1.3 The ‘Bohr model’: Atoms, ions and isotopes

done

Taught 15 Sep – 23 Sep · Booklet pp. 16–26 · Homework week 3

By the end of this, you should be able to:

Recall the names of the three fundamental particles which make up an atom and state their relative charge and mass
Understand and define atomic number (Z) and mass number (A)
Appreciate that the atomic number identifies an element and locates its position in
Use the symbol notation to describe an atom of an element X 𝑍 𝐴𝑋
Describe and draw the structure of the atom according to the ‘Bohr’ model (nucleus at the centre with electrons orbiting in shells or orbits)
Determine and draw (using a Bohr diagram) the arrangement of electrons for the first twenty elements of
Understand that when an atom loses or gains an electron, it becomes an ion and determine the charge of an ion from its atomic number and the number of electrons.
Understand what is meant by the term 'isotope' and describe the similarities and differences between different isotopes of an element

What an atom is made of

What you need to understand

Everything around you is made of atoms. An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons that occupy shells (energy levels). The nucleus is unimaginably small but holds almost all of the atom's mass; the rest of the atom is mostly empty space.

Why the atom looks the way it does

Why it works

Why is an atom neutral? It contains equal numbers of positive protons and negative electrons, so the two charges cancel out exactly. (This is the rule we lean on again and again: in a neutral atom, electrons = protons.)

Why do we say it is "mostly empty space"? In Ernest Rutherford's famous gold-foil experiment, nearly all the tiny particles fired at a thin sheet of gold passed straight through; just a few bounced sharply back. The only way to explain that: almost all the mass and all the positive charge sit in a very small, dense nucleus, with the electrons far away and empty space in between.

Reading a helium atom. Helium has 2 protons, 2 neutrons and 2 electrons. The 2 protons and 2 neutrons sit in the nucleus; the 2 electrons fill the first shell. Check the charge: 2 × (+ 1) from protons and 2 × (− 1) from electrons gives 0 — neutral, exactly as a normal atom should be.

Going deeper

If the whole atom were the size of a sports stadium, the nucleus would be about the size of a pea in the centre — that is how much of an atom is empty space. Later (IB Diploma) you will replace shells with orbitals and energy sub-levels, and you will see direct evidence for them. The key habit to take forward: a scientific model is judged by how well it explains and predicts — and it is always open to improvement.

Counting the particles

What you need to understand

We write them around the symbol like this: Li-7 — mass number on top, atomic number below.

The one part everyone mixes up

Why it works

This is the idea most students get tangled, so go slowly.

Electrons = protons — always (for a neutral atom). An atom has no overall charge, so the negative electrons must exactly balance the positive protons. If there are 3 protons, there are 3 electrons. This one is guaranteed.

Neutrons are the independent one. Neutrons add mass and help hold the nucleus together, but their number is not fixed by the protons — which is exactly why isotopes exist (same element, different neutrons). So you must work neutrons out with A − Z. Never assume neutrons = protons; sometimes they happen to be equal (carbon-12 has 6 and 6), but often they are not (lithium-7 has 3 protons and 4 neutrons).

So what does each particle control?

Lithium-7 Li-7. Z = 3, A = 7.

Protons = Z = 3. Electrons = protons = 3. Neutrons = A − Z = 7 − 3 = 4.

Notice: electrons match protons (both 3), but neutrons (4) do not — you had to calculate them.

A sodium ion Na-23⁺. An ion has lost or gained electrons, so it is charged.

Protons = 11 (unchanged). Neutrons = 23 − 11 = 12. The "+" means it lost one electron, so electrons = 10.

Check the charge: protons − electrons = 11 − 10 = +1. ✓

Two isotopes of chlorine Cl-35 and Cl-37.

Both have Z = 17, so both have 17 protons and 17 electrons — they are the same element. But neutrons = 35 − 17 = 18 versus 37 − 17 = 20. Same element, different neutrons = isotopes.

Going deeper

Because a real sample of chlorine is a mixture of Cl-35 and Cl-37 (about 3 to 1), its relative atomic mass is a weighted average — 35.5, not a whole number. That is why the periodic table shows decimals. Later you will measure these proportions directly with a mass spectrometer (IB Diploma). And the idea of ions leads straight into ionic bonding next year: metals lose electrons and non-metals gain them, and the charges you worked out here decide the formulae.

Diagrams and worked examples are in the Knowledge Book PDF below — they don't reproduce well on a page.

Now test yourself

Say it out loud before you tap.

Words you need

atom define it

the smallest particle of an element that still behaves like that element

E.G.one Fe atom is still iron and behaves like iron

NOTa lone proton — that is a part of an atom

nucleus define it

the tiny, dense centre of an atom; holds the protons and neutrons

E.G.carbon's nucleus: 6 protons + 6 neutrons

NOTthe shells — those hold the electrons, outside the nucleus

proton define it

positive particle in the nucleus — relative charge +1, relative mass 1

E.G.every carbon atom has exactly 6 — that is what makes it carbon

NOTa neutron (no charge) or an electron (negative, almost no mass)

neutron define it

neutral particle in the nucleus — relative charge 0, relative mass 1

E.G.carbon-12 has 6 of them; carbon-14 has 8

NOTthe particle that decides which element it is — change the protons for that

electron define it

negative particle in the shells — relative charge #sym.minus 1, almost no mass

E.G.a neutral carbon atom has 6, arranged 2, 4

NOTcounted in the mass number — far too light to matter

shell (energy level) define it

a region around the nucleus where electrons are found

E.G.sodium fills 2, 8, 1 across three shells

NOTa solid surface or a track — nothing is physically there

Bohr model define it

a simple, useful model with electrons in fixed circular shells

E.G.drawing sodium as three rings holding 2, 8, 1

NOTa photograph of a real atom — nobody has ever seen electrons in rings

orbital define it

the more accurate idea: a region where an electron is likely to be

E.G.a fuzzy cloud, densest near the nucleus

NOTa fixed circular path like a planet's orbit

atomic number (Z) define it

the number of protons; also called the proton number — defines which element it is

E.G.Z = 6 is always carbon; Z = 7 is always nitrogen

NOTthe neutron count, and not the mass number

mass number (A) define it

the number of protons + neutrons; also called the nucleon number (protons and neutrons together are called nucleons)

E.G.sodium: 11 protons + 12 neutrons → A = 23

NOTa mass in grams — and it does not count electrons

ion define it

an atom that has lost or gained electrons, so it carries a charge

E.G.Na⁺ (lost one), Cl− (gained one), Mg²⁺ (lost two)

NOTa neutral Na atom; and never an atom that gained a proton

isotope define it

atoms of the same element (same Z) with different numbers of neutrons

E.G.carbon-12 and carbon-14 — both 6 protons, 6 vs 8 neutrons

NOTcarbon and nitrogen — different protons, so different elements

relative atomic mass define it

the average mass of an element's atoms, allowing for its isotopes

E.G.chlorine 35.45 — the average of Cl-35 and Cl-37

NOTthe mass number of any one atom — no chlorine atom weighs 35.45

electron arrangement define it

how the electrons are spread across the shells, e.g. 2, 8, 1 for sodium

E.G.sodium 2, 8, 1 · chlorine 2, 8, 7

NOT8, 2, 1 — shells fill from the nucleus outwards

Easy to get wrong

TRAP Electrons sit inside the nucleus. true or false?

TRUTH Only protons and neutrons are in the nucleus. Electrons are in shells around it.

TRAP An atom is a solid little ball. true or false?

TRUTH An atom is mostly empty space — the nucleus is tiny compared with the whole atom.

TRAP The Bohr model is exactly how atoms are. true or false?

TRUTH It is a model — useful but simplified. Real electrons occupy orbitals (probability clouds), not neat rings.

TRAP Neutrons always equal protons. true or false?

TRUTH Not true. Only sometimes (carbon-12: 6 and 6). Lithium-7 has 3 protons but 4 neutrons. Always use A #sym.minus Z.

TRAP Electrons equal neutrons. true or false?

TRUTH No — electrons equal protons in a neutral atom. Neutrons are separate and can differ.

TRAP Isotopes are different elements. true or false?

TRUTH No — same element, same protons. Only the neutron number (and so the mass number) differs.

TRAP The mass number is the atom's mass in grams. true or false?

TRUTH No — it is a count of protons + neutrons (a relative figure), not a mass in grams.

TRAP A "+" ion has gained something. true or false?

TRUTH A positive ion has lost electron(s); a negative ion has gained electron(s). Charge = protons #sym.minus electrons.

Check yourself

The nucleus contains: A. protons only B. protons & electrons C. protons & neutrons D. electrons answer

Try this

PhET Build an Atom opens in a new tab RSC Structure of the atom opens in a new tab PhET Isotopes & Atomic Mass opens in a new tab

Test yourself

Arrangement ladder early

Arrangement ladder late

Isotopes ladder early

Isotopes ladder late

Lewis

Lewis diagrams and ions

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