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By the end of this topic, you should be able to:
Inside every atom, there are tiny particles called electrons that exist around a central nucleus. These electrons do not just float around anywhere — they are only allowed to exist at certain fixed distances from the nucleus. Each of these allowed positions is called an energy level.
Think of it like floors in a building. An electron can stand on the ground floor, the first floor, the second floor, and so on — but it cannot stand between floors. It must be on one floor or another. This is what we mean when we say energy levels are discrete — they are separate, specific, and not continuous.
💡 Key idea: Electrons in atoms can only have certain specific amounts of energy — not any random amount. These fixed energy amounts are the energy levels.
Energy levels are drawn as horizontal lines, stacked on top of each other like shelves. Each shelf represents a different energy level. The energy values are always negative (this will be explained below).
Here is what the energy levels of atomic hydrogen look like:
| Level | Name | Energy (eV) |
|---|---|---|
| n = 1 | Ground state | −13.6 eV |
| n = 2 | 1st excited state | −3.4 eV |
| n = 3 | 2nd excited state | −1.5 eV |
| n = 4 | 3rd excited state | −0.85 eV |
| n = 5 | 4th excited state | −0.54 eV |
| Ionisation level | Free electron | 0 eV |
💡 Why are the energies negative? When an electron is very far from the nucleus (at infinity), its energy is defined as zero. The electric field of the nucleus does work to pull the electron in closer, which lowers its energy. So electrons inside the atom have less than zero energy — that is why we use negative values. The closer the electron is to the nucleus, the more negative its energy, and the more tightly it is held.
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