Grade 9

The Periodic Table

Why the elements fall into a table at all, and how a position in that table tells you how a substance will behave.

6 lessons

  1. 1What did Mendeleev actually notice when he arranged the elements?He arranged the known elements by atomic mass and saw their properties repeat at regular intervals — a similar element turning up again after a fixed number of steps. That is what makes a property periodic. He trusted the pattern enough to leave gaps in the table for elements nobody had found, and described their properties in advance.30m
  2. 2What does the atomic number count, and why does it fix an element's identity?The atomic number counts the protons in the nucleus, and nothing else. The number of protons is what decides which element it is: an atom with six protons is carbon, however many neutrons it carries. When the table was ordered by proton number instead of by mass, exactly the places where the mass order looked wrong were put right.35m
  3. 3Why does the periodic table have exactly the rows it has?Because electrons fill energy shells in a fixed order and each shell holds a maximum. The first holds two electrons and the second and third hold eight each. The moment a shell is full the next electron starts a new shell — and in the table, a new row begins. The rows are not a decision made by whoever drew it.35m
  4. 4Why do elements in the same group behave the same way chemically?Because they have the same number of valence electrons, and it is the outer electrons that take part in reactions. Lithium, sodium and potassium each have one outer electron, and all of them react with water and release hydrogen. A group is not an arbitrary grouping — it is a set of atoms that look the same from the outside.35m
  5. 5What changes as you move across a single row of the periodic table?Everything, and quickly. A row opens with soft metals that give electrons away easily, passes through metalloids in the middle, and reaches brittle non-metals that take electrons in. It ends at a noble gas that does not react. Across that row the proton count rises, so the atom gets smaller, not larger.35m
  6. 6How do you predict a compound's formula from two positions in the table?Check whether each element gives electrons away or takes them in. A metal with a non-metal makes an ionic bond: the metal gives and the non-metal takes. Two non-metals share electrons in a covalent bond. The group number gives you the charge on each ion, and then you balance the charges to zero — that is the formula.40m