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3.1.1 Atomic Structure

Fundamental Particles, Isotopes and Relative Atomic Mass

AQA A-level Chemistry

The model of the atom

  • A central nucleus containing protons and neutrons.
  • Electrons arranged in energy levels around it.
  • Most of the atom is empty space, with almost all of its mass in the nucleus.

For differences from an older model write the first two only.

Counting protons, neutrons and electrons

The mass number A is the number of protons plus neutrons, never a mass or an average; the atomic number Z is the number of protons. Relative charges are +1, 0 and −1 on the proton, neutron and electron, and electrons = Z minus the charge. The Periodic Table is in order of atomic number, not relative atomic mass.

READING THE SYMBOL 4622 Ti mass number, A atomic number, Z COUNTING protons = Z neutrons = A minus Z electrons = Z minus the charge ATOM2+ ION 4622 Ti 4922 Ti2+ 22 protons24 neutrons 22 electrons 22 protons27 neutrons 20 electrons
The mass number is the total number of protons and neutrons. The atomic number is the number of protons. In a 2+ ion two electrons have been lost, so there are two fewer electrons than protons.

Isotopes

  • Atoms with the same number of protons and a different number of neutrons.
  • They react identically: same electron configuration, and electrons determine chemical properties.
Two similarity questions, opposite answers

Same chemical properties: same electron configuration or same number of electrons. Protons do not explain how an atom reacts.

A similarity in the numbers of fundamental particles: same number of protons or electrons. A configuration is not a number of a particle.

Relative atomic mass

Relative atomic mass is the average (mean) mass of one atom of the element divided by one twelfth of the mass of one atom of carbon-12. Both halves must be on the same scale.

THE DEFINITION average (mean) mass of one atom of the element 1/12 of the mass of one atom of carbon-12 ATOM ATOM WHAT NOT TO WRITE mass of one atom over 1/12 the mass of one mole of carbon-12 these two lines are on different scales
Both lines must be about atoms, or both must be about moles. Write both halves on the same scale.

Relative isotopic mass is the same construction for one isotope, with no averaging.

Calculating relative atomic mass from abundances

Ar = (mass × abundance) added up for every isotopetotal abundance

The denominator is the total abundance, which is 100 only where the figures are percentages: under a relative intensity column, divide by their actual total. Give Ar to one decimal place, 52.1 and not 52.

Working backwards to a missing abundance or mass number

Method: an unknown in the table
  1. Call one unknown abundance x, and a second (total − x).
  2. Set the Ar expression in x equal to the Ar given, then solve.
  3. Find any remaining abundance by subtraction.
  4. An unknown mass number is rounded to a whole number.

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