Subatomic Particles, Isotopes & Relative Atomic Mass

Subatomic particles

• An atom has no overall charge because the number of protons equals the number of electrons
• The atomic number is the number of protons. Every atom of an element has the same number of protons, and different elements have different numbers of protons
• The mass number is the total number of protons plus neutrons
• Number of neutrons $=$ mass number $-$ atomic number

$${}^{23}_{11}\text{Na} \quad\text{mass number 23, atomic number 11: 11 protons, 12 neutrons, 11 electrons}$$

• An ion is an atom that has lost or gained electrons, so it has a charge
  • Losing electrons gives a positive ion; gaining electrons gives a negative ion
  • $\text{Mg}^{2+}$ has 12 protons, 12 neutrons and 10 electrons
• Atoms are very small: radius about 0.1 nm ($1 \times 10^{-10}$ m)
  • The nucleus is less than $\frac{1}{10\,000}$ of that, about $1 \times 10^{-14}$ m, but holds almost all of the mass