• Atoms interact with each other through their electrons.

• Chemical bonds involve only electrons, so electrons determine how atoms combine into compounds.

• Danish physicist Neils Bohr (1885–1962) proposed the concept of energy levels.

Electrons

• Electrons further away from the nucleus have more energy and electrons closer to the nucleus have less energy.

• The energy of an electron must match one of the energy levels in the atom.

• Each energy level can hold only a certain number of electrons, and no more.

• As electrons are added to an atom, they settle into the lowest unfilled energy level.

In the Bohr model of the atom, the first energy level can accept up to 2 electrons. The second and third energy levels hold up to 8 electrons each. The fourth and fifth energy levels hold up to 18 electrons each.

Energy Levels

The electrons in the outermost energy level are called VALENCE ELECTRONS.

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These electrons interact with different atoms.

Valence Electrons

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