In simple terms
A friendly intro before the formal notes — no formulas yet.
The Atom's Blueprint
Atoms are built from simple rules involving protons, neutrons, and electrons. These rules determine an element's identity, mass, and size.
Imagine a solar system. The sun is the nucleus, its mass and gravitational pull determined by the protons and neutrons inside. The planets are electrons in different orbits (shells). A stronger sun (more protons) pulls the same number of orbits in closer, making the solar system smaller.
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Atoms contain protons and neutrons in the nucleus, electrons in shells.
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Atomic number Z = proton number; mass number A = protons + neutrons.
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Ions form when electrons are gained or lost — charge = protons − electrons.
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Atomic radius decreases across a period (increasing nuclear charge).
Explore the concept
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Atoms contain protons and neutrons in the nucleus, electrons in shells
Atoms contain protons and neutrons in the nucleus, electrons in shells.
Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Subatomic Particles
Atoms are composed of three main types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found together in a dense, central core called the nucleus. Electrons orbit the nucleus in specific energy levels or shells. The properties of these particles are crucial to understanding all of chemistry.
Proton (p): Relative mass = 1, Relative charge = +1
Neutron (n): Relative mass = 1, Relative charge = 0
Electron (e⁻): Relative mass ≈ 1/1840 (negligible), Relative charge = -1
Defining an Element: Atomic and Mass Numbers
The identity of an element is determined solely by its atomic number, Z, which is the number of protons in its nucleus. The mass number, A, represents the total number of nucleons (protons and neutrons) in the nucleus. We can represent any nuclide using the standard notation:
In this notation, X is the element symbol, A is the mass number, and Z is the atomic number. The number of neutrons can be calculated by . In a neutral atom, the number of electrons is equal to the number of protons (Z).
Ions: When Atoms Gain or Lose Electrons
Atoms are electrically neutral because they have an equal number of protons (+) and electrons (-). However, atoms can gain or lose electrons to form charged particles called ions. Losing electrons results in a positive ion (cation), while gaining electrons results in a negative ion (anion).
Charge of ion = (Number of protons) − (Number of electrons)
Trends in Atomic Radius
The atomic radius is a measure of the size of an atom, typically defined as half the distance between the nuclei of two identical atoms bonded together. This property shows clear trends in the Periodic Table.
Across a Period (e.g., Li to Ne): Atomic radius decreases. As you move across, a proton is added to the nucleus and an electron is added to the same principal energy shell. The increasing nuclear charge attracts the electrons more strongly, pulling the shell closer to the nucleus. The shielding effect from inner electrons is relatively constant.
Down a Group (e.g., Li to Cs): Atomic radius increases. As you move down, electrons occupy successively higher principal energy shells, which are further from the nucleus. Although the nuclear charge increases, this is outweighed by the increased distance and the shielding effect of the additional inner electron shells.
When asked to explain trends in atomic or ionic radius, you must refer to three key factors for full marks: 1. Nuclear charge (number of protons), 2. Electron shielding (repulsion from inner shells), and 3. The number of occupied principal electron shells.
Comparing Ionic Radii
The radius of an ion is different from its parent atom. Cations (positive ions) are always smaller than their parent atoms because they have lost their outermost electrons, leading to a decrease in repulsion and a stronger pull from the nucleus on the remaining electrons. Anions (negative ions) are always larger than their parent atoms because the addition of electrons increases electron-electron repulsion, causing the electron cloud to expand.
Cation radius < Parent atom radius (e.g., is smaller than )
Anion radius > Parent atom radius (e.g., is larger than )
For an isoelectronic series (same number of electrons), the species with the most protons will be the smallest due to the greatest nuclear attraction. E.g., is smaller than , which is smaller than .
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Determine the number of protons, neutrons, and electrons in a neutral atom of chlorine-35 () and its isotope, chlorine-37 ().
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For :
- Protons = Atomic Number (Z) = 17
- Electrons = Number of protons (as it's a neutral atom) = 17
- Neutrons = Mass Number (A) - Atomic Number (Z) = 35 - 17 = 18
A magnesium atom, , loses two electrons to form a magnesium ion. State the symbol for this ion and calculate the number of protons, neutrons, and electrons it contains.
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Symbol: Losing two electrons results in a 2+ charge. The symbol is .
How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
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Revision flashcards
Flip the card. Test yourself before the exam.
What are the relative mass and relative charge of a proton?
Relative mass: 1. Relative charge: +1.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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Proton (p): Relative mass = 1, Relative charge = +1
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Neutron (n): Relative mass = 1, Relative charge = 0
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Electron (e⁻): Relative mass ≈ 1/1840 (negligible), Relative charge = -1
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9701/22 · Q1(a)
Complete Table 1.1 using relevant information from the Periodic Table.
Table 1.1
| nucleon number | proton number | number of electrons |
|---|---|---|
| Mg | 24 | |
| Al | 27 |
9701/22 · Q1(b)(i)
Complete Table 1.1.
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Frequently asked
Checkpoint
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Reading it isn’t knowing it — prove it.
Before you move on: do 9701/22 · Q1(a) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.