Atoms are extremely small
Definition
Nucleus
The tiny central part of an atom that contains protons and neutrons and accounts for almost all of the atom’s mass.
- Atoms have a radius of about 0.10.10.1 nm, which is 1×10−101\times10^{-10}1×10−10 m.
- The nucleus sits at the centre and is much smaller still.
- The radius of the nucleus is less than 110000\frac{1}{10000}100001 of the radius of the atom, about 1×10−141\times10^{-14}1×10−14 m.
- This means an atom is mostly empty space, with a tiny nucleus and electrons far out in shells.
- If an atom were the size of a sports stadium, the nucleus would be about the size of a pea at the centre.
Key Idea
- An atom's radius (1×10−101\times10^{-10}1×10−10 m) is about 10000 times larger than its nucleus (1×10−141\times10^{-14}1×10−14 m).
- Numbers this small are written in standard form to keep them manageable.
Almost all the mass sits in the nucleus
Definition
Relative mass
The mass of a particle compared on a scale where a proton and a neutron each have a relative mass of approximately one.
- Masses of subatomic particles are compared on a relative mass scale.
- A proton and a neutron each have a relative mass of 111.
- An electron has a relative mass of about 11835\frac{1}{1835}18351, so small it is usually treated as almost zero.
- Because protons and neutrons are in the nucleus, almost all the mass of an atom is in the nucleus.
- The electrons add almost nothing to the mass, even though they fill most of the atom's volume.
- So the mass of an atom is set by its number of protons and neutrons, not its electrons.
Self review
- What is the approximate radius of an atom in metres?
- How does the size of the nucleus compare with the whole atom?
- What are the relative masses of a proton, neutron and electron?
- Why is almost all the mass of an atom in the nucleus?
