(c) Atomic structure
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
1.14Atoms and moleculesKnow what is meant by the terms atom and molecule1.15Atomic structureKnow the structure of an atom in terms of the positions, relative masses and relative charges of sub-atomic particles1.16Atomic number and isotopesKnow what is meant by the terms atomic number, mass number, isotopes and relative atomic mass (Ar)1.17Relative atomic massBe able to calculate the relative atomic mass of an element (Ar) from isotopic abundances
Distinguish atoms from molecules
An atom is the smallest particle of an element that retains that element's chemical identity. Every atom has a nucleus surrounded by electrons.
A molecule is a discrete group of two or more atoms held together by covalent bonds. Its formula shows both the elements present and the number of atoms, so one molecule of methanol, CHX3OH, contains six atoms in total.
| Species | Atom or molecule? | Reason |
|---|---|---|
| Ar | One atom | Argon exists as individual atoms. |
| FX2 | Molecule of an element | Two fluorine atoms are bonded together. |
| HX2O | Molecule of a compound | Two hydrogen atoms and one oxygen atom are bonded together. |
A molecule need not be a compound: FX2 contains only one element. Conversely, an individual noble-gas atom is not a molecule because it is not a bonded group of atoms.
Locate and compare subatomic particles
An atom has a tiny central nucleus containing protons and neutrons, with electrons occupying shells around the nucleus. Nearly all the atom's mass is concentrated in the nucleus.
| Particle | Position | Relative mass | Relative charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells around the nucleus | 1/2000 (about 0.0005) | −1 |
A neutral atom has equal numbers of protons and electrons, so their positive and negative charges cancel. Neutrons add mass but no charge; electrons contribute very little to the total mass.
Do not place electrons inside the nucleus or assign them a relative mass of 1. The nucleus is small compared with the whole atom even though it contains almost all the mass; shell diagrams are schematic and not drawn to scale.
Connect atomic number, mass number and isotopes
Atomic number identifies the element; mass number identifies one isotope. In nuclide notation XZAX2Z2AX, Z is the atomic number and A is the mass number.
| Term | Meaning | Consequence |
|---|---|---|
| Atomic number, Z | Number of protons in the nucleus | Fixes the element; a neutral atom also has Z electrons. |
| Mass number, A | Total number of protons and neutrons | Number of neutrons = A−Z. |
| Isotopes | Atoms of the same element with the same proton number but different neutron numbers | Same atomic number, different mass numbers. |
| Relative atomic mass, Ar | Weighted mean mass of the atoms of an element relative to 1/12 of the mass of a carbon-12 atom | Reflects both isotope masses and their abundances. |
For X81205X2812205Tl: protons = 81, neutrons = 205−81=124, and a neutral atom has 81 electrons. Another thallium isotope must still have 81 protons but can have a different number of neutrons.
Atomic number is not the total number of particles, and mass number is not the same as Ar. Mass number belongs to one isotope and is a whole number; Ar is an abundance-weighted mean and is often not a whole number.
Calculate relative atomic mass from isotopic abundance
Relative atomic mass is a weighted mean: a more abundant isotope contributes more strongly to the final value than a less abundant isotope.
Ar=100∑(isotope mass×percentage abundance)
For isotopes 24, 25 and 26 with abundances 79.2%, 10.0% and 10.8%: Ar=[(24×79.2)+(25×10.0)+(26×10.8)]/100=24.316, which is 24.3 to one decimal place.
The answer must lie between the lightest and heaviest isotope masses and should be closest to the mass of the most abundant isotope. Keep the unrounded value until the final requested precision.
Do not take a simple average unless the isotopes have equal abundance. Percentages must total 100 and require division by 100; decimal abundances such as 0.792 can instead be used directly without that final division.