Section 1 of 6Structure of the atom
The nucleus
- The nucleus At the centre of every atom is a tiny, dense nucleus containing protons and neutrons.
- Protons are positive and neutrons are neutral, so the nucleus has an overall positive charge.
Electrons
- Electrons Negatively charged electrons move around the nucleus at particular distances.
- A neutral atom has the same number of electrons as protons, so the charges cancel.
Mostly empty space
- Mostly empty space An atom has a radius of about 1 × 10−10 m, while the nucleus is roughly 10 000 times smaller.
- Almost all of the mass of the atom is concentrated in the nucleus.
Proton number defines the element
- Proton number defines the element The number of protons decides which element an atom is,
- so the nuclear charge is characteristic of each element: helium has +2 and uranium has +92.
Section 2 of 6Electron energy levels
1Fixed levels
- Fixed levels Electrons occupy energy levels at set distances from the nucleus: up to 2 in the first, then up to 8 in the second and third.
2Absorb radiation
- Absorb radiation An electron that absorbs electromagnetic radiation of the right energy moves to a higher level (excitation).
3Emit radiation
- Emit radiation When an electron falls to a lower level (de-excitation) it releases the difference in energy as electromagnetic radiation.
Section 3 of 6Ionisation
Ion formation
Positive ions
- Positive ions If an outer electron is removed, the atom has more protons than electrons and becomes a positive ion.
Negative ions
- Negative ions If an atom gains an extra electron, it has more electrons than protons and becomes a negative ion.
How it happens
- How it happens Electrons can be removed by friction, or knocked out by high-energy radiation that ionises the atom.
- Ionising radiation is dangerous because ions in living cells can damage molecules such as DNA.
Section 4 of 6Isotopes
1
Atoms of the same element always have the same number of protons.
2
Atoms with the same number of protons but different numbers of neutrons are called isotopes.
3
Isotopes have the same chemical properties because they have the same number of electrons, but different masses.
4
Some isotopes are unstable because of an imbalance of protons and neutrons, and may decay. For example, hydrogen has the rarer isotopes deuterium (1 neutron) and tritium (2 neutrons).
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Nucleus | The small, dense, positively charged centre of an atom made of protons and neutrons. |
| Energy level | A fixed region at a set distance from the nucleus where electrons can be found. |
| Excitation | An electron moving to a higher energy level after absorbing radiation. |
| Ionisation | The gain or loss of electrons by an atom to form an ion. |
| Isotope | An atom of the same element with the same number of protons but a different number of neutrons. |
Section 6 of 6Exam technique
1
Saying that isotopes have different numbers of protons — they have the same proton number and different neutron numbers.
2
Drawing electrons inside the nucleus; the nucleus holds only protons and neutrons.
3
Thinking a positive ion forms by gaining protons; it forms by losing electrons.
4
Describing the atom as solid. Most of the volume is empty space, with the mass concentrated in the nucleus.