What you'll learn
- How scientists’ ideas about atoms changed as new evidence was discovered.
- The structure of an atom: nucleus, protons, neutrons, electrons and shells.
- How to use atomic number and mass number to find numbers of particles.
- Why isotopes make some relative atomic masses non-whole numbers.
Why atomic models changed
A scientific model is a simplified way of representing something we cannot easily see directly. Atoms are far too small to see in normal microscopes, so scientists use evidence from experiments to build and improve models.
John Dalton’s model described atoms as tiny, solid, indivisible spheres. This was a useful starting point, but later discoveries showed that atoms contain smaller particles.
The model changed roughly like this:
- Dalton model: atoms are solid spheres and cannot be split.
- Discovery of electrons: atoms must contain tiny negative particles, so they are not indivisible.
- Nuclear model: experiments showed atoms have a tiny, dense, positive centre called the nucleus.
- Electron shells: electrons are arranged in shells around the nucleus.
- Discovery of neutrons: the nucleus contains neutral particles as well as positive protons.
Models change with evidence
The atomic model changed because scientists discovered subatomic particles and gathered evidence that atoms are mostly empty space with a tiny nucleus.
The structure of an atom
An atom is the smallest particle of an element that can still be recognised as that element.
Subatomic particles
A subatomic particle is a particle smaller than an atom. The three GCSE particles you need are protons, neutrons and electrons.
In the current GCSE model:
- The nucleus is the tiny central part of the atom.
- The nucleus contains protons and neutrons.
- Electrons move around the nucleus in shells, which are energy levels around the nucleus.
Relative charge and relative mass
“Relative” means compared with something else. At GCSE, we compare particle charges and masses using simple relative values.
| Particle | Where it is found | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells | -1 | about 1/1840 |
Electrons have a tiny mass compared with protons and neutrons, so almost all the mass of an atom is in the nucleus.
Where the mass is
The nucleus is tiny compared with the whole atom, but it contains nearly all the atom’s mass because it contains the protons and neutrons.
Atoms are extremely small. A typical atomic radius is about 1×10−10 m1 \times 10^{-10}\ \text{m}1×10−10 m. The nucleus is much smaller again, around 1×10−14 m1 \times 10^{-14}\ \text{m}1×10−14 m across. That means most of the atom is empty space.
Why atoms are neutral
An atom has no overall charge. This is because it contains equal numbers of protons and electrons.
Each proton has charge +1. Each electron has charge -1. If there are equal numbers of them, the charges cancel out.
Checking the charge of an atom
A sodium atom has 11 protons, 11 electrons and 12 neutrons. Explain why it has no overall charge.
- Work out the total positive charge: 11 protons each have charge +1, so the total positive charge is +11.
- Work out the total negative charge: 11 electrons each have charge -1, so the total negative charge is -11. Neutrons have charge 0, so they do not affect the charge.
- Combine the charges: +11+(−11)=0+11 + (-11) = 0+11+(−11)=0, so the sodium atom is neutral.
Atoms and ions
Do not say every particle has equal numbers of protons and electrons. Atoms do, but ions are charged particles formed when atoms lose or gain electrons.
Atomic number and mass number
The atomic number is the number of protons in the nucleus of an atom.
This number is unique to each element. For example, every atom with 6 protons is carbon. If it has 7 protons, it is nitrogen instead.
Mass number
The mass number is the total number of protons and neutrons in the nucleus of an atom.
You can remember this as:
mass number=number of protons+number of neutrons\text{mass number} = \text{number of protons} + \text{number of neutrons}mass number=number of protons+number of neutronsSo:
number of neutrons=mass number−atomic number\text{number of neutrons} = \text{mass number} - \text{atomic number}number of neutrons=mass number−atomic numberFor a neutral atom:
number of electrons=number of protons\text{number of electrons} = \text{number of protons}number of electrons=number of protons
Finding protons, neutrons and electrons
An aluminium atom has atomic number 13 and mass number 27. Calculate the number of protons, neutrons and electrons.
- Use the atomic number to find protons: aluminium has atomic number 13, so it has 13 protons.
- Use the mass number to find neutrons: 27−13=1427 - 13 = 1427−13=14, so it has 14 neutrons.
- Because it is an atom, it is neutral: electrons = protons, so it has 13 electrons.
Quick particle check
Atomic number gives protons. Mass number gives protons plus neutrons. Electrons equal protons only if the question says it is an atom or shows no charge.
Isotopes
Atoms of the same element always have the same number of protons. However, they do not always have the same number of neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
For example:
- Carbon-12 has 6 protons and 6 neutrons.
- Carbon-14 has 6 protons and 8 neutrons.
Both are carbon because both have 6 protons. They are different isotopes because they have different numbers of neutrons.
What changes in isotopes
Isotopes of the same element differ in neutrons, not protons. If the number of protons changes, it becomes a different element.
Relative atomic mass and isotopes
The relative atomic mass, written as ArA_\text{r}Ar, is the average mass of atoms of an element compared with carbon-12.
Because many elements exist as a mixture of isotopes, the relative atomic mass is often not a whole number. It is a weighted mean, meaning the more abundant isotopes affect the average more strongly.
For example, chlorine has atoms that are mostly chlorine-35, with some chlorine-37. Its relative atomic mass is about 35.5, not exactly 35 or 37.
Abundance
The abundance of an isotope tells you how common that isotope is in a sample, often given as a percentage.
This next calculation is Higher Tier only, but the idea is useful for everyone: relative atomic mass is an average that takes account of abundance.
Ar=∑(isotope mass×abundance)∑abundancesA_\text{r} = \frac{\sum(\text{isotope mass} \times \text{abundance})}{\sum \text{abundances}}Ar=∑abundances∑(isotope mass×abundance)If abundances are percentages, the total abundance is 100.
Calculating relative atomic mass
Chlorine contains 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine.
-
Use a weighted mean because the isotopes are not equally common: multiply each isotope mass by its percentage abundance.
-
Substitute the values into the formula:
Ar=(35×75)+(37×25)100A_\text{r} = \frac{(35 \times 75) + (37 \times 25)}{100}Ar=100(35×75)+(37×25) -
Calculate the numerator:
(35×75)+(37×25)=2625+925=3550(35 \times 75) + (37 \times 25) = 2625 + 925 = 3550(35×75)+(37×25)=2625+925=3550 -
Divide by the total abundance:
Ar=3550100=35.5A_\text{r} = \frac{3550}{100} = 35.5Ar=1003550=35.5 -
Interpret the answer: 35.5 is closer to 35 than 37 because chlorine-35 is more abundant.
Averaging isotopes wrongly
Do not just add the isotope masses and divide by 2 unless the isotopes are equally abundant. Use the abundances given in the question.
In the exam
- For particle-count questions, start with atomic number = protons, then use mass number minus atomic number for neutrons.
- If the particle is a neutral atom, electrons = protons; if it has a charge, think carefully because electrons may have changed.
- For isotope questions, remember: same element means same protons; different isotope means different neutrons.
Check yourself
- Why did Dalton’s solid-sphere model of the atom need to change?
- An atom has atomic number 9 and mass number 19. How many protons, neutrons and electrons does it have?
- Why can the relative atomic mass of an element be a decimal?
