What is an atom made of, how are electrons arranged, and how is the periodic table organised?
The structure of the atom in terms of protons, neutrons and electrons, atomic number and mass number, isotopes, electron arrangement in shells, and how the periodic table is organised into groups and periods including the trends in Groups 1, 7 and 0.
A focused CCEA GCSE Single Award Science answer on atomic structure, covering protons, neutrons and electrons, atomic number and mass number, isotopes, electron arrangement, and how the periodic table is organised into groups and periods with the trends in Groups 1, 7 and 0.
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What this dot point is asking
CCEA wants you to describe the atom in terms of protons, neutrons and electrons, use atomic number and mass number to count particles, explain isotopes, write electron arrangements, and explain how the periodic table is organised including the trends in Groups 1, 7 and 0.
The three subatomic particles
Almost all the mass of an atom is in the nucleus. A neutral atom has equal numbers of protons and electrons, so the charges cancel.
Atomic number, mass number and isotopes
From these numbers: protons equals the atomic number, electrons equals the atomic number (in a neutral atom), and neutrons equals the mass number minus the atomic number.
Electron arrangement
Electrons are arranged in shells around the nucleus. The first shell holds up to 2 electrons, then the next shells hold up to 8 each. For example sodium (atomic number 11) is 2, 8, 1.
The periodic table
The number of outer electrons equals the group number (for the main groups), which is why elements in a group behave alike.
Trends in Groups 1, 7 and 0
- Group 1 (alkali metals) are soft, reactive metals that react with water to give an alkali and hydrogen. Reactivity increases down the group, because the single outer electron is further out and lost more easily.
- Group 7 (halogens) are coloured non-metals that form -1 ions. Reactivity decreases down the group, and a more reactive halogen displaces a less reactive one.
- Group 0 (noble gases) have full outer shells, so they are very unreactive.
Examples in context
Example 1. Why isotopes behave the same chemically. Carbon-12 and carbon-14 both have 6 protons and 6 electrons, differing only in neutrons. Because chemical reactions depend on electrons, the two isotopes react in exactly the same way; they differ only in mass. This is why an element can have a relative atomic mass that is not a whole number, such as chlorine at 35.5, the average of its isotopes.
Example 2. Using the noble gases. The noble gases are unreactive because their outer shells are full. This is useful: argon fills light bulbs so the hot filament does not burn away, and helium fills balloons because it is light and will not react or catch fire. Linking the inertness to the full outer shell is exactly the reasoning CCEA rewards.
Try this
Q1. State the relative charge of a proton, a neutron and an electron. [1 mark]
- Cue. Proton +1, neutron 0, electron -1.
Q2. Write the electron arrangement of an atom with atomic number 12. [1 mark]
- Cue. 2, 8, 2.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20194 marksAn atom of sodium is written as sodium-23 with atomic number 11. State the number of protons, neutrons and electrons it contains, and explain how you worked out the neutrons.Show worked answer →
Four marks for the three counts and the method for neutrons.
Protons equals the atomic number, which is 11.
Electrons equals the number of protons in a neutral atom, which is 11.
Neutrons equals the mass number minus the atomic number: 23 minus 11 equals 12.
So sodium-23 has 11 protons, 12 neutrons and 11 electrons. Markers reward the three correct counts and the neutron working shown as mass number minus atomic number.
CCEA SAS 20213 marksExplain why the reactivity of the Group 1 metals increases going down the group.Show worked answer →
Three marks for the structure-based explanation.
Each Group 1 atom has one outer electron, which it loses when it reacts.
Going down the group the atom is larger, so the outer electron is further from the nucleus and shielded by more inner shells.
The outer electron is therefore held less strongly, so it is lost more easily, making the metal more reactive down the group.
Markers reward the larger atom, more shielding and the outer electron being lost more easily.
Related dot points
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A focused CCEA GCSE Single Award Science answer on bonding, covering ionic bonding and the giant ionic lattice, covalent bonding in simple molecules, dot-and-cross diagrams, and how the structure explains melting points and electrical conductivity.
- Writing chemical formulae by balancing ion charges, turning reactions into word equations and balanced symbol equations with state symbols, and the conservation of mass in a chemical reaction.
A focused CCEA GCSE Single Award Science answer on chemical formulae and equations, covering writing formulae by balancing ion charges, word and balanced symbol equations with state symbols, how to balance, and the conservation of mass.
- Acids, bases and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation to make a salt and water, the reactions of acids with metals, oxides, hydroxides and carbonates, and simple tests for hydrogen and carbon dioxide.
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- The reactivity series, the reactions of metals with water and acid, displacement reactions, how reactivity decides extraction by carbon reduction or electrolysis, the extraction of iron in the blast furnace, and the rusting of iron and its prevention.
A focused CCEA GCSE Single Award Science answer on metals, covering the reactivity series, reactions with water and acid, displacement reactions, extraction by carbon reduction or electrolysis, the blast furnace, and the rusting of iron and how it is prevented.
Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)