S1.2
The nuclear atom
2 SL · +1 HL (SL 1.2.1-1.2.2) (HL 1.2.3)
Unverified
← View on the mapParent topic: Models of the particulate nature of matter
Guiding question
Guiding questionHow do the nuclei of atoms differ?
The one big idea
Every atom has a tiny, dense, positively charged nucleus of protons and neutrons. The proton number defines the element; the neutron number distinguishes isotopes. At HL, mass spectra reveal isotopic composition directly.
What this rests on
Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.
No prerequisites recorded.
What rests on this
Core concepts that must be mastered
- The nuclear atom(1.2.1)Every atom has a small, dense, positively charged nucleus containing protons and neutrons (collectively, nucleons), with electrons occupying the space outside. The proton number Z defines the element: all atoms with Z = 6 are carbon, regardless of how many neutrons they carry. The nucleon number A is the total count of protons plus neutrons. The nuclear symbol AZX encodes both: for ¹²₆C, A = 12 and Z = 6, so there are 6 protons, 6 neutrons, and 6 electrons. For an ion such as ²³₁₁Na⁺, the charge tells you one electron has been lost: 11 protons, 12 neutrons, 10 electrons. For an anion such as ¹⁶₈O²⁻, the 2− charge tells you two electrons have been gained: 8 protons, 8 neutrons, 10 electrons. Relative masses and charges of the subatomic particles are in the data booklet — the electron mass is negligible compared to the proton and neutron, which is why A is effectively the mass number. The nucleus is extremely small relative to the atom: if the nucleus were a marble at the centre of a stadium, the electrons would be in the stands — most of the atom is empty space.
- Isotopes and relative atomic mass(1.2.2)Isotopes are atoms of the same element with different numbers of neutrons. Because they share the same electron configuration, isotopes are chemically identical — ³⁵Cl and ³⁷Cl undergo the same reactions. Their different masses mean physical properties differ slightly (density, rate of diffusion), and this is exploited in isotope labelling and mass spectrometry. Because natural samples contain a mixture of isotopes, relative atomic mass Ar is usually non-integer: chlorine is 35.45, not 35 or 37. Ar is the weighted mean of isotope masses over their natural abundances, on the ¹²C = 12 scale. You must be able to calculate Ar from isotopic abundance data, and work backwards: given Ar and the isotope masses, deduce the relative abundance of each isotope. For example, boron has Ar = 10.81 from two isotopes ¹⁰B and ¹¹B: if x is the fraction of ¹⁰B, then 10x + 11(1−x) = 10.81, giving x = 0.19, so boron is 19% ¹⁰B and 81% ¹¹B. This is a direct application of the mole concept (S1.4) — you are weighting by amount, not by number of atoms alone.
- HL: Mass spectra and isotopic composition(1.2.3)At HL, mass spectra determine relative atomic masses from isotopic composition. A mass spectrum plots relative abundance (y-axis) against mass-to-charge ratio m/z (x-axis). Each peak corresponds to an isotope (or a fragment, for molecular samples). The m/z value of a peak gives the mass of that isotope; the relative heights of the peaks give the relative abundance. From a mass spectrum you can calculate Ar directly: multiply each isotope mass by its fractional abundance and sum. For example, a chlorine spectrum showing peaks at m/z 35 (75.8%) and m/z 37 (24.2%) gives Ar = 35 × 0.758 + 37 × 0.242 = 35.45. The operational details of the mass spectrometer — ionisation method, acceleration, deflection, detection — are not assessed. What you must do is read the spectrum: identify which peaks are isotopes, and use their positions and heights to calculate relative atomic mass. This skill returns in Structure 3.2, where mass spectra of organic compounds show fragmentation patterns used in structure determination — the molecular ion gives the molecular mass, and fragment peaks reveal structural features.
- How this sub-topic connectsThe nuclear atom is the foundation for electron configurations (S1.3), which explain the periodic table (S3.1) and all of bonding (S2). Isotopic abundance calculations are a direct application of the mole concept (S1.4). At HL, mass spectrometry bridges to Structure 3.2 (organic analysis, where the molecular ion peak gives molecular mass and fragmentation patterns reveal structure) and to the Nature of Science theme — isotope tracers provide evidence for reaction mechanisms (Reactivity 3.4) and radiocarbon dating relies on the ¹⁴C isotope. The distinction between mass number (an integer for one isotope) and relative atomic mass (a weighted mean) is a common exam trap: mass number is a count of nucleons in a specific atom, Ar is an average across all naturally occurring isotopes.
Quantitative non-negotiables
- Use the nuclear symbol (mass number A over atomic number Z, as in ¹²₆C) to deduce the numbers of protons, neutrons and electrons in any atom or ion.
- Perform calculations involving non-integer relative atomic masses and isotopic abundance.
Common failure modes
M-01 — Electrons orbit the nucleus in circles like planets.
Confidence: verified
Why it’s wrong: The Bohr model is a superseded approximation; electrons occupy orbitals, which are probability distributions with no defined trajectory.
Correction: An orbital is a region where there is a high probability of finding an electron; it has a shape and an energy, not a path.
M-05 — Relative atomic mass is the number of protons plus neutrons.
Confidence: verified
Why it’s wrong: That is the mass number of one isotope.
Correction: Ar is the weighted mean of isotope masses over natural abundance, on the ¹²C = 12 scale; that's why chlorine is 35.45.
What "HL standard" actually looks like
HLHL adds mass spectrometry: interpret mass spectra in terms of the identity and relative abundance of isotopes, and determine relative atomic masses from isotopic composition. Operational details of the mass spectrometer are not assessed.
Linking questions
Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.
- Official IB
What determines the different chemical properties of atoms?
- Official IB
How does the atomic number relate to the position of an element in the periodic table?
- Official IB
How can isotope tracers provide evidence for a reaction mechanism?
- Official IB
How does the fragmentation pattern of a compound in the mass spectrometer help in the determination of its structure?
- Official IB
How do emission spectra provide evidence for the existence of different elements?
- Official IB
How has the organization of elements in the periodic table facilitated the discovery of new elements?
Bridge: GCSE → IB HL
Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.