What an atom is made of, how big it and its nucleus are, what isotopes and ions are, and how the model of the atom changed over time.
What you will learn
- Work out protons, neutrons and electrons from a nuclide
- Spot isotopes and explain what makes them isotopes
- Use standard form for the sizes of atoms and nuclei
- Explain how alpha scattering led to the nuclear model
Inside an atom
An atom has a tiny, positively charged nucleus made of protons and neutrons.
Electrons move around the nucleus at set distances called energy levels.
- proton: charge +1, relative mass 1, in the nucleus
- neutron: charge 0, relative mass 1, in the nucleus
- electron: charge −1, very small mass (about 1/2000), around the nucleus
An atom has the same number of electrons as protons, so it has no overall charge.
Sizes. The radius of an atom is about 1 × 10⁻¹⁰ m. The radius of the nucleus is about 10 000 times smaller, about 1 × 10⁻¹⁴ m, yet almost all the mass is there. Most of an atom is empty space.
- A sodium atom has a radius of 0.18 nm, and 1 nm = 1 × 10⁻⁹ m
- 0.18 × 10⁻⁹ m = 1.8 × 10⁻¹⁰ m
- nucleus: 1.8 × 10⁻¹⁰ ÷ 10 000 = 1.8 × 10⁻¹⁴ m
Dividing by 10 000 is dividing by 10⁴: take 4 from the power. 10⁻¹⁰ becomes 10⁻¹⁴, not 10⁻⁶.
To find how many times bigger one size is than another, divide the bigger by the smaller: divide the numbers in front, and subtract the powers of ten.
- An atom is 3.6 × 10⁻¹⁰ m across; its nucleus is 4.0 × 10⁻¹⁵ m across.
- numbers: 3.6 ÷ 4.0 = 0.9
- powers: 10⁻¹⁰ ÷ 10⁻¹⁵ = 10⁻¹⁰⁻⁽⁻¹⁵⁾ = 10⁵
- 0.9 × 10⁵ = 90 000 times bigger
A scale model uses the same factor: a model nucleus 1 cm (0.01 m) across would need a model atom 0.01 × 90 000 = 900 m across.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Inside an atom
- Atomic number, mass number, isotopes and ions
- How the model of the atom changed
- Try it: Find the numbers in the periodic table
- Try it: Your first nuclide
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Nucleus, Energy levels, Sizes, Mass number, Atomic number, Isotopes, Positive ion
In this unit: Atomic structure and radioactivity
The nuclear atom and how its model developed, isotopes, alpha, beta and gamma radiation, nuclear equations, half-life, the hazards and uses of radiation, and fission and fusion.