Atom

An atom is the smallest indivisible unit of an element that can participate in a chemical reaction ¹. Every neutral atom consists of a tiny central nucleus orbited by negatively charged electrons.

Atomic Structure

In the IGCSE model, an atom consists of three subatomic particles ¹:

         Electron Shell (e-)
             . - ~ ~ - .
         . '      e-     ' .
       /                     \
      |        Nucleus:       |
      |     (Protons: +1)     |
      |     (Neutrons: 0)     |
       \                     /
         . '      e-     ' .
             ' - ~ ~ - '

Atomic Identifiers

• Proton Number (Atomic Number, $Z$): The number of protons in the nucleus; uniquely defines the chemical element. In a neutral atom, number of protons = number of electrons.
• Nucleon Number (Mass Number, $A$): The total count of protons plus neutrons in the nucleus ($A = Z + N$).

Atoms can bond together by sharing electrons to form molecules or by transferring electrons to form ions.

Related

• Particle - General microscopic entity encompassing atoms, molecules, and ions.
• Molecule - Group of two or more atoms chemically joined by covalent bonds.
• Kinetic Particle Theory - Motion and thermodynamic behavior of atomic particles.
• Pure Substance - Matter consisting of identical atoms or uniform compounds.
• Chemistry - Scientific study of atomic interactions and transformations.