Atomic Structure
Edexcel International GCSE ChemistryΒ· 1.14β1.17Β· 20 min read
1. Key Definitions: Atoms and Moleculesβ βββββ± 5 min
All matter is made of particles, with atoms and molecules being the two fundamental units you need to distinguish for this specification.
Atom and Molecule
An atom is the smallest unit of an element that can take part in a chemical reaction. A molecule is two or more atoms chemically bonded together, either of the same element (e.g. Oβ) or different elements (e.g. HβO).
Classify the following as atoms or molecules: (a) Ne, (b) COβ, (c) Nβ
- 1
- Recall that an atom is a single neutral particle of an element, while a molecule has 2+ bonded atoms.
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- (a) Ne is a single, unbonded atom of neon, so it is an atom.
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- (b) COβ has 1 carbon and 2 oxygen atoms bonded, so it is a molecule.
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- (c) Nβ has 2 nitrogen atoms bonded, so it is a molecule.
Exam tip:
When given particle diagrams in exam questions, count the number of bonded circles to quickly distinguish atoms (1 circle) from molecules (2+ circles).
2. Subatomic Particle Structureβ β ββββ± 6 min
All atoms consist of a dense, positively charged central nucleus surrounded by negatively charged electrons orbiting in energy shells. The nucleus contains two types of particles: protons and neutrons.
Subatomic Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
Proton | +1 | 1 | Nucleus |
Neutron | 0 | 1 | Nucleus |
Electron | -1 | ~1/1840 (negligible) | Electron shells outside nucleus |
State the number of subatomic particles in a neutral atom of lithium, which has atomic number 3 and mass number 7.
- 1
- Atomic number = number of protons: 3 protons.
- 2
- Neutral atoms have equal numbers of protons and electrons: 3 electrons.
- 3
- Number of neutrons = mass number - atomic number = 7 - 3 = 4 neutrons.
Exam tip:
Neutral atoms have no overall charge because the positive charge from protons exactly cancels the negative charge from electrons.
3. Atomic Number, Mass Number and Isotopesβ β ββββ± 5 min
Two key numbers define the composition of an atom: atomic number (given on the periodic table for every element) and mass number (varies between isotopes of the same element).
Isotopes
Atoms of the same element with the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons).
Isotopes have identical chemical properties because they have the same number of electrons, but different physical properties like mass, density or radioactivity.
Oxygen has three isotopes: oxygen-16, oxygen-17 and oxygen-18. All have atomic number 8. State the number of neutrons in each isotope.
- 1
- Neutron count = mass number - atomic number for each isotope.
- 2
- Oxygen-16: 16 - 8 = 8 neutrons.
- 3
- Oxygen-17: 17 - 8 = 9 neutrons.
- 4
- Oxygen-18: 18 - 8 = 10 neutrons.
Exam tip:
Questions often ask why isotopes have the same chemical properties: always reference the equal number of electrons in your answer to get full marks.
4. Calculating Relative Atomic Mass (Ar)β β β βββ± 7 min
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The relative atomic mass (Ar) of an element is a weighted average of the masses of its naturally occurring isotopes, accounting for how abundant each isotope is.
Bromine has two isotopes: 50.7% of bromine atoms are bromine-79, 49.3% are bromine-81. Calculate the Ar of bromine to 3 significant figures.
- 1
- Calculate the weighted mass of each isotope:
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- 3
- Add the weighted masses together:
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- Divide by 100:
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- 7
- Round to 3 significant figures: Ar = 80.0
Exam tip:
Never calculate a simple average of isotope masses, as this does not account for differences in abundance and will give you an incorrect answer.
5. Common Pitfalls
Wrong move:
Counting electrons when calculating mass number
Why:
Electron mass is negligible, ~1/1840 of a proton mass, so it is not included in mass number calculations
Correct move:
Only add the number of protons and neutrons to calculate mass number
Wrong move:
Assuming the number of neutrons equals the number of protons
Why:
This is only true for some light elements, and isotopes of the same element have different neutron counts
Correct move:
Calculate neutron count as mass number minus atomic number for each atom/isotope
Wrong move:
Calculating a simple mean of isotope masses for Ar
Why:
Isotopes are not equally abundant in nature, so their masses must be weighted by their percentage abundance
Correct move:
Use the weighted mean formula: Ar = sum (isotope mass Γ % abundance) Γ· 100
Wrong move:
Stating isotopes have different chemical properties
Why:
Chemical properties are controlled by the number of electrons, which is identical for all isotopes of the same element
Correct move:
Remember isotopes only differ in physical properties like mass, density or radioactivity
Wrong move:
Using relative charge values for mass number calculations
Why:
Charge and mass are independent properties of subatomic particles
Correct move:
Only use relative mass values for any calculation of total atomic mass
6. Quick Reference Cheatsheet
Concept | Key Fact / Formula |
|---|---|
Atom | Smallest unit of an element; nucleus + orbiting electrons |
Molecule | 2+ atoms chemically bonded together |
Subatomic Particles | Proton: +1 charge, mass 1 (nucleus); Neutron: 0 charge, mass 1 (nucleus); Electron: -1 charge, mass ~1/1840 (shells) |
Atomic number (Z) | Number of protons = number of electrons (neutral atoms) |
Mass number (A) | Number of protons + number of neutrons |
Isotopes | Same atomic number, different mass number; same chemical properties |
Relative Atomic Mass (Ar) | Weighted mean mass; A_r = \frac{\text{Sum of (isotope mass} \times \text{% abundance)}}{100} |
7. Frequently Asked
Are electron masses included when calculating mass number?
No, the relative mass of an electron is ~1/1840, which is negligible, so only protons and neutrons are counted in mass number.
Why do isotopes have identical chemical properties?
Chemical properties are controlled by the number and arrangement of electrons, which is the same for all isotopes of the same element. Isotopes only differ in physical properties like mass or density.
What's Next
Now that you have mastered core atomic structure, you are ready to move on to electron arrangement in shells, which explains the structure of the periodic table and how elements form bonds. You will also use your understanding of relative atomic mass to calculate relative formula mass and mole quantities in upcoming quantitative chemistry topics. Practice Ar calculation questions regularly to avoid common weighting errors in your exam.