Earth Science

Materials of Earth’s Crust

Atoms to Molecules

Study Tip
All atoms of a certain element are identical in atomic number, but different isotopes have different masses

The Atom Itself

  • Atoms make up almost everything in the universe, whether they are standalone particles or puzzling compositions of them.
  • An element is in its simplest form: it cannot be made any simpler by chemical means.
  • An atom is the smallest unit of a chemical element.
  • There are two parts of an atom: the nucleus and the electrons.
  • Protons and neutrons make up the nucleus. They are of the same size, but a neutron has no charge, and a proton has a positive charge.
  • The number of protons in an atom’s nucleus determines what element it is.
  • Electrons are much smaller than either protons or neutrons and have a negative electrical charge.
Study Tip
All atoms of a certain element are identical in atomic number, but different isotopes have different masses
Atomic structure
The nucleus and the electrons are the main parts of an atom.

Atomic Properties

  • Because the mass of an electron is negligible, the atomic mass of an atom is the mass of its protons and neutrons.
  • Two atoms of the same element have the same number of protons, but the number of neutrons in each nucleus can be different. Atoms of an element with different amounts of neutrons are called isotopes.
  • When an atom either gains or loses electrons, it becomes an ion. An atom with more protons than electrons is positively charged, and vice versa.
  • When two or more atoms link up, they form a molecule with a molecular mass of the sum of all the atoms in the molecule.
  • The molecular mass is the sum of all of the masses of the atoms in the molecule.
Concept Check
  • What are the different parts of the atom, and what are their differences?
  • What is atomic mass?
  • How does an atom become an ion?