent and 4 tetravalent.
Element
Outer electrons
Pairs
Single dots = valency
Na
1
0
1
Mg
2
0
2
Al
3
0
3
Si
4
0
4
P
5
1
3
S
6
2
2
Cl
7
3
1
Ar
8
4
0
Worked examples
Sulfur has 6 outer electrons. Four dots go on the four sides, one each. The last two dots pair up with two of them. That makes 2 pairs and 2 single dots, so the valency is 2, which agrees with 8 − 6 = 2.
Magnesium is 2, 8, 2, so it has 2 outer electrons and two single dots. Its valency is 2, the same as its group number 2A.
Argon has 8 outer electrons, which make four pairs and no single dot. The valency is 0. This fits with noble gases hardly joining with other atoms.
Class activity
Dot cards. Draw the symbols Si, P, S and Cl on four cards. Use beads or a pencil to put the dots around each one: one on each side first, then pair them. Count the single dots, write the valency, and check it with the 8 minus rule for groups 5A to 0.
Practice
1
What is the valency of aluminium (group 3A)?
3.
2
Use 8 minus the outer electrons to find the valency of chlorine (7 outer electrons).
8 − 7 = 1.
3
How many dots does a group 5A atom have, and how many are single?
