Atomic structure & the periodic table
Atoms, elements and compounds, the development of the atomic model, electronic structure, and the periodic table and groups.
Learn
Atomic structure & the periodic table, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Describe atoms, elements and compounds
Everything is made of atoms. An element contains just one kind of atom and is shown by a symbol like O or Fe; a compound is two or more elements chemically joined, like water (H₂O). A mixture is substances together but not chemically bonded, so it can be separated by physical means such as filtering or distillation.
Describe the sub-atomic particles and their relative mass and charge
An atom has a tiny central nucleus of protons (charge +1, mass 1) and neutrons (charge 0, mass 1), surrounded by electrons (charge −1, almost no mass). The number of protons - the atomic number - defines the element, and atoms are neutral overall because protons and electrons balance. Isotopes are atoms of the same element with different numbers of neutrons.
Describe how the model of the atom has changed over time
Ideas about the atom developed with new evidence. Dalton pictured solid spheres; then the "plum pudding" model had electrons dotted in positive matter; Rutherford's gold-foil experiment showed a small dense nucleus; Bohr placed electrons in shells. This is a good example of how scientific models are revised when experiments reveal something new.
Work out electronic structure
Electrons occupy shells around the nucleus, filling from the inside out: up to 2 in the first shell, then 8, then 8. Sodium has 11 electrons, arranged 2, 8, 1. The number of electrons in the outer shell equals the group number and controls how the element reacts.
Describe the periodic table and how it is arranged
Elements are listed in order of atomic number, with those of similar properties lining up in columns called groups. Rows are periods. Metals sit on the left and centre, non-metals to the right, with a staircase between. Mendeleev's genius was leaving gaps for undiscovered elements, which turned up with the properties he predicted.
Describe the trends in Groups 1, 7 and 0
Group 1 (alkali metals) react with water, getting more reactive down the group as the outer electron is lost more easily - sodium fizzes, potassium bursts into flame. Group 7 (halogens) get less reactive down the group. Group 0 (noble gases) have full outer shells, so they are unreactive and exist as single atoms.
Atomic structure & the periodic table key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Atom
- The smallest part of an element that can exist. Atoms have a tiny central nucleus of protons and neutrons, surrounded by electrons.
- Element
- A substance made of only one type of atom.
- Compound
- A substance made of two or more elements chemically combined in fixed proportions. Compounds can only be separated by chemical reactions.
- Mixture
- Two or more substances not chemically combined, separable by physical processes such as filtration or distillation.
- Isotope
- Atoms of the same element with the same number of protons but different numbers of neutrons.
- Ion
- An atom or group of atoms with an electric charge, formed by losing electrons (positive) or gaining electrons (negative).
- Atomic number
- The number of protons in an atom. It defines which element the atom is.
- Mass number
- The total number of protons and neutrons in an atom.
- Group
- A column of the periodic table. Elements in the same group have the same number of outer-shell electrons, so they react in similar ways.
- Period
- A row of the periodic table. Moving across a period, atoms gain one more electron shell electron at a time.
Practice
Try a Atomic structure & the periodic table question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
Chlorine has two isotopes: 75% of atoms are chlorine-35 and 25% are chlorine-37. Work out the relative atomic mass of chlorine.
- 35.5
- 36
- 35
- 26.25
Show the answer and the working
Answer: 35.5
Relative atomic mass is the weighted mean: (75 × 35 + 25 × 37) ÷ 100 = 35.5.
- Relative atomic mass is a weighted mean of the isotope masses.
- Multiply each mass number by its abundance: 75 × 35 = 2625 and 25 × 37 = 925.
- Add them: 2625 + 925 = 3550.
- Divide by 100: Ar = 35.5.