Chemistry: Atomic Structure

Chemistry: Atomic Structure

Structure of Atoms

  • An atom is the smallest unit of matter that retains all of the chemical properties of an element.
  • Atoms consist of three types of particles: protons, neutrons, and electrons.
  • The nucleus of an atom contains protons and neutrons. Protons carry a positive charge while neutrons have no charge.
  • Electrons, which carry a negative charge, are located in energy levels outside the nucleus, known as electron shells.

Atomic Number and Atomic Mass

  • The atomic number, or proton number, of an atom is the number of protons found in the nucleus. It identifies the element.
  • The atomic mass, or mass number, is the sum of protons and neutrons in the nucleus.
  • Elements are uniquely defined by their atomic number.

Electron Configuration

  • Electrons within an atom are arranged in energy levels, also known as electron shells.
  • The first shell can hold up to two electrons, and each subsequent shell can hold up to eight electrons.
  • The electron configuration of an atom can be written using numbers and letters that represent the energy levels and orbitals.
  • For instance, the configuration 2,8,8 means there are 2 electrons in the first shell, 8 in the second, and 8 in the third.


  • Isotopes are variants of a chemical element that have the same number of protons and electrons, but different numbers of neutrons.
  • Isotopes of an element have identical chemical properties but different physical properties, such as atomic mass.


  • Atoms can gain or lose electrons to form charged particles called ions.
  • An atom that gains one or more electrons becomes a negatively charged ion, or anion.
  • An atom that loses one or more electrons forms a positively charged ion, or cation.

Relative Atomic Mass

  • The relative atomic mass (Ar) of an element is the average mass of its atoms, compared to 1/12th the mass of a carbon-12 atom. This takes into account the different isotopes of the element.

The Periodic Table

  • The Periodic Table is a chart that organises all known elements by their properties.
  • Elements are arranged in order of increasing atomic number, from left to right and top to bottom.
  • The rows of the table are called periods and the columns are called groups. Elements in the same group have similar properties.
  • There are different blocks in the Periodic Table: s-block, p-block, d-block, and f-block. Each block is associated with a specific type of atomic orbital: s, p, d, or f.

Electron Configuration and the Periodic Table

  • The arrangement of electrons in an atom determines many of the element’s chemical properties, including its reactivity and the types of bonds it can form.
  • The electron configuration of an atom can generally be predicted by the atom’s position in the periodic table.
  • Each period in the table corresponds to the filling of a distinct electron shell.