Atom: Smallest unit of an element, retains chemical properties.

Subatomic Particles:

• Protons: Positive charge (+1), located in the nucleus.

• Neutrons: Neutral (0 charge), located in the nucleus.

• Electrons: Negative charge (–1), found in orbitals/shells around the nucleus.

Electron Arrangement (Configuration)

• Shells/Energy Levels: Electrons occupy energy levels (n = 1, 2, 3…).

• Maximum electrons per shell: 2n²

  • 1st shell: 2

  • 2nd shell: 8

  • 3rd shell: 8 (up to 18 for transition metals)

• Valence Electrons: Electrons in the outermost shell; determine chemical properties