الأسبوع 4 · الدرس الثاني «الجدول الدوري لتصنيف العناصر» (٢): أشباه الفلزات والدورة والمجموعة والتكافؤ وتراكيب لويس وأنماط المجموعات · 3 دروس
1
Period number and outer electrons: the table’s clue
Science Prep 1 Term 1, Unit 1: pp. 14–17
GoalUse the electron arrangement to find an element’s period and group, and tell metals, nonmetals and metalloids apart.
New words
outer electrons · the electrons in the outermost energy levelmetalloids · elements along the dividing line between metals and nonmetalsmetals and nonmetals · two big classes of elements with different properties
Explanation
The electron arrangement is a clue to where an element sits in the table. The number of occupied energy levels equals the period number. For the groups 1A to 7A, the number of electrons in the outer level equals the group number. For group 0, helium has 2 and the other noble gases have 8. Look at some examples: sodium 2, 8, 1 is in period 3, group 1A. Phosphorus 2, 8, 5 is in period 3, group 5A. Argon 2, 8, 8 is in period 3, group 0. Potassium 2, 8, 8, 1 and calcium 2, 8, 8, 2 are in period 4. Krypton 2, 8, 18, 8 is also in period 4. Metals usually have 1 to 3 outer electrons, and nonmetals usually have 5 to 7. These are tendencies, not strict laws. The metalloids (B, Si, Ge, As, Sb, Te) lie along the line between metals and nonmetals, and we cannot tell them by their outer electrons.
Element
Arrangement
Period
Group
Mg
2, 8, 2
3
2A
S
2, 8, 6
3
6A
Cl
2, 8, 7
3
7A
K
2, 8, 8, 1
4
1A
Worked examples
Aluminium has 13 electrons: 2, 8, 3. It has three occupied levels, so it is in period 3. It has 3 outer electrons, so it is in group 3A, and with 3 outer electrons it is probably a metal.
The noble gases: He is 2, Ne is 2, 8, Ar is 2, 8, 8 and Kr is 2, 8, 18, 8. The outer level holds 2 in helium and 8 in the others, and these atoms hardly join with others. They sit together in group 0.
Silicon has 14 electrons: 2, 8, 4. That puts it in period 3, group 4A. Four outer electrons does not tell us whether it is a metal or a nonmetal, and silicon is a metalloid on the dividing line.
Class activity
Bead and cup game. Use beads for electrons and cups for levels (2, 8, 18 beads at most). Build atoms with 5, 12 and 16 electrons. For each one, write its arrangement, count the cups in use to get the period, and count the beads in the last cup to get the group. Then check with a periodic table.
Practice
1
An atom is 2, 8, 6. Find its period and group.
Period 3 (three levels) and group 6A (six outer electrons).
2
An element is in period 2 and group 3A. How many electrons does its atom have?
2 in the first level and 3 in the second: 5 electrons.
3
How many outer electrons does calcium (2, 8, 8, 2) have, and is it more likely a metal or a nonmetal?
