9.Development of the Periodic Table

1. Mendeleev’s Periodic Table

• Dmitri Mendeleev: A Russian chemist who organized known elements in the mid-19th century.

• Predicting Elements: His table was so well-structured that it even predicted the properties of elements that hadn't been discovered yet.

• Organization: The table is read from left to right and top to bottom, arranged in order of increasing atomic number (number of protons).

2. Groups and Periods

• Periods: These are the horizontal rows. A new row is formed every time the pattern of chemical properties repeats.

• Groups: These are the vertical columns.

  • Elements in the same group have similar chemical properties.

  • This is because they all have the same number of electrons in their outer shell.

  • The group number (1 to 7) tells you exactly how many electrons are in the outer shell (e.g., Group 2 elements have 2 outer electrons).

3. Key Groups and Regions

• Group 1 (Alkali Metals): Highly reactive; they become more reactive as you go down the group. They all have 1 electron in their outer shell.

• Group 7 (Halogens): All have 7 electrons in their outer shell. They become less reactive as you go down the group.

• Group 0 (Noble Gases): Found on the far right. They have full outer shells, making them very stable and unreactive.

• Transition Metals: A large block of metals located between Group 2 and Group 3.

• Metals vs. Non-metals: A "zigzag" line (from Aluminium to Polonium) separates metals (on the left) from non-metals (on the right). Most elements are metals.

4. Reading the Symbols

Within each box of the periodic table, you will find:

• Elemental Symbol: The one or two-letter code (e.g., Na for Sodium).

• Atomic Number: The smaller number, representing the number of protons.

• Mass Number: The larger number, representing the total number of protons and neutrons.