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Constituents of the atom

The proton, neutron and electron are set out by charge and mass in SI and in relative units, and specific charge is charge divided by mass. Nuclide notation carries the nucleon number A and the proton number Z, and isotopes of one element share Z while their A differs.

Constituents of the atom

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Year 12AQA 3.2.1.1CIE 11.1.2, 11.1.3, 11.1.4, 11.1.5OCR A 6.4.1(b), 6.4.1(d)EDEXCEL 130

Builds on SI base and derived units.

IN THIS TOPIC

  • Quote the charge and mass of the proton, neutron and electron in SI and relative units.
  • Calculate the specific charge of particles, nuclei and ions.
  • Use nuclide notation with Z and A, and explain what isotopes are and why isotopic data is useful.

COMMON MISCONCEPTION

Atoms of the same element are identical.

Atoms of one element share a proton number but not necessarily a neutron number: isotopes of the same element have different nucleon numbers and masses.

The three particles and their units

The simple model of the atom has three particles. A proton carries +1.60 × 10−19 C and a mass of 1.67 × 10−27 kg. A neutron carries no charge and has the same mass to three figures, a shade heavier in truth. An electron carries −1.60 × 10−19 C and a mass of 9.11 × 10−31 kg, nearly two thousand times lighter than either.

Alongside the SI values sit relative units, charges of +1, 0 and −1 in units of e, and masses of 1, 1 and about 1/1800. Relative units keep nuclear equations tidy while the SI values do the calculating. Quote whichever the question uses, and be ready to translate on the spot.

Specific charge

The specific charge of a particle is its charge divided by its mass.

specific charge=Qm\text{specific charge} = \frac{Q}{m}NOT IN THE AQA DATA BOOKLET: LEARN IT

in coulombs per kilogram. It works for a lone particle, a bare nucleus or an ion alike. For a nucleus, Q is the proton count times e and m is very nearly the nucleon count times the nucleon mass. For an ion, count the electrons that are still there. Values are quoted as magnitudes, the convention this course and the data booklet follow; a negative particle's signed specific charge is negative, the electron's being −1.76 × 1011 C kg−1. In magnitude the electron holds the record at about 1.76 × 1011 C kg−1, since nothing else carries a whole unit of charge on so little mass.

WORKED EXAMPLE

The specific charge of a carbon-12 nucleus

Find the specific charge of a 12C nucleus (Z = 6).

Count the charge and the mass separately. Six protons give Q = 6 × 1.60 × 10−19 = 9.6 × 10−19 C. Twelve nucleons at one atomic mass unit each give m = 12 × 1.66 × 10−27 = 1.99 × 10−26 kg.

Specific charge = Q/m = 9.6 × 10−19/(1.99 × 10−26) = 4.8 × 107 C kg−1.

Sense check it against a landmark. The proton alone reaches 9.6 × 107 C kg−1, and this answer has to come in below that, because six neutrons added mass without adding any charge.

The notation

The nuclide notation: a large element symbol X with the nucleon number A at its upper left in cyan and the proton number Z at its lower left in amber, with helium-4 worked as the example.
FIG. 1Nuclide notation: A, the nucleon number, sits above; Z, the proton number, below. Helium-4 unpacks as 2 protons and 2 neutrons.

A nuclide is written with the nucleon number A (protons plus neutrons) as a leading superscript and the proton number Z as a leading subscript on the element symbol. Z identifies the element, so changing Z changes the element. The neutron count is never written, because it is always A − Z. The atomic mass unit is treated fully in the Year 13 Nuclear unit; here it is used only as a convenient nucleon mass, with every answer staying in kilograms.

Isotopes

Three hydrogen nuclei side by side: a lone cyan proton, then a proton with one grey neutron, then a proton with two. Z stays one throughout while A climbs from one to three.
FIG. 2Hydrogen three ways: one proton every time, with zero, one or two neutrons. Same element, different nuclide.

Isotopes are atoms with the same Z but different A. Same element, same chemistry, different neutron count, and therefore a different nuclear character. That last difference is what makes isotopic data useful. The ratio of carbon-14 to carbon-12 in dead material falls with age and dates archaeological finds, and the isotopic signature of a sample can say where it came from. Isotopes are near-identical chemically, because chemistry is set by the shared electron structure; what tells them apart is mostly physical, a mass spectrometer above all, though isotope effects make reaction rates differ slightly.

GUIDED PRACTICE

An ion's specific charge

An aluminium-27 atom (Z = 13) loses three electrons. Count the ion's charge and mass, then find its specific charge. Electron masses may be neglected.

Show the working

The nucleus keeps 13 protons but the ion now holds only 10 electrons, so its net charge is +3e = 4.8 × 10−19 C. The mass is 27 nucleons: 27 × 1.66 × 10−27 = 4.48 × 10−26 kg.

Specific charge = 4.8 × 10−19/(4.48 × 10−26) = 1.1 × 107 C kg−1. An ion's specific charge counts the electrons it lost, not the ones its neutral atom would have.

ASSESSMENT FOCUS

  • Relative charge and mass answer one kind of question, SI values another. Read which one is wanted, because a proton quoted at “+1 C” is wrong by nineteen orders of magnitude.
  • Specific charge is charge over mass, with both counted correctly. An ion has lost or gained electrons and you must say how many. A bare nucleus has none at all.
  • The neutron's specific charge is zero, because it has mass and no charge. One line saying so is a complete answer.
  • In nuclide notation the top number is A, the bottom is Z, and the neutron number is A − Z. A question that asks for neutrons is asking for that subtraction.
  • Isotopes have the same proton number and a different nucleon number. Both halves of that sentence are needed, every time.

CHECK YOURSELF

Calculate the specific charge of an iron-56 nucleus (Z = 26). Take e = 1.60 × 10−19 C and the nucleon mass as 1.67 × 10−27 kg.

Show a hint

Charge counts protons only; mass counts every nucleon.

Show the answer

Charge first. Q = 26 × 1.60 × 10−19 = 4.16 × 10−18 C.

Then mass. m = 56 × 1.67 × 10−27 = 9.35 × 10−26 kg.

Divide, and Q/mQ/m = 4.4 × 107 C kg−1. That comes in below a lone proton's figure, as it should, because thirty neutrons added mass and no charge.

Z is the proton number and identifies the element.

A is the nucleon number.

Specific charge is Q/m.

WORKBOOK

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Question workbook

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Answer book

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  • Quote the charge and mass of the proton, neutron and electron in SI and relative units.
  • Calculate the specific charge of particles, nuclei and ions.
  • Use nuclide notation with Z and A, and explain what isotopes are and why isotopic data is useful.

Open the full revision checklist to track your progress across the whole unit.