The Periodic Table — 18 Groups, 7 Periods and Four Trends

A Map, Not a List

There are 118 known elements. Nobody memorises 118 sets of properties — you memorise a map, and then read properties off the position.

The map was built in two stages, and papers ask about both.

• ›Mendeleev's periodic law (1869) — the properties of elements are a periodic function of their atomic masses. He left gaps rather than force a fit, and predicted the missing elements.
• ›Modern periodic law (Moseley, 1913) — the properties of elements are a periodic function of their atomic numbers.

That one word swap, mass to number, is the single most asked fact in the chapter. It also repaired the anomalies Mendeleev could not explain, such as argon (mass 40) sitting before potassium (mass 39): by atomic number, 18 before 19, the order is obviously right.

Once the map is correct, four trends explain almost everything else.

1. The modern periodic law states that the properties of elements are a periodic function of their:

2. Mendeleev arranged the elements in his periodic table primarily in order of increasing:

3. The noble gases are placed in which group of the modern periodic table?

4. The element Mendeleev predicted as 'eka-silicon' was later discovered and named:

5. On moving from left to right across a period, the atomic radius of elements generally:

6. Who demonstrated that atomic number, rather than atomic mass, should be the basis for arranging elements?

7. Which of the following is the most electronegative element?

8. How many periods are there in the modern periodic table?