Calculate the number of protons, neutrons and electrons in an atom using the atomic number and mass number.
Classify protons, neutrons and electrons in terms of mass, charge and location in the atom.
Inside the Atom ‘Atom’
The word ‘atom’ comes from the Greek for ‘cannot be divided’.
While Dalton’s ideas about atoms are very useful, he was wrong in thinking that atoms could not be broken into smaller pieces.
After many scientific investigations, we now know that an atom is made up of even smaller particles called subatomic particles.
The atom is made up of three subatomic particles: zz
z z z z
Protons Neutrons Electrons
Neutron Nucleus
Proton
Electron
z
Fig. 10.2.1 Atomic structure
The three subatomic particles are quite different from each other. The following table summarises the properties of the three subatomic particles.
Subatomic particle Proton
Remembering charge
Protons are positive and neutrons are neutral.
176 176 zz
Neutron Electron
Note that: zz
Mass 1 1
Positive (+1) Neutral (0)
Charge Location in the atom Nucleus Nucleus
1/1,836 Negative (–1) Orbiting the nucleus Protons and neutrons have the same mass.
Electrons have so little mass that scientists treat their mass as zero. It takes almost 2,000 electrons to have the same mass as a proton or a neutron.
