Periodic table: periods and groups
Find an element's address from its electrons
1. Periodic table: periods and groups: from zero
2. An address for every element
The book places the first transition series 'in the fourth period after calcium'. How do calcium's electrons tell you its period?
Calcium (Z = 20): 2, 8, 8, 2. Predict:
Pick the right answer
Which period is calcium in?
- A. Period 2
- B. Period 4
- C. Period 20
الشرح
Calcium's arrangement 2, 8, 8, 2 uses 4 main energy levels, and the period number equals the number of occupied main energy levels, so calcium is in period 4. Its 2 valence electrons give its group, and 20 is its atomic number.
3. Periods: the rows
The modern periodic table arranges the elements in order of increasing atomic number.
Period
A horizontal row of the periodic table. The period number = the number of occupied main energy levels.
Element table
period 4
- Ca: 2, 8, 8, 2
4. Groups: the columns
Group
A vertical column of the periodic table. In the main (A) groups, elements in the same group have similar properties.
The table has 18 vertical columns. In the main (A) groups, the Roman numeral = the number of valence electrons.
Element table
group 2, group 17
- Cl: 2, 8, 7
- Mg: 2, 8, 2
True or false?
Calcium and bromine are both in period 4, so they have similar chemical properties.
الشرح
Ca and Br share period 4, but they are in different A groups (IIA and VIIA). In the main (A) groups, similar properties go with the same group, not the same period. Group VIII is different: you will meet it in Ch.1.
5. Find the address
Pick the right answer
Atom X has the arrangement 2, 8, 8, 1. Where is it in the periodic table?
- A. Period 4, group IA (1)
- B. Period 1, group IVA
- C. Period 3, group IA
- D. Period 4, group 19
الشرح
The arrangement 2, 8, 8, 1 has 4 occupied main energy levels, so X is in period 4. The last number, 1 valence electron, puts it in group IA (1).
True or false?
Sodium (2, 8, 1) and chlorine (2, 8, 7) are in the same period, so they have similar chemical properties.
الشرح
In the main (A) groups, elements with the same number of valence electrons (same group) behave alike.
Need a hint?
- Count the numbers in the arrangement: that is the number of levels.
- The last number is the valence electrons.
6. Summary
- Elements are ordered by increasing atomic number: 7 periods, 18 columns.
- Period number = number of occupied main energy levels.
- Main (A) groups: group number = number of valence electrons.
| Element | Arrangement | Period | Group |
|---|---|---|---|
| Mg | 2, 8, 2 | 3 | IIA |
| Cl | 2, 8, 7 | 3 | VIIA |
| K | 2, 8, 8, 1 | 4 | IA |
| Ca | 2, 8, 8, 2 | 4 | IIA |
Can you explain why potassium and calcium are in the same period but in different groups?