In this lesson, we will learn:
• The definition of an atom.
• The particles that make up an atom and where in the atom they are found.
• The charge and mass of an atom's constituent particles.
• The atom is the smallest indivisible unit of matter, comprised of protons, neutrons and electrons.
• It's known as the smallest indivisible unit of matter
because an atom
is the smallest unit of a material that still shows all the properties
of the bulk material. It is also the smallest unit of matter that is stable in its own right
• Atoms are comprised of three main particles
– protons and neutrons
, which make up the central core of the atom known as the nucleus, and electrons
, which are found in shells (or orbitals) occupying space outside the nucleus.
• A brief overview of the particle properties is below:
• The nucleus contains almost all of the mass of an atom
as it contains protons and neutrons, which both have a relative mass of 1 AMU. The nucleus also has an overall positive charge
, as neutrons have no charge and protons have a relative charge of +1.
• The mass of an electron
is (relative to protons and neutrons) so small that it is often ignored
; it is around 1/1840 AMU. An electron has a relative charge of -1
so an electron has an equal but opposite charge to a proton.
• In an atom of an element
, the number of protons is always equal
to the number of electrons. This means the overall charge of an atom is zero and is therefore neutral
• Because the atomic mass of an electron is approximately 0, we can say that the atomic mass
, or mass number, of an element tells us the number of protons and neutrons in an atom
because only these two particles contribute to the mass of the atom.
• Using the last two points of information, we can look at the periodic table and predict the number of protons, electrons and neutrons in an atom of any element!