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Periodic Table
Periodic Table: The number of protons in the atom of an element determines its place in the Periodic Table.
The number of electrons in an atom is the same as the number of protons in periodic table. These electrons are arranged in shells or ‘energy levels’ around the nucleus.
The arrangement of electrons determines the chemical properties of an element.
Electron arrangement of the first 20 elements
Electrons are arranged in shells at different distances around the nucleus.
As we move across each row of the periodic table the proton number increases by one for each element. This means the number of electrons also increases by one for each element.
Starting from the simplest element, hydrogen, and moving through the elements in order we can see how the electrons fill the shells.
The the innermost shell (or lowest energy level) of electrons is filled first. This shell can contain a maximum of two electrons.
Next, the second shell fills with electrons. This can hold a maximum of eight electrons. When this is filled, electrons go into the third shell, which also holds a maximum of eight electrons. Then the fourth shell begins to fill.
The table shows the number of electrons in each shell for the first 20 elements in the periodic table.
Number of electrons in each shell
Element Symbol Electrons in shell 1 Electrons in shell 2 Electrons in shell 3 Electrons in
Number of electrons in each shell
| Element | Symbol | Electrons in shell 1 | Electrons in shell 2 | Electrons in shell 3 | Electrons in shell 4 |
|---|---|---|---|---|---|
| Hydrogen | H | 1 | – | – | – |
| Helium | He | 2 | – | – | – |
| Lithium | Li | 2 | 1 | – | – |
| Beryllium | Be | 2 | 2 | – | – |
| Boron | B | 2 | 3 | – | – |
| Carbon | C | 2 | 4 | – | – |
| Nitrogen | N | 2 | 5 | – | – |
| Oxygen | O | 2 | 6 | – | – |
| Fluorine | F | 2 | 7 | – | – |
| Neon | Ne | 2 | 8 | – | – |
| Sodium | Na | 2 | 8 | 1 | – |
| Magnesium | Mg | 2 | 8 | 2 | – |
| Aluminium | Al | 2 | 8 | 3 | – |
| Silicon | Si | 2 | 8 | 4 | – |
| Phosphorus | P | 2 | 8 | 5 | – |
| Sulfur | S | 2 | 8 | 6 | – |
| Chlorine | Cl | 2 | 8 | 7 | – |
| Argon | Ar | 2 | 8 | 8 | – |
| Potassium | K | 2 | 8 | 8 | 1 |
| Calcium | Ca | 2 | 8 | 8 | 2 |
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