4.1.3Atomic Structure
Atomic and Mass Number — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 4 of 23
Part 1 of 7Structure of the atom
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | In the nucleus |
| Neutron | 1 | 0 | In the nucleus |
| Electron | Very small (about 1/2000) | −1 | In shells around the nucleus |
Part 2 of 7Size and make-up
Tiny radius
- Tiny radius An atom has a radius of about 1 × 10−10 m, and the nucleus is about 10 000 times smaller.
Mass in the nucleus
- Mass in the nucleus Nearly all the mass of the atom is concentrated in the nucleus, since electrons are so light.
Neutral overall
- Neutral overall An atom has equal numbers of protons and electrons, so the charges cancel and it has no net charge.
Part 3 of 7Reading nuclear notation
1
The atomic number (Z), written at the bottom, is the number of protons in the nucleus; it identifies the element.
2
The mass number (A), written at the top, is the total number of protons and neutrons.
3
Number of neutrons = mass number − atomic number.
4
In a neutral atom, number of electrons = number of protons. For carbon-14 (A = 14, Z = 6): 6 protons, 8 neutrons, 6 electrons.
Part 4 of 7Worked examples
| Atom | Atomic number | Mass number | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| Lithium-7 | 3 | 7 | 3 | 4 | 3 |
| Oxygen-16 | 8 | 16 | 8 | 8 | 8 |
| Sodium-23 | 11 | 23 | 11 | 12 | 11 |
| Chlorine-35 | 17 | 35 | 17 | 18 | 17 |
| Chlorine-37 | 17 | 37 | 17 | 20 | 17 |
| Uranium-238 | 92 | 238 | 92 | 146 | 92 |
Part 5 of 7Isotopes
Same element, different mass
Definition
- Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Example
- Example Carbon-12 has 6 neutrons; carbon-14 has 8 neutrons. Both have 6 protons.
Chemical properties
- Chemical properties Isotopes react identically because they have the same electron arrangement.
Stability
- Stability Some isotopes have unstable nuclei and are radioactive.
Part 6 of 7Where the numbers are found
Periodic table
- Periodic table Elements are arranged in order of increasing atomic number, so the position of an element tells you its number of protons.
Relative atomic mass
- Relative atomic mass The mass shown for most elements is an average of all of its isotopes,
- which is why it is often not a whole number.
- (e.g. chlorine, 35.5)
- which is why it is often not a whole number.
Electron arrangement
- Electron arrangement The electrons fill shells from the inside outwards (2, 8, 8),
- and this arrangement controls the chemical behaviour of the element.
Part 7 of 7Using the periodic table
Finding protons
- Finding protons Look up the element and read its atomic number;
- this equals the number of protons and, in a neutral atom, the number of electrons.
Finding neutrons
- Finding neutrons Subtract the atomic number from the mass number of the particular isotope you are given,
- because different isotopes of one element have different neutron numbers.
Key terms
| Term | Meaning |
|---|---|
| Atomic number | The number of protons in the nucleus of an atom. |
| Mass number | The total number of protons and neutrons in the nucleus. |
| Isotope | An atom of an element with the same number of protons but a different number of neutrons. |
| Nucleon | A particle in the nucleus, either a proton or a neutron. |
| Nucleus | The small, dense, positively charged centre of an atom. |
| Relative atomic mass | The average mass of the atoms of an element, taking account of its isotopes, compared with carbon-12. |
Avoid these mistakes
1
Swapping atomic and mass numbersthe larger number at the top is the mass number.
2
Subtracting from the wrong number to find neutrons — use mass number minus atomic number.
3
Saying isotopes have different numbers of electrons or protons; only neutrons differ.
4
Giving the electron a relative mass of 1 rather than a very small value.
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