Protons, Neutrons and Electrons

In short: Atoms are made of protons (charge +1, in the nucleus), neutrons (no charge, in the nucleus) and electrons (charge −1, in shells). The atomic number is the number of protons, the mass number is protons plus neutrons, and isotopes are atoms of one element with different numbers of neutrons.

The three subatomic particles

Protons and neutrons are together called nucleons.

Real atoms are far too light to weigh in grams easily. A carbon-12 atom has a mass of about $1.99\times 10^{-23}$ g. So chemists use the atomic mass unit (amu, also written u):

$$1\text{ amu}=\frac{1}{12}\text{ of the mass of a carbon-12 atom}\approx 1.66\times 10^{-24}\text{ g}$$

A proton and a neutron each have a mass of about 1 amu. That is where the "relative mass 1" in the table comes from.

Atomic number and mass number

• Atomic number ($Z$) = number of protons. It decides which element it is.
• Mass number ($A$) = number of protons + neutrons.

So:

$$\text{neutrons}=A-Z$$

In a neutral atom, the positive and negative charges balance, so

$$\text{electrons}=\text{protons}=Z$$

Nuclear symbol notation

We write an atom like this, with the mass number on top and the atomic number underneath:

$${}_Z^A\mathrm{X}\text{for example}{}_{11}^{23}\mathrm{Na}$$

Isotopes

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. So they have different mass numbers.

Isotopes of an element have the same chemical properties, because chemistry depends on electrons, and they have the same electrons. They have slightly different physical properties, such as mass and density. Some, like carbon-14, are radioactive. Scientists use carbon-14 to find the age of ancient bones and wood.

Ions

An ion is an atom (or group of atoms) that has gained or lost electrons, so it has a charge. The protons do not change.

• Lose electrons: more protons than electrons, so the ion is positive. A positive ion is a cation. Metals form cations.
• Gain electrons: more electrons than protons, so the ion is negative. A negative ion is an anion. Non-metals form anions.

$$\text{charge}=\text{protons}-\text{electrons}$$

Quick summary

• Proton: $+1$, mass 1, in the nucleus. Neutron: $0$, mass 1, in the nucleus. Electron: $-1$, mass almost 0, in shells.
• Atomic number $Z$ = protons. Mass number $A$ = protons + neutrons.
• Neutrons = $A-Z$. In a neutral atom, electrons = protons.
• Isotopes: same protons, different neutrons. Same chemistry.
• Ions: atoms that lost electrons (positive cations) or gained electrons (negative anions).

Quiz