Atomic structure — GCSE Physics Revision
Everything you need to revise atomic structure for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
The Basic Structure of an Atom
Atoms are the fundamental building blocks of all matter. Every atom consists of a tiny, dense central nucleus surrounded by electrons. The nucleus contains protons and neutrons. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. The overall charge of an atom is zero because the number of protons (positive charges) equals the number of electrons (negative charges). This balance ensures the atom is electrically neutral. The electrons orbit the nucleus in specific energy levels or shells.
Atomic Number and Mass Number
The atomic number (Z) represents the number of protons in an atom's nucleus. It uniquely identifies an element. All atoms of a particular element have the same atomic number. The mass number (A) represents the total number of protons and neutrons in the nucleus. To find the number of neutrons, you subtract the atomic number from the mass number (A - Z). The mass of an electron is negligible compared to protons and neutrons, so it's not included in the mass number calculation.
Isotopes: Variations of an Element
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because they have the same number of protons, isotopes have the same atomic number and thus belong to the same element. However, due to the different number of neutrons, they have different mass numbers. For example, Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons. Isotopes often have different physical properties but identical chemical properties.
The Development of the Atomic Model
Our understanding of the atom has evolved over time. Dalton's model proposed indivisible spheres. Thomson's 'plum pudding' model suggested positive matter with embedded electrons. Rutherford's gold foil experiment led to the nuclear model, with a dense, positive nucleus and orbiting electrons. Bohr then refined this, proposing electrons orbit in specific energy shells. Finally, the modern model suggests electrons exist in 'clouds' of probability around the nucleus, not fixed orbits, reflecting quantum mechanical principles.
Key facts
- Atoms are neutral; number of protons equals number of electrons.|Atomic number (Z) = number of protons.|Mass number (A) = protons + neutrons.|Isotopes have same protons, different neutrons.|Electrons orbit in shells/energy levels.|Rutherford's experiment proved a dense, positive nucleus.
Key terms
- Atomic number::The number of protons in the nucleus of an atom, defining the element.|Mass number::The total number of protons and neutrons in the nucleus of an atom.|Isotopes::Atoms of the same element with the same number of protons but different numbers of neutrons.|Ion::An atom or molecule that has lost or gained one or more electrons, giving it an electric charge.|Nucleus::The small, dense, positively charged centre of an atom, containing protons and neutrons.|Electron shells::Regions around the nucleus where electrons are found, each with a specific energy level.
Common mistakes
- Confusing atomic number with mass number when calculating neutrons.|Assuming all atoms of an element have the same mass number (forgetting isotopes).|Stating that the nucleus contains electrons or that electrons have significant mass.|Incorrectly describing the charges of subatomic particles (e.g., neutron is negative).
Exam tips
- Clearly state the charge and relative mass of each subatomic particle.|Remember Rutherford's experiment and its key findings (alpha particle scattering).|Practice calculating protons, neutrons, and electrons from atomic and mass numbers.|Be able to describe how atomic models have changed over time and why.
Quick quiz
1. Which subatomic particle has a relative charge of +1?
- Electron
- Neutron
- Proton
- Ion
Show answer
Proton — Protons are positively charged, with a relative charge of +1. Electrons have a relative charge of -1, and neutrons have no charge.
2. An atom has an atomic number of 17 and a mass number of 35. How many neutrons does it have?
- 17
- 18
- 35
- 52
Show answer
18 — Number of neutrons = Mass number - Atomic number = 35 - 17 = 18.
3. What did Rutherford's alpha particle scattering experiment conclude about the atom?
- Atoms are solid, indivisible spheres.
- Atoms are a uniform sphere of positive charge with electrons embedded.
- Atoms have a tiny, dense, positively charged nucleus.
- Electrons orbit the nucleus in fixed energy levels.
Show answer
Atoms have a tiny, dense, positively charged nucleus. — The experiment showed that most alpha particles passed through, but some were deflected at large angles, indicating a small, dense, positive nucleus.
4. Which statement correctly describes isotopes of an element?
- They have different numbers of protons.
- They have different numbers of electrons.
- They have the same number of protons but different numbers of neutrons.
- They have different atomic numbers.
Show answer
They have the same number of protons but different numbers of neutrons. — Isotopes are atoms of the same element, meaning they have the same number of protons (same atomic number), but they differ in their number of neutrons, leading to different mass numbers.
5. In a neutral atom, which two subatomic particles are present in equal numbers?
- Protons and neutrons
- Neutrons and electrons
- Protons and electrons
- Nucleus and electrons
Show answer
Protons and electrons — For an atom to be electrically neutral, the positive charges from protons must balance the negative charges from electrons, so their numbers are equal.
Exam-style questions
Describe the main difference between an atom and an ion. [2 marks]
Show mark scheme
- An atom has an equal number of protons and electrons, making it electrically neutral.
- An ion has either lost or gained electrons, giving it an overall positive or negative charge.
Explain why isotopes of an element have the same chemical properties but can have different physical properties. [3 marks]
Show mark scheme
- Chemical properties are determined by the number of electrons (or electron configuration).
- Isotopes have the same number of protons and therefore the same number of electrons (in neutral atoms), leading to identical chemical properties.
- Physical properties (e.g., mass, density) are affected by the total mass, which differs due to the different number of neutrons.
An atom of element X has a mass number of 40 and contains 20 neutrons. Calculate the atomic number of element X and identify the element. [3 marks]
Show mark scheme
- Atomic number = Mass number - number of neutrons.
- Atomic number = 40 - 20 = 20.
- The element with atomic number 20 is Calcium (Ca).
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