Atomic Structure and the Periodic Table
Atomic structure and the periodic table is the GCSE chemistry topic covering the structure of atoms (protons, neutrons and electrons), isotopes and relative atomic mass, and how the modern periodic table arranges elements by atomic number into groups with similar properties. It also covers the historical development of atomic models and separation techniques for mixtures.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the relative charge and relative mass of protons (+1, mass 1), neutrons (0, mass 1) and electrons (-1, mass almost 0), and that the nucleus contains almost all the mass of the atom.
- Use the mass number and atomic number to find the number of protons, neutrons and electrons: protons = atomic number; neutrons = mass number - atomic number; electrons = atomic number, for a neutral atom.
- For relative atomic mass calculations, multiply each isotope's mass number by its percentage abundance, add the results together, then divide by 100.
- Learn electronic structures using the rule that the first shell holds up to 2 electrons and the second and third shells hold up to 8 electrons each, and know that the number of outer-shell electrons equals the group number.
- Learn the reactivity trends in Group 1 (reactivity increases down the group, as the outer electron is further from the nucleus and more shielded) and Group 7 (reactivity decreases down the group), and the properties of Group 0 (unreactive, full outer shell).
- Learn the physical separation techniques (filtration, evaporation, distillation, chromatography) and choose the correct one based on whether substances are soluble, insoluble, or have different boiling points.
Worked example
An element has two isotopes: 70% of atoms have a mass number of 20, and 30% of atoms have a mass number of 22. Calculate the relative atomic mass of the element. Give your answer to 1 decimal place.
- Multiply the first isotope's mass number by its percentage abundance: 20 x 70 = 1400.
- Multiply the second isotope's mass number by its percentage abundance: 22 x 30 = 660.
- Add the two results together: 1400 + 660 = 2060.
- Divide by 100, the total percentage: 2060 / 100 = 20.6.
- Final answer: the relative atomic mass is 20.6.
Practice questions
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Q1State the relative charge of a neutron.Show answer
Answer: 0 (no charge)
Q2An atom has atomic number 11 and mass number 23. State the number of protons.Show answer
Answer: 11
Q3An atom has atomic number 9 and mass number 19. Calculate the number of neutrons.Show answer
Answer: 10 (19 - 9)
Q4Write the electronic structure of an atom with 12 electrons.Show answer
Answer: 2,8,2
Q5A sample contains 90% of an isotope with mass number 10 and 10% of an isotope with mass number 11. Calculate the relative atomic mass, to 1 decimal place.Show answer
Answer: 10.1 ((10x90 + 11x10)/100 = 1010/100)
Q6Explain, in terms of electrons, why reactivity decreases going down Group 7.Show answer
Answer: Going down the group, the outer shell is further from the nucleus and more shielded by inner shells, so the attraction between the nucleus and an incoming electron is weaker, making it harder to gain an electron, so the element is less reactive.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
State the number of electrons an atom of magnesium (atomic number 12) has, and explain why this equals the number of protons.
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Boron has two isotopes. In a sample, 20% of atoms are boron-10 and 80% of atoms are boron-11. Calculate the relative atomic mass of boron in this sample. Give your answer to 1 decimal place.
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In the alpha particle scattering experiment, most alpha particles passed straight through a thin sheet of gold foil, a small number were deflected through large angles, and a very few bounced almost straight back. Explain how these observations led scientists to replace the plum pudding model with the nuclear model of the atom.
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Free printable worksheet
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This topic is chapter 1 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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