The modern periodic table (AQA GCSE Chemistry): Revision Notes
The modern periodic table
How the modern periodic table is arranged
The modern periodic table organises elements in a very specific way. After scientists discovered particles inside atoms, they realised the best way to arrange elements was by atomic number (number of protons). This was different from earlier attempts that used atomic mass.
The shift from organising by atomic mass to atomic number was a crucial breakthrough that solved many inconsistencies in earlier periodic tables.
The periodic table has:
- Periods - horizontal rows
- Groups - vertical columns (numbered 1-7, plus group 0)
Each element has:
- An atomic symbol (like H for hydrogen)
- An atomic number (number of protons)
- A relative atomic mass
Metals vs non-metals
The periodic table clearly separates metals and non-metals, each with very different properties.
Understanding the fundamental difference between metals and non-metals is crucial: metals lose electrons while non-metals gain electrons when forming compounds.
Metals
Metals have these key features:
- Few electrons in their outer shell
- Lose electrons to form positive ions in ionic compounds
- All metals except mercury are solid at room temperature
- All metals conduct electricity
Non-metals
Non-metals behave very differently:
- Few electrons short of a full outer shell
- Gain electrons to form negative ions in ionic compounds
- Share electrons in covalent compounds
- Some are gases or solids with low melting points at room temperature
- Some have giant structures with high melting points
- Most do not conduct electricity (except some forms of carbon)
Groups in the periodic table
Elements in the same group (vertical column) have very similar properties. This happens because they have the same number of outer electrons and therefore similar chemical properties.
Example: Group 1 Elements
All group 1 elements (lithium, sodium, potassium, etc.) have 1 outer electron and show similar behaviour:
- They all react vigorously with water
- They all form +1 ions
- They all have similar chemical reactions
Why Mendeleev's table needed changing
Mendeleev originally arranged elements by atomic weight, but this caused problems.
Example: The Tellurium-Iodine Problem
- Tellurium (Te) and iodine (I) seemed in the wrong order
- Mendeleev put tellurium before iodine even though tellurium had a higher atomic weight
- This was because their chemical properties fitted better this way
When scientists discovered atomic structure, they realised the correct order should be by atomic number, not atomic weight. This solved the problems with Mendeleev's table.
How to work out which group an element belongs to
You can use the electronic structure to find which group an element belongs to:
Worked Example: Finding Group Number
Step 1: Count the number of outer shell electrons Step 2: This number tells you the group number
For example, if an element has 6 outer electrons, it belongs to group 6.
Key Points to Remember:
- The modern periodic table arranges elements by atomic number (number of protons)
- Metals lose electrons to form positive ions, non-metals gain electrons to form negative ions
- Elements in the same group have the same number of outer electrons and similar properties
- The group number equals the number of outer electrons
- Using atomic number instead of atomic weight solved the problems in Mendeleev's original table