Size and mass of atoms
What you'll learn
- How small atoms and nuclei are, using nanometres and standard form.
- Why almost all the mass of an atom is in the nucleus.
- What atomic number and mass number mean.
- How to calculate the numbers of protons, neutrons and electrons in atoms, ions and isotopes.
The basic model of an atom
An atom is the smallest part of an element that still has the chemical properties of that element. At GCSE, you use a simple model where an atom has a tiny central nucleus with electrons arranged around it in shells.
Subatomic particles
Subatomic particles are the smaller particles that make up atoms: protons, neutrons and electrons. Protons and neutrons are found in the nucleus, while electrons are found around the nucleus in shells.
The diagram below shows the key idea: the atom is tiny, but the nucleus is far, far smaller than the atom as a whole.
How small is an atom?
A typical atom has a radius of about 0.1 nanometres.
Radius
The radius of an object is the distance from its centre to its edge.
A nanometre is a very small unit of length. The prefix nano- means one billionth.
1 nm=1×10−9 m1\text{ nm} = 1 \times 10^{-9}\text{ m}1 nm=1×10−9 mSo 0.1 nm is:
0.1 nm=1×10−10 m0.1\text{ nm} = 1 \times 10^{-10}\text{ m}0.1 nm=1×10−10 mStandard form
Standard form writes very large or very small numbers as a number from 1 up to, but not including, 10 multiplied by a power of ten. For example, 1×10−10 m1 \times 10^{-10}\text{ m}1×10−10 m is standard form.
The nucleus has a radius of about 1×10−14 m1 \times 10^{-14}\text{ m}1×10−14 m, which is less than one ten-thousandth of the radius of the atom.
Comparing atom and nucleus radii
An atom has radius 1×10−10 m1 \times 10^{-10}\text{ m}1×10−10 m. Its nucleus has radius 1×10−14 m1 \times 10^{-14}\text{ m}1×10−14 m. How many times larger is the atom’s radius than the nucleus’s radius?
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Set up the comparison as a ratio:
atom radiusnucleus radius=1×10−101×10−14\frac{\text{atom radius}}{\text{nucleus radius}} = \frac{1 \times 10^{-10}}{1 \times 10^{-14}}nucleus radiusatom radius=1×10−141×10−10 -
Divide the numbers and subtract the powers of ten:
1×10−101×10−14=1×104\frac{1 \times 10^{-10}}{1 \times 10^{-14}} = 1 \times 10^{4}1×10−141×10−10=1×104 -
Interpret the answer: 1×1041 \times 10^{4}1×104 means 10 000, so the atom’s radius is about 10 000 times larger than the nucleus’s radius.
Atomic scale
Atoms are about 1×10−10 m1 \times 10^{-10}\text{ m}1×10−10 m in radius, while nuclei are about 1×10−14 m1 \times 10^{-14}\text{ m}1×10−14 m in radius. The nucleus is tiny compared with the whole atom.
A stadium-sized atom
If an atom were scaled up so its radius was about 100 m, the nucleus would have a radius of about 1 cm. Most of the atom would be empty space in this simple model.
Where is the mass of an atom?
Almost all the mass of an atom is in the nucleus. This is because protons and neutrons have much more mass than electrons.
Relative mass
Relative mass compares particle masses without using extremely small values in kilograms. At GCSE, a proton has relative mass 1, a neutron has relative mass 1, and an electron has a very small relative mass.
| Particle | Where it is found | Charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | Positive | 1 |
| Neutron | Nucleus | No charge | 1 |
| Electron | Shells around nucleus | Negative | Very small |
Because electrons have such a small mass, you usually ignore their mass when finding the mass number of an atom.
Mass is concentrated in the nucleus
The nucleus is tiny, but it contains the protons and neutrons, so almost all the mass of the atom is found there.
Atomic number and mass number
To describe atoms properly, you need two numbers: the atomic number and the mass number.
Atomic number
The atomic number is the number of protons in the nucleus of an atom. It identifies the element.
For example, every sodium atom has 11 protons. If an atom has 11 protons, it is sodium.
Mass number
The mass number is the total number of protons and neutrons in the nucleus.
So:
number of neutrons=mass number−atomic number\text{number of neutrons} = \text{mass number} - \text{atomic number}number of neutrons=mass number−atomic numberAtoms can be written using nuclear notation. For sodium-23:
1123Na{}^{23}_{11}\text{Na}1123Na- The top number, 23, is the mass number.
- The bottom number, 11, is the atomic number.
- The symbol Na tells you the element is sodium.
Finding protons, neutrons and electrons
Find the numbers of protons, neutrons and electrons in a neutral atom of 1123Na{}^{23}_{11}\text{Na}1123Na.
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Use the atomic number to find protons: the atomic number is 11, so sodium has 11 protons.
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Use the mass number to find neutrons:
neutrons=23−11=12\text{neutrons} = 23 - 11 = 12neutrons=23−11=12 -
The atom is neutral, so the number of electrons equals the number of protons. It has 11 electrons.
Mixing up the two numbers
The mass number is the larger top number and means protons plus neutrons. The atomic number is the smaller bottom number and means protons only.
Atoms and ions
A neutral atom has no overall charge. That means it has the same number of protons and electrons.
An ion is a charged particle formed when an atom gains or loses electrons. The nucleus does not change when an ion forms, so the numbers of protons and neutrons stay the same.
Ion
An ion is an atom or group of atoms with an overall charge because it has gained or lost electrons.
- A positive ion has lost electrons.
- A negative ion has gained electrons.
Electron shortcut for ions
Positive charge means fewer electrons than protons. Negative charge means more electrons than protons.
Finding electrons in an ion
Find the numbers of protons, neutrons and electrons in 1224Mg2+{}^{24}_{12}\text{Mg}^{2+}1224Mg2+.
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Use the atomic number to find protons: magnesium has atomic number 12, so it has 12 protons.
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Use the mass number to find neutrons:
neutrons=24−12=12\text{neutrons} = 24 - 12 = 12neutrons=24−12=12 -
The ion has a 2+ charge, so it has lost 2 electrons:
electrons=12−2=10\text{electrons} = 12 - 2 = 10electrons=12−2=10
Changing protons instead of electrons
When atoms become ions, they gain or lose electrons, not protons. Changing the number of protons would change the element.
Isotopes
Atoms of the same element always have the same number of protons. However, they can have different numbers of neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Because isotopes have different numbers of neutrons, they have different mass numbers.
Identifying isotopes
Decide whether 1735Cl{}^{35}_{17}\text{Cl}1735Cl and 1737Cl{}^{37}_{17}\text{Cl}1737Cl are isotopes of the same element.
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Compare the atomic numbers: both atoms have atomic number 17, so both have 17 protons and are chlorine atoms.
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Calculate the number of neutrons in each atom:
35Cl: 35−17=18 neutrons37Cl: 37−17=20 neutrons\begin{aligned} {}^{35}\text{Cl}:&\ 35 - 17 = 18 \text{ neutrons} \\ {}^{37}\text{Cl}:&\ 37 - 17 = 20 \text{ neutrons} \end{aligned}35Cl:37Cl: 35−17=18 neutrons 37−17=20 neutrons -
They have the same number of protons but different numbers of neutrons, so they are isotopes.
What makes isotopes different
Isotopes are the same element because they have the same number of protons, but they have different masses because they have different numbers of neutrons.
Quick calculation method
When you are given an atom or ion, use this order:
- Atomic number gives the number of protons.
- Mass number minus atomic number gives the number of neutrons.
- Charge tells you how many electrons have been gained or lost.
For a neutral atom:
electrons=protons\text{electrons} = \text{protons}electrons=protonsFor ions, adjust the electrons only.
In the exam
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In notation like ZAX{}^{A}_{Z}\text{X}ZAX, the bottom number ZZZ is the atomic number, so it gives the number of protons.
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Find neutrons using mass number−atomic number\text{mass number} - \text{atomic number}mass number−atomic number, not by using the charge.
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For ions, change the number of electrons only: positive ions have lost electrons, negative ions have gained electrons.
Check yourself
- What is the approximate radius of an atom in metres, written in standard form?
- How many protons, neutrons and electrons are in 1327Al3+{}^{27}_{13}\text{Al}^{3+}1327Al3+?
- Why are 612C{}^{12}_{6}\text{C}612C and 614C{}^{14}_{6}\text{C}614C isotopes of the same element?