Inside the atom: particles, atomic number and isotopes: WASSCE Chemistry

For over two thousand years people argued about whether matter could be cut up forever. John Dalton's atomic theory (1808) said no: there are indivisible atoms. A century of experiments then showed that atoms themselves contain smaller particles. This lesson traces that story and teaches you to read the numbers that describe every atom.

Learning objectives

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

state Dalton's atomic theory and the modifications made to it
describe the discoveries of the electron, nucleus and neutron
state the relative masses and charges of protons, neutrons and electrons
define atomic number and mass number and use nuclide notation
work out the numbers of particles in atoms and ions
define isotopes and give examples

Dalton's atomic theory

All matter is made of tiny, indivisible particles called atoms.
Atoms can be neither created nor destroyed.
Atoms of the same element are identical in mass and properties; atoms of different elements differ.
Atoms combine in simple whole-number ratios to form compounds.
Chemical reactions rearrange atoms.

Modifications to the theory

Dalton said	We now know
Atoms are indivisible	Atoms contain protons, neutrons and electrons
Atoms of an element are identical	Isotopes of an element have different masses
Atoms cannot be created or destroyed	True in chemical reactions, but nuclear reactions change atoms into other atoms
Atoms combine in simple ratios	Large organic molecules have complex ratios (e.g. C₁₂H₂₂O₁₁)

The theory still explains the law of conservation of mass and the law of constant composition: atoms are rearranged, not lost, in reactions.

Discovering the particles

Scientist	Experiment	Discovery
J. J. Thomson (1897)	Cathode rays bent by electric and magnetic fields	The electron, a negative particle in all atoms; "plum pudding" model
Ernest Rutherford (1911)	Alpha particles fired at gold foil	A tiny, dense, positive nucleus
Niels Bohr (1913)	Spectra of hydrogen	Electrons move in fixed energy levels (shells)
James Chadwick (1932)	Bombarding beryllium with alpha particles	The neutron, a neutral particle in the nucleus

Alpha particles fired at gold foil: most pass straight through, a few are deflected and a very few bounce back

Figure 1. Rutherford's gold foil experiment. WASSCE Tutor diagram.

What the gold foil results showed:

Most alpha particles passed straight through, so the atom is mostly empty space.
A few were deflected, so there is a positive centre that repels the positive alpha particles.
A very few bounced back, so this centre, the nucleus, is very small but holds almost all the mass.

The structure of the atom

A sodium atom with a nucleus of protons and neutrons surrounded by electrons in shells 2, 8, 1, beside a table of the masses and charges of the particles

Figure 2. The particles in an atom. WASSCE Tutor diagram.

Particle	Location	Relative mass	Relative charge
Proton	Nucleus	1	+1
Neutron	Nucleus	1	0
Electron	Shells around the nucleus	$\tfrac{1}{1840}$ (almost 0)	−1

A neutral atom has equal numbers of protons and electrons, so its charges cancel.

Atomic number and mass number

Definition

Atomic number (Z): the number of protons in the nucleus of an atom. It identifies the element.
Mass number (A): the total number of protons + neutrons (nucleons) in the nucleus.
Number of neutrons $= A - Z$ .

Nuclide notation writes the mass number at the top left and the atomic number at the bottom left of the symbol:

{}^{A}_{Z}\text{X} \qquad \text{e.g.} \qquad {}^{23}_{11}\text{Na}

\begin{gathered}{}^{A}_{Z}\text{X} \\[6pt] \text{e.g.} \\[6pt] {}^{23}_{11}\text{Na}\end{gathered}

Worked example

Counting particles.

Species	Protons	Neutrons	Electrons
${}^{23}_{11}\text{Na}$ atom	11	23 − 11 = 12	11 (neutral atom)
${}^{23}_{11}\text{Na}^+$ ion	11	12	11 − 1 = 10 (lost one)
${}^{16}_{8}\text{O}^{2-}$ ion	8	16 − 8 = 8	8 + 2 = 10 (gained two)
${}^{27}_{13}\text{Al}^{3+}$ ion	13	27 − 13 = 14	13 − 3 = 10 (lost three)

A positive ion has lost electrons; a negative ion has gained electrons. The protons never change in a chemical reaction.

Isotopes

Definition

Isotopes are atoms of the same element with the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons).

Element	Isotopes
Hydrogen	¹H (protium, 0 neutrons), ²H (deuterium, 1 neutron), ³H (tritium, 2 neutrons)
Carbon	¹²C (6 neutrons), ¹³C (7), ¹⁴C (8, radioactive)
Chlorine	³⁵Cl (18 neutrons), ³⁷Cl (20 neutrons)
Oxygen	¹⁶O, ¹⁷O, ¹⁸O

Isotopes have the same chemical properties, because chemistry depends on electrons, and isotopes have the same number of electrons.
They differ in physical properties that depend on mass, such as density and rate of diffusion.
Some isotopes, such as carbon-14, are radioactive (next lesson).

Common mistake

Isotopes differ in the number of neutrons, not protons. If the number of protons changes, the atom becomes a different element.

In Sierra Leone

The diamonds mined in Kono are pure carbon, made mostly of carbon-12 atoms with about 1% carbon-13. Every carbon atom, in a diamond, in charcoal or in your body, has exactly 6 protons.

Key points

Dalton's theory pictured indivisible atoms; it was modified after the discovery of subatomic particles, isotopes and nuclear reactions.
Thomson found the electron, Rutherford the nucleus, Chadwick the neutron.
Protons (+1, mass 1) and neutrons (0, mass 1) are in the nucleus; electrons (−1, mass 1/1840) are in shells.
Atomic number = protons; mass number = protons + neutrons; neutrons = A − Z.
Isotopes: same protons, different neutrons; same chemical properties.

WASSCE examination focus

WAEC tests this

Expect to state Dalton's postulates and their modifications, to explain the gold foil results, to calculate protons, neutrons and electrons in atoms and ions from nuclide notation, and to define isotopes with examples. Show A − Z working for neutrons.

Quick check

State two postulates of Dalton's atomic theory that have been modified.
Which observation in the gold foil experiment showed that the atom is mostly empty space?
How many protons, neutrons and electrons are in ${}^{39}_{19}\text{K}^+$ ?
Why do isotopes of an element have the same chemical properties?

Answers:

Any two: atoms are indivisible; atoms of the same element are identical; atoms cannot be created or destroyed; atoms combine in simple ratios.
Most alpha particles passed straight through the foil.
19 protons, 39 − 19 = 20 neutrons, 19 − 1 = 18 electrons.
They have the same number of electrons, and chemical properties depend on electrons.

Inside the atom: particles, atomic number and isotopes