The modern periodic table (AQA GCSE Chemistry Combined Science): Revision Notes
The modern periodic table
How the modern periodic table is arranged
The modern periodic table organises elements differently from the original version. After scientists discovered sub-atomic particles, they realised elements should be arranged by atomic number (number of protons) rather than atomic weight.
This arrangement by atomic number solved several problems that existed in Mendeleev's original table, where some elements appeared to be in the wrong order when arranged by atomic weight alone.
Problem Solved: Iodine and Tellurium
When arranged by atomic weight:
- Tellurium (atomic weight 127.6) would come after Iodine (atomic weight 126.9)
- But their properties suggested they should be the other way around
When arranged by atomic number:
- Tellurium (atomic number 52) correctly comes before Iodine (atomic number 53)
- Their properties now match their positions perfectly
Key features of the modern periodic table
Groups (vertical columns)
Groups are the vertical columns in the periodic table. Elements in the same group share important features:
- They have the same number of outer shell electrons
- They have similar chemical properties
- Group numbers tell you how many outer electrons each element has
Group 1 Elements Example
All elements in Group 1 have 1 outer electron and react in similar ways:
- Lithium (Li): 1 outer electron, reacts vigorously with water
- Sodium (Na): 1 outer electron, reacts vigorously with water
- Potassium (K): 1 outer electron, reacts vigorously with water
This pattern continues for all Group 1 elements.
Periods (horizontal rows)
Periods are the horizontal rows across the periodic table. As you move across a period, the atomic number increases by one each time.
Metals and non-metals
The periodic table is divided into two main types of elements:
Metals (found on the left side):
- Have only a few electrons in their outer shell
- Lose electrons easily to form positive ions
- Are solid at room temperature (except mercury)
- Conduct electricity
Non-metals (found on the right side):
- Are few electrons short of having a full outer shell
- Gain electrons to form negative ions, or share electrons
- Can be gases, liquids, or solids at room temperature
- Do not conduct electricity (except some forms of carbon like graphite)
Why atomic number matters
Using atomic number instead of atomic weight gives us the correct order of elements. This arrangement is crucial because:
- Elements with similar properties appear in the same groups
- The patterns in properties make sense
- We can predict how elements will behave
Electronic structure and groups
The electronic structure of an element shows us which group it belongs to. The last number in the electronic structure tells you the group number.
Determining Group from Electronic Structure
An element with electronic structure 2,8,18,32,18,6:
- The last number is 6
- This means it has 6 outer electrons
- Therefore, it belongs to Group 6
This method works for all elements in the periodic table.
Key Points to Remember:
- The modern periodic table arranges elements by atomic number, not atomic weight
- Groups (columns) contain elements with the same number of outer electrons and similar properties
- Metals are on the left and lose electrons easily; non-metals are on the right and gain electrons
- The electronic structure determines which group an element belongs to
- This arrangement allows us to predict how elements will behave