What is an atom made of, and how do we describe its particles?
Atomic structure: protons, neutrons and electrons; atomic number and mass number; electron arrangement of the first 20 elements; isotopes and relative atomic mass.
An SQA National 5 Chemistry answer on atomic structure, covering protons, neutrons and electrons, atomic number and mass number, electron arrangements of the first 20 elements, isotopes, and how relative atomic mass is a weighted average of isotope masses.
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What this key area is asking
The SQA wants you to describe the three subatomic particles and their charge and mass, work out the numbers of protons, neutrons and electrons from atomic number and mass number, write the electron arrangement of any of the first 20 elements, define isotopes, and explain why relative atomic mass is usually not a whole number. These ideas underpin bonding, formulae and the whole of the course.
The three subatomic particles
Atomic number and mass number
From these two numbers you can find every particle count:
- number of protons = atomic number
- number of electrons = atomic number (in a neutral atom)
- number of neutrons = mass number atomic number
Electron arrangement
Electrons occupy energy levels (shells) around the nucleus. The first level holds up to 2 electrons, and the next levels each hold up to 8 for the first 20 elements. You fill from the inside out.
Isotopes
Isotopes of an element behave the same chemically, because chemistry depends on the electrons, and isotopes have the same number of electrons. They differ only slightly in mass.
Relative atomic mass
The relative atomic mass of an element is the average mass of its atoms, weighted by how common each isotope is. Because it is an average of different isotope masses, it is often not a whole number.
Worked example: a weighted average
Examples in context
Carbon dating relies on isotopes: living things take in a fixed proportion of radioactive carbon-14 alongside ordinary carbon-12, and the carbon-14 decays slowly after death, so its amount measures age. Smoke detectors contain a tiny source of the isotope americium-241. In each case the chemistry of the element is unchanged by which isotope is present, because the electron arrangement is the same; only the nucleus differs.
Try this
Q1. An atom has 19 protons and 20 neutrons. State its mass number and the number of electrons in the neutral atom. [2 marks]
- Cue. Mass number ; electrons (it is potassium).
Q2. Write the electron arrangement of an atom of sulfur (atomic number 16). [1 mark]
- Cue. .
Q3. Explain why isotopes of the same element react in the same way. [2 marks]
- Cue. They have the same number of electrons, and chemistry depends on the electrons, not the neutrons.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2018 style3 marksAn atom of an element has 17 protons and 18 neutrons. State its atomic number, its mass number, and the number of electrons in a neutral atom of this element.Show worked answer →
Markers reward each of the three values, found from the definitions.
The atomic number is the number of protons, so the atomic number is 17.
The mass number is the number of protons plus neutrons, so it is .
A neutral atom has equal numbers of protons and electrons, so the number of electrons is 17. (This element is chlorine.)
A common error is to add electrons into the mass number; only protons and neutrons count towards mass.
SQA N5 2020 style3 marksChlorine has two isotopes, chlorine-35 and chlorine-37. Explain what is meant by isotopes, and explain why the relative atomic mass of chlorine in the data booklet is 35.5 rather than a whole number.Show worked answer →
A 3 mark answer needs a correct definition of isotopes and a clear weighted-average explanation.
Isotopes are atoms of the same element, so they have the same atomic number (same number of protons), but they have different mass numbers because they contain different numbers of neutrons. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons.
The relative atomic mass is the average mass of the atoms of an element, weighted by how common each isotope is. Chlorine is a mixture of about three parts chlorine-35 to one part chlorine-37, so the weighted average sits closer to 35 and works out at 35.5. Because it is an average of two different masses, it is not a whole number.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)