GCSE · Chemistry · AQA · Spec 8462

Size and mass of atoms; isotopes

Most of an atom's mass is packed into a nucleus thousands of times smaller than the atom itself.

What you need to know

  • An atom has a radius of about 0.1 nm (1 × 10⁻¹⁰ m), while a nucleus is about 1 × 10⁻¹⁴ m and less than 1/10 000 of the atom's radius.
  • Almost all atomic mass is in the nucleus; protons and neutrons each have relative mass 1, while an electron's relative mass is very small.
  • Mass number is the number of protons plus neutrons, and isotopes are atoms of the same element with different numbers of neutrons.
  • In chemical-symbol notation, mass number goes top-left and atomic number bottom-left; these values can be used to work out particle numbers.

The big picture

Atoms are extremely small, with a radius of about 0.1 nm, or 1 × 10⁻¹⁰ m, while a nucleus has a radius of about 1 × 10⁻¹⁴ m and is less than one ten-thousandth of the atom's radius. Almost all of an atom's mass is in its nucleus; protons and neutrons each have relative mass 1, while an electron's relative mass is very small. Mass number is the total number of protons and neutrons, and chemical-symbol notation places mass number at the top-left and atomic number at the bottom-left. Atoms of the same element with different numbers of neutrons are isotopes.

TONIGHT'S REVISION

Size and mass of atoms; isotopes

Zoom from the tiny atom to its nucleus, then use particle counts to understand mass number and isotopes.

How small is an atom?

Read the scale from the nucleus out to a familiar one-metre length.

10⁻¹⁴10⁻¹⁰10⁻⁶10⁻²1Radius or length (m) — log scaleNucleus radius

Nucleus radius

about 1 × 10⁻¹⁴ m

The nucleus is less than 1/10 000 of the atom's radius, so it occupies an extraordinarily small central scale.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Relative massMass significance
Proton
Neutron
Electron

Each cell hides a short answer and the reason behind it. Predict before you tap.

Two numbers, two jobs

Mass numbervsAtomic number

Read the position first, then ask what that number counts.

Focus

What does it count?

Mass number

Protons + neutrons

Atomic number

Protons

The insight

Mass number combines two particle counts, whereas atomic number gives the proton count directly.

Position by symbol

Mass number

Top-left

Atomic number

Bottom-left

What can you find?

Mass number

Use with atomic number to find neutrons

Atomic number

Gives the number of protons

Read the symbol, find the particles

Use a generic atom written as ²³₁₁X.

Spot the isotopes

Compare what stays the same and what changes.

Two atoms of element X are written as ²⁰₈X and ²²₈X. Which statement is correct?

Key points

1Atom radius ≈ 0.1 nm = 1 × 10⁻¹⁰ m.
2Nucleus radius ≈ 1 × 10⁻¹⁴ m and is less than 1/10 000 of the atom's radius.
3Proton relative mass = 1, neutron = 1, electron = very small.
4Mass number = total number of protons and neutrons.
5Isotopes are atoms of the same element with different numbers of neutrons.

Worked example

Problem

A neutral atom is written as ²³₁₁X. How many protons, neutrons and electrons does it have?

🧠

Memory hook

Top is total: the top-left mass number counts protons plus neutrons; the bottom-left atomic number gives protons.

⚠ Watch out

Adding atomic number and mass number together instead of subtracting to find neutrons.

✓

Check yourself

Two atoms have the same chemical symbol but different mass numbers. What particle number must be different for them to be isotopes?

Flashcards

(9)
What is the approximate radius of an atom?
About 0.1 nm, which is 1 × 10⁻¹⁰ m.
What is the approximate radius of a nucleus?
About 1 × 10⁻¹⁴ m, less than 1/10 000 of the radius of the atom.
Where is almost all of an atom's mass?
In the nucleus.
What are the relative masses of a proton, neutron and electron?
Proton 1, neutron 1, electron very small.
What is mass number?
The total number of protons and neutrons in an atom.
What is an isotope?
An atom of the same element as another atom but with a different number of neutrons.
Where are mass number and atomic number written beside a chemical symbol?
Mass number is top-left; atomic number is bottom-left.
How do you find the number of neutrons from mass number and atomic number?
Subtract the atomic number from the mass number.
How can atomic size be compared with a one-metre scale?
An atom radius of 1 × 10⁻¹⁰ m is ten billion times smaller than 1 m.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Size and mass of atoms; isotopes properly — interactive practice, marked questions and flashcards.

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