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Atoms & Elements

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Structure of the Atom

An atom consists of a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in energy levels (shells). The atomic number is the number of protons. The mass number is the total of protons + neutrons.

Elements

An element is a substance that contains only one type of atom. There are about 118 known elements, organized in the periodic table by increasing atomic number.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties but different physical properties (e.g., density, radioactivity).

Electronic Configuration

Electrons fill energy levels (shells) starting from the one closest to the nucleus. The first shell holds up to 2 electrons, the second and third hold up to 8. The electronic configuration of sodium (Na, atomic number 11) is 2,8,1.

Key Points

  • Protons: positive, in nucleus. Neutrons: neutral, in nucleus. Electrons: negative, in shells.
  • Atomic number = number of protons = number of electrons (in neutral atom)
  • Mass number = protons + neutrons
  • Isotopes = same protons, different neutrons

Exam Tips

  • Always state the charge and location of each subatomic particle
  • For isotopes, emphasize "same element" and "different neutrons"
  • Draw electron shell diagrams neatly with correct electrons per shell