Sub-atomic particles and their relative masses and charges, atomic number and mass number, isotopes and their identical chemical properties, and the determination of relative atomic mass from mass spectra.

What this topic is asking

OCR specification point 2.1.1 wants you to describe the atom in terms of protons, neutrons and electrons with their relative masses and charges, use atomic number and mass number, define and recognise isotopes, and calculate relative atomic mass and relative isotopic mass from mass spectrometry data.

Sub-atomic particles

Protons and neutrons sit in the nucleus; electrons occupy shells around it. Their relative masses and charges are:

The proton and neutron dominate the mass; the electron is negligible by mass. A neutral atom has equal numbers of protons and electrons, so the overall charge is zero.

Isotopes

Isotopes of an element have identical chemical properties because chemical behaviour depends on the number and arrangement of electrons, which is the same for every isotope. They differ only in physical properties that depend on mass, such as density and rate of diffusion.

Relative masses

OCR expects precise definitions of the relative-mass terms, each measured against carbon-12.

• Relative isotopic mass: the mass of an atom of an isotope compared with one twelfth the mass of an atom of carbon-12.
• Relative atomic mass ($A_r$): the weighted mean mass of an atom of an element compared with one twelfth the mass of an atom of carbon-12.
• Relative molecular mass ($M_r$): the weighted mean mass of a molecule compared with one twelfth the mass of an atom of carbon-12 (use relative formula mass for ionic compounds).

Mass spectrometry and relative atomic mass

A mass spectrometer ionises a sample, separates the ions by their mass-to-charge ratio ($m/z$), and records the relative abundance of each. For singly charged ions the $m/z$ value equals the relative isotopic mass. The relative atomic mass is the weighted mean:

Examples in context

Example 1. Chlorine in mass spectra. A molecule of chlorine, $\text{Cl}_2$, gives three molecular-ion peaks at $m/z = 70$, $72$ and $74$ because each molecule can contain two $^{35}\text{Cl}$, one of each, or two $^{37}\text{Cl}$. The $70 : 72 : 74$ peak ratio of roughly $9 : 6 : 1$ comes straight from the $3 : 1$ abundance of the two isotopes, a recurring exam pattern that shows isotope abundance controlling the spectrum.

Example 2. Carbon dating. The ratio of radioactive $^{14}\text{C}$ to stable $^{12}\text{C}$ in a once-living sample falls predictably as the $^{14}\text{C}$ decays. A mass spectrometer can measure this ratio, the same instrument used to find relative atomic masses in the laboratory, linking the abstract definition of isotopes to a real dating technique.

Try this

Q1. State the relative mass and relative charge of a neutron. [1 mark]

• Cue. Relative mass $1$, relative charge $0$.

Q2. Explain why the two isotopes of chlorine react in the same way with sodium. [2 marks]

• Cue. They have the same number of electrons and the same electron configuration, and chemical reactions depend on electrons, not neutrons.