Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Group number
For elements in Groups I to VII, the group number equals the number of outer-shell electrons. Sodium 2,8,1 is in Group I, magnesium 2,8,2 in Group II and chlorine 2,8,7 in Group VII.
Noble gases
Group VIII elements have full outer shells. Helium has 2 electrons in its only shell; neon and argon have 8 in the outer shell. This stable arrangement explains their very low reactivity.
Period number
The period number equals the number of occupied electron shells. Magnesium has three shells and is in Period 3. Calcium has four shells and is in Period 4.
Similar group properties
Elements in the same group have similar chemical properties because they have the same outer-electron number. The detailed reactivity trends are studied in a later volume.
Ions and noble-gas configurations
Many atoms form ions with noble-gas configurations. A sodium ion has the same electron arrangement as neon, but it remains sodium because its proton number is still 11.
Practical or data skill
Identify group and period from configurations, then predict a plausible configuration from a group and period for an element within Z=1-20.
Examination tip
Group VIII is the noble-gas group in this syllabus.
Review questions and suggested answers
Question 1
For 2,8,3, state group and period.
Suggested answer
Group III, Period 3.
Question 2
Why are group members similar?
Suggested answer
They have the same number of outer electrons.
Question 3
Which group has full outer shells?
Suggested answer
Group VIII.
