(I have taught this chapter for over thirty years, and I still remember the day it clicked for me as a student. My teacher drew a single atom on the board — a tiny nucleus with electrons whizzing around it — and then said, "Now imagine arranging all the atoms in the universe in order of how many protons they have. That arrangement is the periodic table." In that moment, chemistry stopped being a list of facts and became a map. I hope this chapter does the same for you.) Everything you have ever touched — the air, the water, your own hand — is made of atoms. But not all atoms are the same. Some are light, some are heavy. Some react violently, some sit quietly. Some conduct electricity, some do not. The question that puzzled scientists for centuries was simple: Is there a pattern? And the answer, it turned out, was yes. When you arrange all the elements in order of increasing atomic number, they fall into repeating patterns — patterns so reliable that you can predict how an element will behave just by knowing where it sits on the table. In this chapter, we are going to look at the structure of the atom and how that structure explains the periodic table. We will learn about electron shells, atomic number, mass number, isotopes, and the trends that run across periods and down groups. By the end, you will not just know the periodic table — you will understand it. And that understanding will carry you through every chemistry chapter that follows. Let us begin.
An atom is the smallest particle of an element that can exist. It consists of a tiny, dense nucleus surrounded by electrons in energy levels (shells).
The nucleus contains:
The electrons:
In a neutral atom, the number of protons equals the number of electrons.
(If the atom were the size of a football stadium, the nucleus would be a pea at the centre. Almost all the mass is in that pea, and almost all the space is empty. That is the strangest and most wonderful fact in all of chemistry.)
Two numbers define every atom:
From these, you can work out the number of each subatomic particle:
| Particle | How to Find It |
|---|---|
| Protons | = atomic number (Z) |
| Electrons | = protons (in a neutral atom) |
| Neutrons | = mass number (A) − atomic number (Z) |
Example: Sodium has atomic number 11 and mass number 23.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.
| Isotope | Protons | Neutrons | Mass Number | Abundance |
|---|---|---|---|---|
| Chlorine-35 | 17 | 18 | 35 | 75% |
| Chlorine-37 | 17 | 20 | 37 | 25% |
| Carbon-12 | 6 | 6 | 12 | 98.9% |
| Carbon-14 | 6 | 8 | 14 | Trace |
Isotopes have the same chemical properties because they have the same number of electrons. But they have slightly different physical properties because their masses differ.
Uses of isotopes:
Most elements exist as a mixture of isotopes. The relative atomic mass (Ar) is the weighted average mass of the atoms of an element compared to 1/12 the mass of a carbon-12 atom.
Formula:
Ar = (abundance₁ × mass₁ + abundance₂ × mass₂ + …) ÷ total abundance
Example: Chlorine is 75% Cl-35 and 25% Cl-37.
Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5
That is why chlorine has a relative atomic mass of 35.5, not a whole number.
Electrons are arranged in energy levels or shells around the nucleus. Each shell can hold a maximum number of electrons:
| Shell | Maximum Electrons |
|---|---|
| 1st shell | 2 |
| 2nd shell | 8 |
| 3rd shell | 8 (for the first 20 elements) |
| 4th shell | 2 (for the first 20 elements) |
Electrons fill the shells in order: first the 1st, then the 2nd, then the 3rd, then the 4th. The outermost shell is called the valence shell, and its electrons are the valence electrons.
| Element | Atomic Number | Electronic Configuration |
|---|---|---|
| Hydrogen | 1 | 1 |
| Helium | 2 | 2 |
| Lithium | 3 | 2,1 |
| Beryllium | 4 | 2,2 |
| Boron | 5 | 2,3 |
| Carbon | 6 | 2,4 |
| Nitrogen | 7 | 2,5 |
| Oxygen | 8 | 2,6 |
| Fluorine | 9 | 2,7 |
| Neon | 10 | 2,8 |
| Sodium | 11 | 2,8,1 |
| Magnesium | 12 | 2,8,2 |
| Aluminium | 13 | 2,8,3 |
| Silicon | 14 | 2,8,4 |
| Phosphorus | 15 | 2,8,5 |
| Sulphur | 16 | 2,8,6 |
| Chlorine | 17 | 2,8,7 |
| Argon | 18 | 2,8,8 |
| Potassium | 19 | 2,8,8,1 |
| Calcium | 20 | 2,8,8,2 |
The modern periodic table arranges elements in order of increasing atomic number.
The link:
Example: Sodium has the electronic configuration 2,8,1. It has 1 outer electron, so it is in Group 1. It has 3 shells, so it is in Period 3.
(This is the key that unlocks the whole table. Once you can write an electronic configuration, you can predict an element's group, period, and chemical behaviour. That is a superpower — use it.)
| Category | Position | Properties | Examples |
|---|---|---|---|
| Metals | Left and centre | Shiny, malleable, ductile, good conductors, high melting points, form positive ions. | Sodium, iron, copper, gold |
| Non-metals | Right (and hydrogen) | Dull, brittle, poor conductors, low melting points, form negative ions or share electrons. | Oxygen, chlorine, sulphur, carbon |
| Metalloids | Along the staircase | Properties in between — semi-conductors. | Silicon, germanium |
As you move from left to right across a period:
| Property | Trend | Reason |
|---|---|---|
| Metallic character | Decreases | Elements become less likely to lose electrons. |
| Non-metallic character | Increases | Elements become more likely to gain electrons. |
| Atomic radius | Decreases | Nuclear charge increases, pulling electrons closer. |
| Ionisation energy | Increases | Electrons are held more tightly. |
| Electronegativity | Increases | Atoms attract shared electrons more strongly. |
As you move down a group:
| Property | Trend | Reason |
|---|---|---|
| Atomic radius | Increases | A new shell is added at each step. |
| Metallic character | Increases | Outer electrons are further from the nucleus and easier to lose. |
| Ionisation energy | Decreases | Outer electrons are easier to remove. |
| Electronegativity | Decreases | The nucleus has less pull on outer electrons. |
| Reactivity of metals | Increases | Metals lose electrons more easily down the group. |
| Reactivity of non-metals | Decreases | Non-metals gain electrons less easily down the group. |
Lithium, sodium, potassium, rubidium, caesium. They have one outer electron, which they lose easily to form a +1 ion.
| Property | Observation |
|---|---|
| Appearance | Soft, silvery, shiny when freshly cut, tarnish quickly. |
| Softness | Can be cut with a knife. |
| Density | Low — lithium, sodium, potassium float on water. |
| Reaction with water | Lithium fizzes gently; sodium fizzes vigorously; potassium ignites with a lilac flame. |
| Reaction with oxygen | Form metal oxides (e.g., 4Na + O₂ → 2Na₂O). |
| Reaction with chlorine | Form white crystalline salts (e.g., 2Na + Cl₂ → 2NaCl). |
Trend: Reactivity increases down the group because the outer electron is further from the nucleus and easier to lose.
Fluorine, chlorine, bromine, iodine. They have seven outer electrons and need one more to complete their shell. They form -1 ions.
| Property | Fluorine | Chlorine | Bromine | Iodine |
|---|---|---|---|---|
| Colour | Pale yellow | Greenish-yellow | Red-brown | Dark grey |
| State | Gas | Gas | Liquid | Solid |
| Reactivity | Most reactive | Very reactive | Reactive | Least reactive |
Trend: Reactivity decreases down the group because the outer shell is further from the nucleus and attracts electrons less strongly.
Displacement: A more reactive halogen displaces a less reactive one from its salt. Example: Cl₂ + 2KBr → 2KCl + Br₂.
Helium, neon, argon, krypton, xenon. They have full outer shells, so they are unreactive.
| Gas | Use |
|---|---|
| Helium | Balloons, cooling superconducting magnets |
| Neon | Advertising signs |
| Argon | Light bulbs, welding |
| Xenon | Car headlights, anaesthesia |
Question: An atom of aluminium has atomic number 13 and mass number 27. Determine the number of protons, neutrons, and electrons.
Solution:
• Protons = atomic number = 13
• Electrons = protons = 13
• Neutrons = mass number − atomic number = 27 − 13 = 14
Question: Copper has two isotopes: Cu-63 (69%) and Cu-65 (31%). Calculate the relative atomic mass.
Solution: Ar = (69 × 63 + 31 × 65) ÷ 100 = (4347 + 2015) ÷ 100 = 6362 ÷ 100 = 63.62
Question: An element has the electronic configuration 2,8,7. Identify its group and period, and state whether it is a metal or non-metal.
Solution: It has 7 outer electrons, so it is in Group 17 (halogens). It has 3 shells, so it is in Period 3. This is chlorine, a non-metal.
1. State the relative mass and charge of a proton, a neutron, and an electron.
Proton — mass 1, charge +1. Neutron — mass 1, charge 0. Electron — mass 1/1836, charge -1.
2. Define atomic number and mass number.
Atomic number is the number of protons in the nucleus. Mass number is the total number of protons and neutrons.
3. An atom has 17 protons and 18 neutrons. Determine its atomic number, mass number, and number of electrons.
Atomic number = 17. Mass number = 35. Electrons = 17.
4. What are isotopes? Give one example.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Example: chlorine-35 and chlorine-37.
5. Write the electronic configuration of chlorine (atomic number 17).
2,8,7.
6. State the trend in reactivity down Group 1 and explain why it occurs.
Reactivity increases down Group 1 because the outer electron is further from the nucleus and shielded, so it is easier to lose.
7. Why are noble gases unreactive?
They have full outer electron shells, so they do not need to gain or lose electrons to achieve stability.
Question 1 [10 marks]
(a) Describe the structure of an atom, naming the subatomic particles and their locations. [4]
(b) State the relative mass and charge of each subatomic particle. [3]
(c) An atom of magnesium has atomic number 12 and mass number 24. Determine the number of protons, neutrons, and electrons. [3]
(a) An atom has a central nucleus containing protons and neutrons. Electrons orbit the nucleus in shells. (4 marks: 1 for nucleus, 1 for protons, 1 for neutrons, 1 for electrons in shells.)
(b) Proton — mass 1, charge +1. Neutron — mass 1, charge 0. Electron — mass 1/1836, charge -1. (3 marks: 1 for each.)
(c) Protons = 12. Neutrons = 24 − 12 = 12. Electrons = 12. (3 marks: 1 for each.)
Question 2 [10 marks]
(a) Define the term "isotope". [2]
(b) Chlorine consists of two isotopes: Cl-35 (75%) and Cl-37 (25%). Calculate the relative atomic mass of chlorine. [3]
(c) Explain why isotopes of the same element have the same chemical properties but different physical properties. [3]
(d) State one use of a radioactive isotope. [2]
(a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. (2 marks: 1 for "same protons", 1 for "different neutrons".)
(b) Ar = (75 × 35 + 25 × 37) ÷ 100 = 35.5. (3 marks: 1 for setup, 1 for calculation, 1 for answer.)
(c) Isotopes have the same number of electrons and the same electronic configuration, so they react the same way chemically. However, they have different masses, which affects physical properties like density and rate of diffusion. (3 marks: 1 for same electrons, 1 for same chemical properties, 1 for different masses.)
(d) Carbon-14 for radiocarbon dating; cobalt-60 for cancer treatment; iodine-131 for thyroid diagnosis. (2 marks: 1 for use, 1 for clarity.)
Question 3 [10 marks]
(a) Write the electronic configuration of: (i) sodium (11), (ii) chlorine (17), (iii) calcium (20). [3]
(b) State the group and period of each of the elements in (a). [3]
(c) Explain how the electronic configuration of an element determines its position in the periodic table. [4]
(a) (i) 2,8,1. (ii) 2,8,7. (iii) 2,8,8,2. (3 marks: 1 for each.)
(b) Sodium — Group 1, Period 3. Chlorine — Group 17, Period 3. Calcium — Group 2, Period 4. (3 marks: 1 for each.)
(c) The number of electrons in the outermost shell determines the group number. The number of shells determines the period number. Elements in the same group have the same number of outer electrons and therefore similar chemical properties. (4 marks: 1 for group = outer electrons, 1 for period = shells, 1 for similar properties, 1 for clarity.)
Challenge 1: An element X has the electronic configuration 2,8,8,1. Identify the element, state its group and period, and predict what ion it will form. Explain your reasoning.
Element X has 19 electrons, so it is potassium. It has 1 electron in its outermost shell, so it is in Group 1. It has 4 shells, so it is in Period 4. It will lose its one outer electron to form a +1 ion (K⁺), achieving the stable configuration of argon (2,8,8). Group 1 metals all lose one electron to form +1 ions because they have one outer electron and achieve a noble gas configuration by losing it.
Challenge 2: Explain why the relative atomic mass of chlorine is 35.5 and not a whole number.
The relative atomic mass of chlorine is 35.5 because chlorine exists as a mixture of two isotopes: chlorine-35 (about 75% abundance) and chlorine-37 (about 25% abundance). The relative atomic mass is the weighted average of these isotopic masses. Since the two isotopes have different masses and different abundances, the weighted average is 35.5, which is not a whole number. If chlorine existed as only one isotope, its relative atomic mass would be a whole number equal to the mass number of that isotope.
Challenge 3: A neutral atom has 16 protons and 18 neutrons. Identify the element, write its electronic configuration, and state whether it is a metal or non-metal. Predict the ion it is likely to form and explain why.
The element has 16 protons, so it is sulphur (atomic number 16). It has 16 electrons. Its electronic configuration is 2,8,6. It is a non-metal because it is on the right side of the periodic table and has 6 outer electrons. It is likely to gain 2 electrons to form a -2 ion (S²⁻), achieving the stable configuration of argon (2,8,8). Non-metals on the right of the periodic table tend to gain electrons to complete their outer shells, while metals on the left tend to lose electrons.
"The atom and the table — two ideas that changed everything. Now you know how they connect."
