Inside the Atom

Inside the Atom ⚛️

Everything is made of **atoms** — and atoms have their own structure. A tiny, dense **nucleus** sits at the centre, with **electrons** whizzing around it. Almost all the atom is empty space: if the atom were a football stadium, the nucleus would be a pea in the centre.

The nuclear model 🔬

An atom is about **1 × 10⁻¹⁰ m** across, and the nucleus is around **10,000 times smaller** still. • The **nucleus** contains **protons** and **neutrons**. • The **electrons** move around the nucleus in **shells** (energy levels).

Label the atom

An interactive activity.

The three particles 🔴

Each subatomic particle has a **charge** and a **relative mass**: • **Proton** — charge **+1**, relative mass **1**. • **Neutron** — charge **0**, relative mass **1**. • **Electron** — charge **−1**, relative mass **very small** (about 1/2000, taken as negligible).

Match each particle to its description

Atomic and mass number 🔢

Two numbers describe any atom: • **Atomic number** = the number of **protons** (it defines the element). • **Mass number** = the number of **protons + neutrons**. So the number of **neutrons = mass number − atomic number**. Carbon-14 has mass number 14 and atomic number 6, so 14 − 6 = 8 neutrons.

Count the neutrons

An interactive activity.

What is the atomic number?

What does the atomic number of an element tell you?

• The number of protons in the nucleus
• The number of neutrons
• The total number of particles
• The mass of the atom in grams

A neutral atom ➖

A complete atom has **no overall charge** — it is **neutral**. That is because it has **equal numbers of protons and electrons**, so the +1 charges exactly cancel the −1 charges. If an atom gains or loses electrons it becomes a charged **ion**, but a plain atom is always neutral.

Isotopes 👯

**Isotopes** are atoms of the **same element** with the **same number of protons** but a **different number of neutrons**. Carbon-12 and carbon-14 are both carbon (6 protons each), but carbon-12 has 6 neutrons and carbon-14 has 8. Same element, different mass number.

What makes an isotope?

Two isotopes of the same element always have the same number of protons but a different number of what?

• Neutrons
• Protons
• Electrons
• Nuclei

Order the discoveries

An interactive activity.

Neutrons again

An interactive activity.

The atom rules

The atomic number equals the number of _____ in the nucleus. In a neutral atom, the number of protons equals the number of _____. Isotopes have the same number of protons but a different number of _____.