What you'll learn
- How Mendeleev built an early periodic table and used it to make predictions.
- What atomic number, periods and groups mean.
- How to identify metals and non-metals from their position and atomic structure.
- How to write electronic configurations for the first 20 elements, such as 2.8.1.
The big idea: the periodic table is a pattern-finder
The periodic table is not just a list of elements. It organises elements so that patterns in their properties become easier to see.
Periodic table
The periodic table is a chart of all known elements, arranged in order of increasing atomic number, with elements of similar properties placed in the same vertical columns.
An element is a substance made from only one type of atom. For example, oxygen contains only oxygen atoms, and copper contains only copper atoms.
Mendeleev’s periodic table
What Mendeleev did
In 1869, Dmitri Mendeleev arranged the elements known at the time into a table. He mainly used:
- the properties of the elements, such as whether they reacted strongly or weakly
- the properties of their compounds, such as oxides and chlorides
- what he believed was increasing relative atomic mass
Relative atomic mass
Relative atomic mass, often written as ArA_rAr, is the average mass of atoms of an element compared with one twelfth of the mass of a carbon-12 atom.
Mendeleev’s clever move was that he did not force every element into strict mass order if that ruined the pattern of properties. He placed elements with similar chemical behaviour in the same columns.
Why Mendeleev’s table mattered
Mendeleev arranged elements by patterns in properties, left gaps for undiscovered elements, and predicted the properties of those missing elements.
Predicting missing elements
Mendeleev left gaps where he thought an undiscovered element should go. He then predicted its properties by looking at the elements around the gap.
For example, he predicted an element below aluminium, which was later discovered and called gallium. Its properties were close to his predictions.
Using a group pattern to predict a missing element
A missing element is in the same group as lithium and sodium. Lithium and sodium are soft metals that react with water and form compounds with one chlorine atom, such as LiCl and NaCl. Predict two properties of the missing element.
- Elements in the same group have similar chemical properties, so the missing element should behave like lithium and sodium.
- Lithium and sodium are metals, so the missing element is likely to be a metal.
- Lithium and sodium form chlorides with one chlorine atom, so the missing element is likely to form a chloride with a similar formula pattern, such as XCl.
- A good prediction is: the missing element is a reactive metal and forms a chloride with one chlorine atom per atom of the element.
Why relative atomic mass was not always enough
Mendeleev thought he had arranged elements in increasing relative atomic mass. Mostly, this worked — but not always.
This is because relative atomic mass depends on the isotopes of an element and how common each isotope is.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Some pairs of elements appear “out of order” by relative atomic mass because their isotopes have different relative abundances. The modern periodic table fixes this by using atomic number instead.
Mass order is not the modern rule
Do not say the modern periodic table is arranged by relative atomic mass. It is arranged by increasing atomic number.
Atomic number and position
Atomic number
Atomic number
The atomic number of an element is the number of protons in the nucleus of one atom of that element.
A proton is a positively charged particle found in the nucleus. The nucleus is the tiny, dense centre of an atom.
For example:
- hydrogen has atomic number 1, so it has 1 proton
- carbon has atomic number 6, so it has 6 protons
- sodium has atomic number 11, so it has 11 protons
The atomic number identifies the element. If an atom has 8 protons, it is oxygen. If it has 17 protons, it is chlorine.
Atomic number identifies the element
The number of protons decides which element an atom is, and elements are arranged in increasing atomic number in the modern periodic table.
Periods and groups
Periods: the rows
Period
A period is a horizontal row in the periodic table.
As you move from left to right across a period, atomic number increases by 1 each time.
For the first 20 elements:
- period 1 contains hydrogen and helium
- period 2 runs from lithium to neon
- period 3 runs from sodium to argon
- period 4 begins with potassium and calcium
Groups: the columns
Group
A group is a vertical column in the periodic table. Elements in the same group have similar chemical properties.
For GCSE, you often focus on groups such as:
- Group 1: alkali metals, such as lithium, sodium and potassium
- Group 7: halogens, such as fluorine, chlorine and bromine
- Group 0: noble gases, such as helium, neon and argon
Group 0 is sometimes called Group 18 on fuller periodic tables.
Metals and non-metals
Elements can be classified as metals or non-metals.
Metal and non-metal
A metal is an element that usually forms positive ions and has metallic properties such as electrical conductivity. A non-metal is an element that usually forms negative ions or shares electrons in covalent bonding.
On the periodic table, metals are mainly found on the left and in the centre. Non-metals are mainly found on the right. There is a stepped dividing line between them.
Explaining the division using atomic structure
The metal/non-metal pattern links to electronic structure — how electrons are arranged in atoms.
- Metal atoms usually have a small number of electrons in their outer shell.
- They often lose outer electrons to form positive ions.
- Non-metal atoms usually have more electrons in their outer shell.
- They often gain electrons or share electrons.
Quick location check
If an element is on the left-hand side of the periodic table, it is usually a metal. If it is on the right-hand side, it is usually a non-metal. Hydrogen is a non-metal even though it is placed on the left.
Electronic configuration
Electron shells
Electrons are arranged around the nucleus in energy levels called shells.
Electronic configuration
An electronic configuration shows how the electrons in an atom are arranged in shells. For example, sodium has the electronic configuration 2.8.1.
For the first 20 elements, use these shell rules:
- the first shell can hold up to 2 electrons
- the second shell can hold up to 8 electrons
- the third shell can hold up to 8 electrons for the first 20 elements
- then the fourth shell begins
So potassium, atomic number 19, is 2.8.8.1 rather than 2.8.9.
First 20 elements rule
At GCSE for the first 20 elements, fill shells as 2, then 8, then 8, then start the fourth shell. Do not put 9 electrons in the third shell for potassium.
Electronic configurations of the first 20 elements
| Atomic number | Element | Symbol | Electronic configuration |
|---|---|---|---|
| 1 | Hydrogen | H | 1 |
| 2 | Helium | He | 2 |
| 3 | Lithium | Li | 2.1 |
| 4 | Beryllium | Be | 2.2 |
| 5 | Boron | B | 2.3 |
| 6 | Carbon | C | 2.4 |
| 7 | Nitrogen | N | 2.5 |
| 8 | Oxygen | O | 2.6 |
| 9 | Fluorine | F | 2.7 |
| 10 | Neon | Ne | 2.8 |
| 11 | Sodium | Na | 2.8.1 |
| 12 | Magnesium | Mg | 2.8.2 |
| 13 | Aluminium | Al | 2.8.3 |
| 14 | Silicon | Si | 2.8.4 |
| 15 | Phosphorus | P | 2.8.5 |
| 16 | Sulfur | S | 2.8.6 |
| 17 | Chlorine | Cl | 2.8.7 |
| 18 | Argon | Ar | 2.8.8 |
| 19 | Potassium | K | 2.8.8.1 |
| 20 | Calcium | Ca | 2.8.8.2 |
Writing an electronic configuration
Calcium has atomic number 20. Work out its electronic configuration.
- A neutral calcium atom has 20 electrons because its atomic number is 20, so it has 20 protons and 20 electrons.
- Fill the first shell with 2 electrons, leaving 18 electrons.
- Fill the second shell with 8 electrons, leaving 10 electrons.
- Fill the third shell with 8 electrons, leaving 2 electrons.
- Put the remaining 2 electrons in the fourth shell, giving the electronic configuration 2.8.8.2.
Linking electronic configuration to the periodic table
This is one of the most useful skills in the topic.
Period number = number of occupied shells
If an atom has electrons in three shells, it is in period 3.
For example:
- sodium is 2.8.1, so it has 3 occupied shells → period 3
- chlorine is 2.8.7, so it has 3 occupied shells → period 3
- calcium is 2.8.8.2, so it has 4 occupied shells → period 4
Group number = number of outer electrons
For Groups 1 to 7, the group number is the number of electrons in the outer shell.
The outer shell is the highest occupied electron shell in an atom.
For example:
- lithium is 2.1, so it has 1 outer electron → Group 1
- magnesium is 2.8.2, so it has 2 outer electrons → Group 2
- oxygen is 2.6, so it has 6 outer electrons → Group 6
- chlorine is 2.8.7, so it has 7 outer electrons → Group 7
Group 0 is slightly special. Noble gases have full outer shells. Helium has 2 electrons in its first shell, while neon and argon have 8 electrons in their outer shells.
Position from electrons
For the first 20 elements, the number of occupied shells gives the period, and the number of outer electrons gives the group for Groups 1 to 7.
Finding an element’s position from its electrons
An atom has the electronic configuration 2.8.6. Predict its period, group and whether it is a metal or non-metal.
- Count the occupied shells: 2.8.6 has electrons in 3 shells, so the element is in period 3.
- Count the outer-shell electrons: the final number is 6, so the element is in Group 6.
- Find the likely type: Group 6 on the right-hand side of period 3 is in the non-metal region, so the element is a non-metal.
- The element with configuration 2.8.6 is sulfur.
Confusing period and group
The period is the row and tells you the number of occupied shells. The group is the column and usually tells you the number of outer electrons.
In the exam
- If asked about Mendeleev, mention both similar properties and gaps for undiscovered elements.
- If asked for modern arrangement, say increasing atomic number, not increasing relative atomic mass.
- For electronic configurations, use the atomic number to get the number of electrons, then fill shells as 2, 8, 8 for the first 20 elements.
- To link electrons to the table, use: occupied shells → period; outer electrons → group.
Check yourself
- Why did Mendeleev leave gaps in his periodic table?
- What is the electronic configuration of potassium, and why is it not 2.8.9?
- An element has electronic configuration 2.8.3. What period and group is it in?
