Period Number
Summary: The period number of an element equals the number of occupied electron shells in its atoms, indicating which horizontal row of the periodic table it belongs to. Tags: igcse chemistry Created: 2026-07-18
The period number in the periodic table is equal to the number of occupied electron shells in an atom of that element. Periods are the horizontal rows of the periodic table, numbered 1 to 7, though at IGCSE level the focus is primarily on Periods 1, 2, and 3 (elements with proton numbers 1 to 18). An element’s period can be read directly from its electronic configuration: simply count how many numbers appear in the configuration. For example, hydrogen (configuration: 1) has 1 occupied shell and is in Period 1; sodium (configuration: 2.8.1) has 3 occupied shells and is in Period 3; potassium (configuration: 2.8.8.1) has 4 occupied shells and is in Period 4. As you move down a period, one new electron shell is added, and as you move across a period from left to right, the outer shell is progressively filled with electrons — which explains why properties change gradually across each period.
Periods 1, 2, and 3
Period 1 — 1 Occupied Shell
Period 1 contains only two elements, both with electrons in the first shell only:
| Element | Symbol | Z | Configuration | Shells Occupied |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 1 |
These are the smallest atoms. The first shell can hold a maximum of 2 electrons, which is why Period 1 has only 2 elements.
Period 2 — 2 Occupied Shells
Period 2 contains eight elements, all with electrons distributed across two shells (the first shell full at 2, and the second shell filling from 1 to 8):
| Element | Symbol | Z | Configuration | Shells Occupied |
|---|---|---|---|---|
| Lithium | Li | 3 | 2.1 | 2 |
| Beryllium | Be | 4 | 2.2 | 2 |
| Boron | B | 5 | 2.3 | 2 |
| Carbon | C | 6 | 2.4 | 2 |
| Nitrogen | N | 7 | 2.5 | 2 |
| Oxygen | O | 8 | 2.6 | 2 |
| Fluorine | F | 9 | 2.7 | 2 |
| Neon | Ne | 10 | 2.8 | 2 |
Period 3 — 3 Occupied Shells
Period 3 contains eight elements, all with electrons in three shells (the first full at 2, the second full at 8, and the third filling from 1 to 8):
| Element | Symbol | Z | Configuration | Shells Occupied |
|---|---|---|---|---|
| Sodium | Na | 11 | 2.8.1 | 3 |
| Magnesium | Mg | 12 | 2.8.2 | 3 |
| Aluminium | Al | 13 | 2.8.3 | 3 |
| Silicon | Si | 14 | 2.8.4 | 3 |
| Phosphorus | P | 15 | 2.8.5 | 3 |
| Sulfur | S | 16 | 2.8.6 | 3 |
| Chlorine | Cl | 17 | 2.8.7 | 3 |
| Argon | Ar | 18 | 2.8.8 | 3 |
Period 4 (First Two Elements)
At IGCSE level, potassium and calcium are the only Period 4 elements typically encountered:
| Element | Symbol | Z | Configuration | Shells Occupied |
|---|---|---|---|---|
| Potassium | K | 19 | 2.8.8.1 | 4 |
| Calcium | Ca | 20 | 2.8.8.2 | 4 |
Relationship Between Period and Group
An element’s group number and period number can both be read directly from its electronic configuration:
- Period number = number of occupied shells (how many numbers)
- Group number = number of electrons in the outermost shell (the last number, for Groups I–VII)
For example, an element with configuration 2.8.6 has 3 occupied shells (Period 3) and 6 outer electrons (Group VI) — this is sulfur (S).
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2 Core 6(c), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”The period number tells you how many electrons an atom has” | The period number tells you the number of occupied electron shells, not the total number of electrons. For example, sodium (Z = 11) is in Period 3 — it has 3 occupied shells, not 3 electrons. |
| ”Larger period number means a larger atomic number, always” | While atomic number generally increases with period number, it is possible for an element in a higher period to have a lower atomic number than one in the next period’s beginning — but within IGCSE scope, the first 20 elements follow this trend cleanly. |
| ”All periods have the same number of elements” | Period 1 has only 2 elements (H, He). Periods 2 and 3 each have 8 elements. Period length increases in higher periods due to the filling of d and f sub-shells. |
| ”Period number and group number mean the same thing” | Period number (horizontal rows) = number of occupied shells. Group number (vertical columns) = number of outer-shell electrons. They describe different aspects of electron arrangement. |
| ”Potassium’s 19th electron goes in the third shell” | At IGCSE level, potassium’s configuration is written as 2.8.8.1 (Period 4). The 19th electron enters the fourth shell after the third has 8, placing potassium in Period 4 rather than Period 3. |