Protons, Neutrons and Electrons

Atoms are the tiny building blocks of matter. Each atom has a very small, heavy centre called the nucleus, with light electrons moving around it. Most of an atom is empty space.

The three subatomic particles

• Proton: charge = +1, relative mass ≈ 1, location: nucleus
• Neutron: charge = 0, relative mass ≈ 1, location: nucleus
• Electron: charge = −1, relative mass ≈ 1/1836 (very small), location: around the nucleus

Protons and neutrons make up almost all the atom’s mass. Electrons give the atom its size.

Atomic number Z and mass number A

We describe a nucleus using two numbers:

$$\text{Proton (atomic) number } Z = \text{number of protons}$$

$$\text{Nucleon (mass) number } A = \text{number of protons} + \text{number of neutrons}$$

Number of neutrons $N$:

$$N = A - Z$$

Nuclear charge is $+Ze$. A neutral atom has $Z$ electrons. The nucleus’s relative mass is about $A$. Nuclide notation: $^{A}_{Z}\text{X}$ (X is the element symbol).

Isotopes are atoms of the same element (same $Z$) with different $A$ because they have different numbers of neutrons.

Ions (charged atoms)

Atoms form ions by gaining or losing electrons only:

• Lose electrons → positive ion (cation)
• Gain electrons → negative ion (anion)

In chemical changes, the number of protons (and usually neutrons) does not change.

Common misconceptions

• Electrons are not in the nucleus; they move around it.
• Neutrons have no charge but do have mass.
• Changing electrons makes ions; changing protons changes the element.
• Mass number $A$ is not the mass in grams; it is a count of protons + neutrons.