Chemistry · Lesson 2

Inside the atom

JAMBWAECNECO

What the syllabus asks

JAMB: Atomic structure and bonding: atomic number, mass number, isotopes, relative atomic mass, electronic configuration, orbitals, Aufbau, Pauli and Hund rules

WAEC: Atomic structure: protons, neutrons, electrons; atomic number and mass number; isotopes and relative atomic mass; electron configuration (s, p, d up to Z = 30); quantum numbers and orbitals; historical models (Dalton, Thomson, Rutherford, Bohr); radioactivity basics

NECO: Atomic structure: protons, neutrons, electrons; atomic number and mass number; isotopes and relative atomic mass; electron configuration (s, p, d up to Z = 30); quantum numbers and orbitals; historical models (Dalton, Thomson, Rutherford, Bohr); radioactivity basics

What you will learn

Describe the structure of the atom and the properties of its three particles.
Use Z and A to find numbers of protons, neutrons and electrons in atoms and ions.
Explain isotopes and calculate relative atomic mass from abundance data.
Write electron configurations in shells and in s, p, d notation up to Z = 30.
Apply the Aufbau principle, Pauli exclusion principle and Hund's rule.
Outline how the atomic model developed from Dalton to the modern view.

Key points

The atom has a small dense positive nucleus of protons and neutrons, with electrons in shells around it.
Proton: +1, mass 1; neutron: 0, mass 1; electron: −1, mass about 1/1840.
Z = protons; A = protons + neutrons; neutrons = A − Z; electrons in an ion = Z − charge.
Isotopes have the same Z and different A; same chemical but different physical properties.
Aᵣ = Σ(mass × % abundance)/100; it lies nearer the most abundant isotope.
Shell n holds up to 2n² electrons; for Z ≤ 20 use 2, 8, 8, 2.
Fill 1s 2s 2p 3s 3p 4s 3d 4p; s holds 2, p holds 6, d holds 10.
Pauli: max two electrons per orbital, opposite spins. Hund: singly first in equal orbitals.
Cr is [Ar] 4s¹ 3d⁵ and Cu is [Ar] 4s¹ 3d¹⁰; ions of transition metals lose 4s electrons first.
Models developed from Dalton to Thomson, Rutherford, Bohr and the modern orbital model.

Start here: What is an atom?

Imagine cutting a piece of copper wire in half, then in half again, and again, millions of times. Eventually you would reach a particle so small that if you cut it further it would no longer be copper. That smallest particle is an atom. A single grain of salt contains roughly a billion billion atoms, so atoms are far too small to see even with a good school microscope.

An atom is the smallest particle of an element that can take part in a chemical reaction.

The word comes from the Greek atomos, meaning "cannot be cut". For a long time people believed atoms really were solid, uncuttable balls. We now know that each atom is made of even smaller subatomic particles. But in chemical reactions, such as burning firewood or rusting of a roofing sheet, atoms are not destroyed. They are only rearranged into new combinations. This is why mass is conserved in chemical reactions.

How big is an atom?

A typical atom has a diameter of about 0.0000000001 m (10⁻¹⁰ m). If you lined up atoms across the width of a ₦50 note, you would need about 700 million of them. The nucleus is even tinier: about 100,000 times smaller than the atom itself. If an atom were blown up to the size of the National Stadium in Abuja, the nucleus would be about the size of a groundnut on the centre spot, and the electrons would be moving in the empty space all the way out to the stands.

This tells us something surprising: an atom is mostly empty space. Nearly all its mass is packed into the nucleus.

Worked example 2.1

Explain why chemical reactions do not change the total mass of substances.

In a reaction, bonds between atoms break and new bonds form.
The atoms themselves are neither created nor destroyed; they are only rearranged.
Since the same atoms are present before and after, the total mass is the same.

Atoms are rearranged, not created or destroyed (law of conservation of mass).

⚠️ Common mistake: saying "an atom is the smallest particle of matter". Electrons, protons and neutrons are smaller. Use the exam definition: the smallest particle of an element that can take part in a chemical reaction.

Try it yourself. Where is most of the mass of an atom found?

Show solution

In the nucleus, which contains the protons and neutrons.

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Practice questions

1. An atom has mass number 23 and atomic number 11. How many neutrons does it have?

Show answer

A: 12
Neutrons = A − Z = 23 − 11 = 12.

2. Isotopes of an element have the same number of

neutrons
nucleons
mass number
protons

Show answer

D: protons
Isotopes differ only in neutron number.

3. The electron configuration of an element with atomic number 17 is

2, 7, 8
2, 8, 6, 1
2, 8, 7
2, 8, 8

Show answer

C: 2, 8, 7
17 electrons: 2 + 8 + 7 (chlorine).

4. Which subatomic particle has a negligible mass?

Nucleus
Electron
Proton
Neutron

Show answer

B: Electron
An electron is about 1/1840 the mass of a proton.

5. Chlorine has isotopes ³⁵Cl (75%) and ³⁷Cl (25%). Its relative atomic mass is

35.5
36.0
35.0
37.0

Show answer

A: 35.5
0.75×35 + 0.25×37 = 26.25 + 9.25 = 35.5.

Key words

Atom: The smallest particle of an element that can take part in a chemical reaction.
Proton: Positively charged particle in the nucleus, relative mass 1.
Neutron: Uncharged particle in the nucleus, relative mass 1.
Electron: Negatively charged particle in shells, relative mass about 1/1840.
Nucleus: The dense, positive centre of the atom.
Atomic number (Z): Number of protons in an atom.
Mass number (A): Total number of protons and neutrons.
Isotopes: Atoms of the same element with different numbers of neutrons.
Relative atomic mass: Average mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom.
Shell (energy level): A region around the nucleus where electrons of similar energy are found.
Orbital: A region of space that can hold at most two electrons.
Valence electrons: Electrons in the outermost shell.

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