Inside the Atom

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 1.14 — The terms atom and molecule
• 1.15 — Atomic structure: positions, relative masses and relative charges of sub-atomic particles
• 1.16 — The terms atomic number, mass number, isotopes and relative atomic mass
• 1.17 — Calculating relative atomic mass from isotopic abundances

Specification section 1(c) Atomic structure — also taught in Electronic Configurations and the Periodic Table.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

An atom is almost entirely empty space. All of its mass is packed into a nucleus so small that if the atom were a football stadium, the nucleus would be a pea on the centre spot — and the electrons would be specks somewhere up in the stands.

The three particles, and what each one decides

1. Protons: in the nucleus, relative mass 1, charge +1. The NUMBER of protons decides which element it is. Change it and you have a different element entirely.
2. Neutrons: in the nucleus, relative mass 1, charge 0. Changing the number gives a different ISOTOPE of the same element.
3. Electrons: in shells around the nucleus, relative mass about 1/1836 (effectively zero), charge −1. Changing the number gives an ION, and electrons decide all the chemistry.

Atomic number is the number of protons. Mass number is protons plus neutrons — the count of the heavy things. So neutrons = mass number − atomic number, which is a subtraction you will do dozens of times this course.

Why relative atomic mass has a decimal point

Chlorine's relative atomic mass is 35.5, yet no chlorine atom weighs 35.5 — they are all either 35 or 37. Think of a class where every student is either 15 or 17 years old. The AVERAGE could easily be 15.5; nobody is 15.5. Relative atomic mass is a weighted average across a natural sample: chlorine is about 75% chlorine-35 and 25% chlorine-37, and (75 × 35 + 25 × 37) ÷ 100 = 35.5.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Atom

The smallest particle of an element that can exist.

Learn this definition · Molecule

Two or more atoms chemically bonded together by covalent bonds. A molecule may be an element (O₂, N₂, Cl₂) or a compound (H₂O, CO₂) — 'molecule' says how the atoms are joined, not whether they are the same.

Learn this definition · Atomic number

The number of protons in the nucleus of an atom. It defines which element the atom is, and it never changes in a chemical reaction.

Learn this definition · Mass number

The total number of protons and neutrons in the nucleus of an atom. Always a whole number, because it is a count of particles.

Learn this definition · Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons.

Learn this definition · Relative atomic mass (Ar)

The weighted mean mass of the isotopes of an element, taking their abundances into account, compared with 1/12 the mass of a carbon-12 atom.

The three sub-atomic particles

Reading a nuclide symbol

Mass number (top)

protons + neutrons

Atomic number (bottom)

protons — this defines the element

Neutrons

mass number − atomic number

Electrons

equal to protons in a neutral atom; adjusted by the charge in an ion

Ion

an atom (or group of atoms) that has lost or gained electrons and therefore carries a charge

Why an atom is neutral

equal numbers of protons (+1 each) and electrons (−1 each), so the charges cancel exactly

Calculating relative atomic mass from abundances — the routine

1. If the abundances are given as a RATIO, convert to percentages first: a 3 : 1 ratio is 75% and 25%.
2. Multiply each isotope's mass number by its percentage abundance.
3. Add the products together.
4. Divide by 100.
5. Check: the answer must lie between the two isotope masses, and nearer the more abundant one.
6. Backwards, from Ar to an unknown abundance: call one x %, the other (100 − x) %, and solve.

Mass number vs relative atomic mass — the pair most often blurred

A. MASS NUMBER belongs to one atom. It counts protons and neutrons, so it is always a whole number. Chlorine-35 has a mass number of 35.

B. RELATIVE ATOMIC MASS belongs to the element as found in nature. It is a weighted mean over all the isotopes, so it is usually a decimal. Chlorine's Ar is 35.5, and no single chlorine atom has that mass.

Counting particles in an ion

1. Protons = atomic number. This never changes.
2. Neutrons = mass number − atomic number.
3. Electrons = protons − charge. A 2+ ion has LOST two; a 1− ion has GAINED one.
4. Check: ²⁴Mg²⁺ has 12 protons, 12 neutrons, 10 electrons.

Ar = Σ(isotope mass × % abundance) ÷ 100

Convert a ratio to percentages first. For an unknown abundance x: Ar × 100 = m₁x + m₂(100 − x)

Units: None — Ar is a ratio.

Model answer [3 marks]

Boron is 19.9% boron-10 and 80.1% boron-11. Calculate Ar to 1 d.p. [3]

Ar = [(10 × 19.9) + (11 × 80.1)] ÷ 100 = (199 + 881.1) ÷ 100 = 10.8. It lies between 10 and 11, nearer 11 because boron-11 is more abundant.

Model answer [2 marks]

Explain why an atom has no overall electrical charge. [2]

An atom contains equal numbers of protons and electrons. A proton has a relative charge of +1 and an electron a relative charge of −1, so the positive and negative charges cancel exactly and the atom is neutral overall.

Model answer [3 marks]

Explain why two isotopes of chlorine react identically with sodium. [3]

Isotopes differ only in their number of neutrons. They have the same number of electrons in the same arrangement, and chemical reactions involve only the electrons, so the chemistry is identical. Only physical properties such as mass and density differ.

Isotope vs ion

Mark-losing trap. Ar has no units — never write grams.

Mark-losing trap. Ar must lie between the lightest and heaviest isotope mass. If not, the abundances are swapped.

Mark-losing trap. Extra neutrons make an isotope; extra electrons make an ion.

Mark-losing trap. 'Average mass' scores one mark. 'Weighted mean, compared with carbon-12' scores both.

Mark-losing trap. Mass comes from the nucleus; all chemical behaviour comes from the electrons.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — How many neutrons are there in an atom with mass number 39 and atomic number 19?
2. Grade 7 · Determine [2 marks] — A magnesium ion is written ²⁴Mg²⁺. Magnesium has atomic number 12. How many electrons does this ion contain?
3. Grade 8 · Explain [3 marks] — Explain why two isotopes of chlorine react in exactly the same way with sodium.
4. Grade 9 · Calculate [3 marks] — Boron exists as two isotopes: boron-10 (19.9%) and boron-11 (80.1%). Calculate the relative atomic mass of boron to one decimal place.
5. 9+ · Analyse [4 marks] — An element has two isotopes, of mass 63 and 65, and a relative atomic mass of 63.5. Which statement about the abundances is correct?

The people behind this science

Two ways into the same idea — the one who explained why the shells exist, and the one who proved the atom is not indivisible. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Niels Bohr — the one who explained why the shells exist

The shells you have just been drawing are Bohr's. He proposed that electrons can only occupy fixed energy levels — an idea that seemed absurd, and which turned out to explain both the shape of the Periodic Table and the exact colours each element emits.

• “Why can electrons only sit in certain shells, and not anywhere they like?”
• “What was wrong with the earlier picture of the atom that made you change it?”
• “How does your model explain why each element gives out its own colours of light?”
• “Why does the first shell hold only two electrons?”
• “If the nucleus is so tiny, why does the atom behave as though it is far bigger?”

Marie Curie — the one who proved the atom is not indivisible

This lesson tells you the atom has parts. For most of chemistry's history that was denied — 'atom' means 'uncuttable'. Curie showed that some atoms break apart on their own and turn into different elements, which is the evidence that forced chemistry to accept an internal structure at all.

• “How did you know radiation was coming from inside the atom itself?”
• “What convinced you that one element was turning into another?”
• “Why was the idea of an atom having parts so hard for chemists to accept?”
• “Two atoms of the same element can behave identically in chemistry yet differently in radioactivity — why?”
• “What does radioactivity tell us about the nucleus that chemistry alone cannot?”

Carry on through the course

• Previous lesson: Pure Substances, Mixtures and How to Separate Them
• Next lesson: Electronic Configurations and the Periodic Table
• The Three States of Matter — What the particles are doing in a solid, a liquid and a gas — and what really changes when ice melts.
• Pure Substances, Mixtures and How to Separate Them — Why a pure substance melts at one temperature, and which separation technique to reach for when.
• Electronic Configurations and the Periodic Table — Why the table has the shape it has: the group number IS the outer-shell electron count, and everything follows from that.
• Solubility and Solubility Curves — Why a solution can be full, why hot water holds more, and how one curve tells you exactly how many crystals you will get back.
• Formulae and Balanced Equations — Why the ash weighs more than the metal, what balancing an equation is really doing, and how to add up a formula mass without losing a bracket.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification