Development of The Periodic Table
The periodic table is a key tool in Chemistry. It organises all known elements in a logical way, showing patterns in their properties, electron structures, and reactivity.
Mendeleev’s periodic table (1869)
Dmitri Mendeleev made a major breakthrough in organising elements:
- He arranged elements in order of increasing atomic mass
- He grouped elements with similar properties into vertical columns
- Most importantly, he left gaps for elements that hadn’t been discovered yet
- He also predicted the properties of these missing elements, and his predictions were later proven correct
💡 Mendeleev sometimes switched elements around to keep them in the correct group based on their chemical properties, even if it meant breaking the order of atomic mass.
His table worked well because he focused on chemical behaviour, not just numbers.
The modern periodic table
The modern periodic table is based on atomic number (number of protons), not atomic mass.
- Elements are arranged in order of increasing atomic number
- It contains groups (vertical columns) and periods (horizontal rows)
This layout helps us predict the properties, reactivity, and electron structure of elements.
Periods and groups
Term | What it shows | Example |
Period | Number of electron shells | Period 2: atoms have 2 shells |
Group | Number of electrons in outer shell | Group 1: all have 1 outer electron |
Elements in the same group have similar chemical properties because they have the same number of outer electrons.
Special groups
- Group 1 – Alkali metals: very reactive, soft metals with 1 outer electron
- Group 7 – Halogens: reactive non-metals with 7 outer electrons
- Group 0 – Noble gases: unreactive gases with full outer shells
These groups show clear patterns in reactivity, boiling points, and states at room temperature.
Questions
- Who developed the early form of the periodic table we use today?
- What key decision did Mendeleev make when placing elements?
- What is the modern periodic table arranged by?
- What does the group number tell you?
- Why do elements in the same group have similar properties?
Summary
- Mendeleev arranged elements by atomic mass and left gaps for undiscovered ones.
- He grouped elements by chemical properties and made successful predictions.
- The modern periodic table is arranged by atomic number.
- Periods show the number of shells; groups show outer electrons.
- Elements in the same group have similar properties.