1.1 - Atoms & Subatomic Particles

The atom and its particles

• Every substance is made of atoms. An atom is the smallest part of an element that can exist.
• Atoms are extremely small - far too small to see even under a microscope.
• Each atom is built from three kinds of particle: protons, neutrons, and electrons.

The nucleus and the shells

• At the centre of the atom is the nucleus, which holds the protons and neutrons packed tightly together.
• The nucleus carries a positive charge (because of the protons), and almost the whole mass of the atom is concentrated there.
• The electrons move around the nucleus in shells (also called energy levels).
• Most of an atom is empty space, with the outermost electrons setting how big the atom is.

The three particles compared

The particles are compared on a relative scale, where a proton and a neutron each have a mass of 1 and a proton has a charge of +1.

Why atoms are neutral

• An atom always has the same number of electrons as protons.
• The total negative charge on the electrons matches the total positive charge on the protons, so they cancel out.

The size of atoms and their nucleus

• An atom has a radius of about 0.1 nanometres, which is 1 × 10^-10 m.
• The nucleus is far smaller: its radius is less than 1/10,000 of the radius of the whole atom, about 1 × 10^-14 m.

Atomic number, mass number and nuclide notation

• Atomic number (also called the proton number) - The number of protons in the atom. Every atom of a given element has the same number of protons.
• Mass number (also called the nucleon number) - The total number of protons and neutrons in the nucleus.

Nuclide notation

• In nuclide notation (or the nuclear symbol), the mass number is above and the atomic number is below, to the left of the element's symbol.
• Example: sodium is written ${}_{11}^{23}\text{Na}$ (11 protons, and 23 protons and neutrons in total).

Working out the particles

• Number of protons = atomic number
• Number of neutrons = mass number - atomic number
• Number of electrons = number of protons (in a neutral atom)

Worked example - The particles in an atom

An atom of iron is written ${}_{26}^{56}\text{Fe}$. How many protons, neutrons, and electrons does it contain?

Counting the particles in an ion

• An ion is an atom, or group of atoms, that has lost or gained electrons.
• Because the number of protons and electrons no longer match, an ion carries an overall charge.
• An atom that loses electrons becomes a positive ion (more protons than electrons).
• An atom that gains electrons becomes a negative ion (more electrons than protons).

The nucleus stays the same

• Only electrons are lost or gained, so the nucleus is untouched.
• An ion has the same number of protons and neutrons as the atom it came from.
• In nuclide notation, the charge is written at the top right, as in ${}_{17}^{35}{\text{Cl}}^{-}$ or the lithium ion ${}_3^7{\text{Li}}^{+}$ shown below.

Working out the particles in an ion

• Number of protons = atomic number
• Number of neutrons = mass number - atomic number
• Number of electrons = atomic number minus the charge (for positive ions) or plus the charge (for negative ions)

For a chloride ion, ${}_{17}^{35}{\text{Cl}}^{-}$: 17 protons, 35 - 17 = 18 neutrons, and 17 + 1 = 18 electrons.