# Periodic table: periods and groups

> Find an element's address from its electrons


## Objectives

- Describe the modern periodic table: elements in order of increasing atomic number, in 7 periods and 18 columns.
- Find the period of an element from the number of its occupied main energy levels.
- Find the A group of a main-group element from its valence electrons.

## 1. Periodic table: periods and groups: from zero




## 2. An address for every element

Which period is calcium in?

- Period 20
- Period 4 ✓
- Period 2

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.

[ar-EG] ترتيب الـ calcium 2, 8, 8, 2 فيه 4 main energy levels، ورقم الـ period بيساوي عدد الـ main energy levels اللي فيها electrons، يبقى الـ calcium في period 4. أما الـ 2 valence electrons فبيحددوا الـ group، و20 ده الـ 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. Period 4: every atom here has electrons in 4 main energy levels (K, L, M, N). There are 7 periods.


## 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. Mg is in group IIA (2); Cl is in group VIIA (17). Calcium and bromine are both in period 4, so they have similar chemical properties.


## 5. Find the address

Atom X has the arrangement 2, 8, 8, 1. Where is it in the periodic table?

- Period 3, group IA
- Period 4, group IA (1) ✓
- Period 4, group 19
- Period 1, group IVA

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).

[ar-EG] الترتيب 2, 8, 8, 1 فيه 4 main energy levels فيها electrons، يبقى X في period 4. وآخر رقم، valence electron واحد، بيحطه في group IA (1).
## 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. Count the levels for the period; read the valence electrons for the A group. Calcium (2, 8, 8, 2): period 4, group IIA. The first transition series follows it in period 4 (Ch.1 p.4). **Period** = a horizontal row. **Group** = a vertical column. Can you explain why potassium and calcium are in the same period but in different groups?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-pt.groups-periods