From Triads to the Modern Table: Quick Revision

A brief note on From Triads to the Modern Table. Read it before a test. Try each question at the end before you open its answer.

The early attempts

Mendeleev's table

• Ordered by atomic mass, with groups of similar properties, tested by the formulas of oxides and hydrides.
• He left gaps and predicted eka-boron, eka-aluminium and eka-silicon: scandium, gallium and germanium.
• Limits: hydrogen had no fixed place; isotopes would need separate places; a few pairs, such as cobalt and nickel, were out of order by mass.

The modern table

• Ordered by atomic number, the number of protons, as Moseley showed.
• $18$ groups and $7$ periods. The period is the number of shells.
• Groups 1 and 2: group $=$ outer electrons. Groups 13 to 18: group $= 10 +$ outer electrons.

A worked example

Find the period and the group of the element with atomic number $10$.

1. Shells. $10 = 2 + 8$: the arrangement is 2, 8.
2. Period. Two shells: period $2$.
3. Group. Eight outer electrons, a full shell: group $10 + 8 = 18$. It is neon.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Saying Mendeleev used atomic numbers. He used atomic masses; atomic numbers came with Moseley.
• Calling the gaps a weakness. They were his strongest move.
• Counting the group from the outer electrons in groups 13 to 18 without adding 10.
• Thinking helium belongs to group 2. Its full shell puts it in group 18.

Check yourself

1. What did Mendeleev predict in the gap below silicon?

2. How many periods does the modern table have?

3. Why does the modern table have no trouble with isotopes?

4. An element is 2, 8, 5. Where is it?