7.1 - Atomic Structure & Isotopes

The nuclear model of the atom

• An atom has a tiny central nucleus surrounded by electrons.
• The nucleus contains protons (positively charged) and neutrons (neutral), giving the nucleus an overall positive charge.
• Electrons are negatively charged and orbit the nucleus at different distances.

The three subatomic particles

Almost all of an atom's mass is concentrated in the nucleus. The rest of the atom is mostly empty space.

Atoms are neutral

An atom has no overall charge because it has equal numbers of protons and electrons, whose equal and opposite charges cancel out.

The size of atoms

• The radius of a typical atom is about 1 × 10^-10 m.
• The nucleus is much smaller - its radius is more than 10,000 times smaller than the radius of the whole atom.
• Small molecules (e.g. O₂) are roughly the size of a single atom, about 1 × 10^-10 m.

Atomic number, mass number and isotopes

The number of protons determines which element an atom is (e.g. every carbon atom has 6 protons).

Atomic number and mass number

• Atomic number (proton number) - The number of protons in the nucleus, symbol $Z$.
• Mass number (nucleon number) - The total number of protons and neutrons in the nucleus, symbol $A$.
• In a neutral atom, the atomic number also equals the number of electrons.
• Number of neutrons = $A-Z$.

Nuclide notation

A nucleus is written with the mass number above the atomic number, in front of the chemical symbol:

$${}_Z^A\text{X}$$

Where:

• $\text{X}$ = Chemical symbol
• $A$ = Mass number
• $Z$ = Atomic number

For example, ${}_6^{14}\text{C}$ has 6 protons and 14 - 6 = 8 neutrons.

Isotopes

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (same atomic number, different mass numbers).
• Example: carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
• Most elements have multiple isotopes, but usually only one or two are stable.