Atomic structure & trends within the periodic table
In this topic you will:
- learn more about the structure of the atom
- learn how the structure of the Periodic Table is related to the structure of the elements.
Key words
- atomic number
- electron shells
- electronic structure
- electrostatic forces
- energy levels
- mass number
- Periodic Table
The Periodic Table
In previous lessons, you learnt about the first 20 elements and their symbols in the Periodic Table. Now you will learn more about the structure of the atoms of these elements.
- The atoms of the elements increase in mass as you progress from left to right (starting with hydrogen) and downwards in the Periodic Table.
- Each element has an atomic number. This tells you how many protons it contains.
- Each element has a mass number. This tells you how many protons and neutrons the atom contains in total.
- Protons have a positive charge. Electrons have a negative charge. Neutrons have no charge.
- An atom has no overall charge because the number of protons equals the number of electrons.
Let’s take a look at the metal lithium as an example.
Worked Example: Calculating Neutrons in Lithium
Lithium (metal):
- Atomic number = 3
- Mass number = 7
- Number of protons = 3
- Number of electrons = 3
- Number of neutrons = ?
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neutrons = mass number − atomic number
neutrons = 7 − 3
neutrons = 4
Mini Question
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neutrons = mass number − atomic number
neutrons = 16 − 8
neutrons = 8
The higher the mass number, the more mass the atom has. Hydrogen has an atomic mass of 1. Carbon has an atomic mass of 12. So, one atom of carbon has 12 times more mass than one atom of hydrogen.
Let’s compare the mass of a piece of sodium measuring 1 cm³ with the mass of a piece of iron measuring 1 cm³. The mass of the piece of sodium is 0.97 g and the mass of the piece of iron is 7.87 g. The mass of the iron is more than the mass of the sodium.
This measure of the mass for a fixed volume of a substance is called the density. The density is given using the unit g/cm³.
The density of iron is 7.87 g/cm³ and that of sodium 0.97 g/cm³. So, iron is more dense than sodium.
For more information on how to find the densities of solids, liquids and gases and to compare them, see Unit 3, Topic 3.1.
Definition: Density
Density is the amount of mass in a given volume. It is calculated using the formula:
density = mass ÷ volume
The unit of density is usually g/cm³.
Mini Question
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Because iron has a higher density than sodium. The same volume of iron contains more mass, so 1 cm³ of iron (7.87 g) weighs more than 1 cm³ of sodium (0.97 g).
Arranging electrons
In Stage 8, you learnt about the development of a model for the structure of the atom. This used ideas from a number of scientists such as J. J. Thompson and Ernest Rutherford. They developed the model shown here:
In 1913, the Danish scientist Niels Bohr developed Rutherford’s model of the atom further. He had the idea and evidence that the electrons move in different electron shells (also called energy levels) around the nucleus. He was awarded the Nobel Prize for his work. His idea changed the model of the structure of the atom to the one we use today.
The electrons are arranged in electron shells around the nucleus. This is the electronic structure. The first electron shell only has room for two electrons. The second and third electron shells have room for up to eight electrons. The crosses represent the electrons. Remember that the electrons are held in place by electrostatic forces.
Look carefully at the diagrams, showing atoms of increasing mass:
The arrangement of the electrons in an atom is often written as numbers. For example, the electronic arrangement for a boron atom can be written as 2,3. This means that there are two electrons in the first electron shell and three in the next electron shell. The first shell always fills up before electrons go into the second shell.
Definition: Electronic Structure
The electronic structure is the way electrons are arranged in shells around the nucleus of an atom. Each shell can hold a set number of electrons. Electrons fill the lowest available energy levels (shells) first.
Mini Question
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2,4 – Two electrons go in the first shell, and four go in the second shell.
Questions
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6 electrons – equal to the atomic number of carbon.
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4 protons – atomic number of beryllium is 4.
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Boron's mass number is 11 and atomic number is 5.
neutrons = 11 − 5 = 6
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Magnesium has atomic number 12. Arrange 12 electrons in shells as 2,8,2 with a nucleus in the center.
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Aluminium – atomic number 13
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Fluorine has 9 electrons. Draw a nucleus and place electrons in shells as 2,7. Label the nucleus, electron shells, and electrons.
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The current model includes electrons arranged in shells (energy levels) around the nucleus, as proposed by Bohr. Rutherford’s model did not include these electron shells.
Building on What You Know
In the last topic, you explored the structure of atoms — including protons, neutrons, electrons, and how they are arranged in shells. You also learned about atomic number, mass number, and how elements are positioned in the Periodic Table.
Now that you understand how atoms are built, it's time to look at how groups of elements behave in similar or predictable ways. This topic focuses on the trends within groups in the Periodic Table — like how atomic size, melting point, and reactivity change as you go down a group.
Understanding these patterns will help you explain and predict the behavior of elements — a key skill in chemistry!
Group 1: The alkali metals
In previous topics, you learnt that the columns in the Periodic Table are called groups. The first group, also known as the alkali metals, includes the elements lithium, sodium and potassium. The elements have some properties in common. The table below contains data about three of the elements in Group 1.
| Element | Atomic number | Mass number | Melting point in °C | Boiling point in °C |
|---|---|---|---|---|
| lithium, Li | 3 | 7 | 180 | 1360 |
| sodium, Na | 11 | 23 | 98 | 900 |
| potassium, K | 19 | 39 | 63 | 777 |
As you can see, the atomic number increases as you go down the group. The mass number also increases as you go down the group. These increasing numbers tell you that the size of the atom is increasing.
When you look at the melting points you can see that they go down as you go down the group. The next element down in the group is the metal rubidium. We can predict it would have a melting point lower than 63 °C.
Definition: Alkali Metals
Alkali metals are the elements in Group 1 of the Periodic Table. They are very reactive metals with low melting points and are soft enough to be cut with a knife. They all have one electron in their outer shell.
Mini Question
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The melting points decrease as you go down Group 1. Lithium has the highest melting point, while potassium has the lowest.
Questions
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Metals are generally found on the left side and in the center of the Periodic Table.
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Boiling points decrease as you go down Group 1.
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Rubidium is below potassium in Group 1, so it will likely have a boiling point lower than 777 °C.
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Sodium has 11 electrons, lithium has 3. So sodium has 8 more electrons than lithium.
The structure of the Group 1 metals
The properties of lithium, sodium and potassium are similar and so is their atomic structure.
Lithium has an atomic number of 3 and a mass number of 7. This atom contains 3 protons, 3 electrons and 4 neutrons. The electrons are arranged as 2,1. This means there are two electrons in the lowest electron shell (which is full) and one electron in the second electron shell.
Sodium has an atomic number of 11 and a mass number of 23. This atom contains 11 protons, 11 electrons and 12 neutrons. The electronic structure is 2,8,1. This means there are two electrons in the lowest electron shell (which is full), eight electrons in the second electron shell (which is full) and one electron in the third electron shell.
Potassium has an atomic number of 19 and a mass number of 39. This atom contains 19 protons, 19 electrons and 20 neutrons. The electronic structure is 2,8,8,1. This means there are two electrons in the lowest electron shell (which is full), eight electrons in the second electron shell (which is full), eight electrons in the third electron shell (which is full) and one electron in the fourth electron shell.
Questions
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The atoms get larger as you go down the group because each new element has more electron shells.
(Look at the electron shell arrangements.)
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They all have one electron in their outer shell. This gives them similar chemical properties.
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Because all the elements in this group have one electron in their outer shell — which is what defines Group 1.
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As you go down the group, the atoms become larger, the melting and boiling points decrease, and the metals become more reactive.
Group 7: The halogens (extension material)
Group 7 is another group in the Periodic Table, known as the halogens. The group includes fluorine, chlorine and bromine. These elements have a number of properties in common. The first two elements (fluorine and chlorine) are gases at room temperature. Bromine is a liquid.
The most reactive is fluorine, then chlorine. Bromine is the least reactive of the three.
| Element | Atomic number | Electronic structure | Mass number | Colour | Melting point in °C | Boiling point in °C |
|---|---|---|---|---|---|---|
| fluorine, F | 9 | 2,7 | 19 | pale yellow | −220 | −188 |
| chlorine, Cl | 17 | 2,8,7 | 35 | yellowish green | −101 | −34 |
| bromine, Br | 35 | 2,8,18,7 | 80 | brown | −7 | 59 |
Questions
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Halogens are non-metals.
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Melting points increase as you go down Group 7.
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The colour of the halogens gets darker as you go down the group.
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Boiling points increase as you go down Group 7.
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Since the trend is increasing, iodine will have a higher melting and boiling point than bromine.
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Less reactive — halogen reactivity decreases as you go down the group.
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7 electrons — all halogens have 7 electrons in their outer shell.
The structure of fluorine and chlorine
Fluorine has an atomic number of 9 and a mass number of 19. This atom contains 9 protons, 9 electrons and 10 neutrons. The electronic structure is 2,7. This means that the first electron shell has 2 electrons and is full. The second electron shell has seven electrons.
Chlorine has an atomic number of 17 and a mass number of 35. This atom contains 17 protons, 17 electrons and 18 neutrons. The electronic structure is 2,8,7. This means that the first electron shell has two electrons and is full. The second electron shell has eight electrons and is full. The third electron shell has seven electrons.
Questions
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The atoms get larger as you go down the group because they gain more electron shells.
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All the atoms in Group 7 have seven electrons in their outer shell.
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Because the elements in Group 7 all have 7 electrons in their outer shell, which gives them similar reactivity.
Group 8: The noble gases
Group 8 includes the elements helium, neon and argon. They are all gases. They are inert (unreactive) and do not form compounds. They are called the noble gases.
| Element | Atomic number | Electronic structure | Mass number | Melting point in °C | Boiling point in °C |
|---|---|---|---|---|---|
| helium, He | 2 | 2 | 4 | −270 | −269 |
| neon, Ne | 10 | 2,8 | 20 | −249 | −246 |
| argon, Ar | 18 | 2,8,8 | 40 | −189 | −186 |
Helium has an atomic number of 2 and a mass number of 4. This atom contains 2 protons, 2 electrons and 2 neutrons. The electrons are arranged with 2 in the first shell. The shell is full.
Neon has an atomic number of 10 and a mass number of 20. This atom contains 10 protons, 10 electrons and 10 neutrons. The electrons are arranged with 2 in the first shell and 8 in the second shell. Both shells are full.
Argon has an atomic number of 18 and a mass number of 40. This atom contains 18 protons, 18 electrons and 22 neutrons. The electrons are arranged with 2 in the first shell, 8 in the second shell and 8 in the third shell. All three shells are full.
Questions
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Melting points increase as you go down Group 8.
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The atoms get larger as you go down the group, due to the addition of more electron shells.
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All the atoms have full outer electron shells, making them stable and unreactive.
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Because the atoms in this group (except helium) have 8 electrons in their outer shell — a full, stable configuration.
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Krypton is lower in the group, so its melting and boiling points will be higher than those of argon.
Think Like a Scientist
Investigation: Observing the Reactions of Group 1 Metals with Water
In this task, you will observe the reactions of lithium, sodium, and potassium with water. Your teacher will carry out the demonstration, but you will be responsible for watching carefully and analyzing what you see.
You will need: safety glasses
Your teacher will need: safety glasses, large trough of water, white tile, scalpel or knife, long forceps, lithium, sodium, potassium (stored safely), and a safety screen.
Steps:
1. Put on your safety glasses and remain behind the safety screen.
2. Watch as your teacher cuts small pieces of lithium, sodium, and potassium, and places them into a trough of water one at a time.
3. Observe how each metal reacts — take notes on the speed, appearance, and any other visible effects.
4. Use your notes to answer the following questions.
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The teacher wore safety glasses, used long forceps, worked behind a safety screen, and used a small amount of each metal to minimize risk.
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Lithium fizzed gently and floated. Sodium moved quickly on the surface and melted. Potassium caught fire with a lilac flame.
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Lithium + water → lithium hydroxide + hydrogen
Sodium + water → sodium hydroxide + hydrogen
Potassium + water → potassium hydroxide + hydrogen
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All metals reacted with water to release gas (bubbling) and produce hydroxide. They all floated and moved on the surface.
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Lithium reacted slowly. Sodium was faster and melted. Potassium reacted violently and caught fire. The reactivity increased down the group.
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They are soft metals, have low density, react with water to produce hydrogen gas and an alkali, and are stored under oil to prevent reaction with air or moisture.
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Rubidium is too reactive and dangerous for classroom use. It may explode or ignite violently in water.
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Because they form alkaline (basic) solutions when they react with water — for example, sodium hydroxide.