Electronic structure
Electron shells
Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. are arranged in different shells around the The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei.. Each successive shell can only hold a certain number of electrons.
The innermost shell is filled first. This shell can contain a maximum of two electrons. The second shell 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.
| Energy shell | Maximum number of electrons |
| First | 2 |
| Second | 8 |
| Third | 8 |
| Energy shell | First |
|---|---|
| Maximum number of electrons | 2 |
| Energy shell | Second |
|---|---|
| Maximum number of electrons | 8 |
| Energy shell | Third |
|---|---|
| Maximum number of electrons | 8 |
A lithium atom, for example, has three electrons. It has two in the first shell and one in the second shell.
A carbon atom has six electrons. It has two in the first shell and four in the second shell.
A calcium atom has 20 electrons. Two are in the first shell, eight in the second shell, eight in the third shell, and two in the fourth shell.