2.1Foundations in Chemistry

Atoms and Reactions

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Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Atoms & Reactions Contents Atomic Structure & Isotopes Atomic Structure & Mass Spectrometry Compounds, Formulae & Equations Page 2 Atomic Struc…

Part 1 of 7Subatomic structure of atoms

Subatomic structure of atoms

Atoms

Atoms the smallest parts of an element that take part in chemical reactions, and mostly empty space.

Nucleus

Nucleus holds the protons and neutrons, so the mass of the atom is concentrated there.

Electrons

Electrons negatively charged, orbit the nucleus and add negligible mass.

Holding the atom together

Holding the atom together electrostatic attraction between the positive nucleus and the negative electrons.

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1836

Part 2 of 7Protons, electrons and neutrons in atoms and ions

Atoms and ions

Atoms and ions an atom is neutral, whereas ions form when atoms gain or lose electrons.

Protons

Protons the atomic (proton) number Z defines the element.
All atoms and ions of an element have the same number of protons, such as 3 for lithium and 4 for beryllium.
Number of protons = mass number − number of neutrons.

Electrons

Electrons a neutral atom has equal protons and electrons.
A cation has lost electrons, so it has fewer electrons than protons.
An anion has gained electrons, so it has more electrons than protons.
Mg²⁺ has 12 protons but only 10 electrons.

Neutrons

Neutrons number of neutrons = mass number A − atomic number Z.
Mg²⁺ (A = 24) and carbon-12 have 12 and 6 neutrons respectively.

Part 3 of 7Isotopes

Isotopes

Isotopes atoms of the same element with the same number of protons and electrons but different numbers of neutrons.

They are also described as atoms of the same element with different mass numbers.

Notation

Notation chemical symbol or name followed by a dash and the mass number.
Carbon-12 and carbon-14 have 6 and 8 neutrons respectively.
Hydrogen has three isotopes with 0, 1 and 2 neutrons.

Chemical properties

Chemical properties identical, because they have the same outer-shell electrons and electrons determine the chemistry.

Physical properties

Physical properties different, because extra neutrons only add mass.
Examples are small differences in mass and density.

Part 4 of 7Relative masses

Standard

Standard carbon-12 is the international standard, fixed at exactly 12 atomic mass units.
One carbon-12 atom has an actual mass of 1.992646538 × 10^-26 kg, which is not practical to use.
The standard for comparison is 1/12th of the mass of a carbon-12 atom.

Relative isotopic mass

Relative isotopic mass the mass of an atom of an isotope relative to 1/12th of the mass of a carbon-12 atom.

A Level work often rounds, for example nitrogen 14.00307401 to 14.0 and oxygen 15.99491464 to 16.0.

Relative atomic mass Ar

Relative atomic mass Ar the weighted mean mass of an atom of an element relative to 1/12th of the mass of a carbon-12 atom.

This is needed because most elements are a mixture of isotopes.

Part 5 of 7Calculating relative atomic mass from isotope abundance

Calculating relative atomic mass from isotope abundance

Multiply each isotope's mass by its relative abundance.

Add the results together.

Divide by the total abundance, usually 100.

Formula

A_r = Σ / total abundance.
- (abundance × isotopic mass)

Abundance data

Abundance data values are given or read from a mass spectrum.

Oxygen check

99.76% ¹⁶O, 0.04% ¹⁷O and 0.20% ¹⁸O give A_r = 16.0044, which is 16.00 to 2 d.p.

Boron check

19.9% at mass 10 and 80.1% at mass 11 give A_r = 10.80.

Part 6 of 7Mass spectrometry

Use
- finds the relative abundance of isotopes in an element and the relative atomic mass.
- A Level spectra are interpreted assuming all ions are singly charged (1+).

The sample is vaporised.

The sample is ionised to form positive ions.

The ions are accelerated; heavier ions move more slowly and are deflected less.

The ions are detected as a mass-to-charge ratio, m/z.

Signal height
- each ion gives a signal proportional to its abundance, so a larger signal means greater abundance.

Part 7 of 7Relative molecular mass and relative formula mass

Relative molecular mass

Relative molecular mass for substances with simple molecules.
Ethanol, C₂H₅OH, has 2 carbon atoms, 6 hydrogen atoms and 1 oxygen atom.

Relative formula mass

Relative formula mass for substances with giant structures.
Calcium chloride, CaCl₂, is an ionic lattice with a repeating ratio of 1 calcium ion to 2 chloride ions.

Calculation

Calculation both are found by adding the relative atomic masses (A_r) of all the atoms in the formula.

Common misuse

Common misuse relative molecular mass is often wrongly applied to all compounds, including ionic ones.

Key terms

Term	Meaning
Isotopes	Atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass	The weighted mean mass of an atom of an element relative to 1/12th of a carbon-12 atom.
Relative isotopic mass	The mass of an atom of an isotope relative to 1/12th of a carbon-12 atom.
Mass number	The total number of protons and neutrons in a nucleus.
Atomic number	The number of protons in the nucleus, which defines the element.
m/z	The mass-to-charge ratio of an ion detected in mass spectrometry.

Avoid these mistakes

Saying isotopes differ in protons — they differ only in neutrons.

Claiming isotopes have different chemical properties — outer electrons are the same, so chemistry is the same.

Using relative molecular mass for ionic compounds — use relative formula mass.

Counting electrons of an ion as equal to protons — cations have fewer and anions have more.