Building the Periodic Table: Quick Revision

A brief note on Building the Periodic Table. Read it before a test. Try each question at the end before you open its answer.

From triads to Mendeleev

• Dobereiner (1829): triads, the middle mass near the mean of the other two.
• Newlands (1865): the law of octaves, every eighth element alike, true only up to calcium.
• Lothar Meyer: physical properties such as the atomic volume rise and fall in waves with the atomic mass.
• Mendeleev (1869): properties are a periodic function of atomic mass. He trusted properties over mass order, left gaps, and predicted eka-aluminium (gallium) and eka-silicon (germanium).

The modern periodic law

• Moseley (1913): $\sqrt{\nu}$ of X-rays is a straight line against the atomic number $Z$.
• Properties are a periodic function of $Z$. The order of tellurium and iodine, or argon and potassium, then comes out right.
• The long form: 7 periods ($2, 8, 8, 18, 18, 32, 32$ elements) and 18 groups.
• Temporary names: nil, un, bi, tri, quad, pent, hex, sept, oct, enn, plus ium. Drop the i of bi and tri before ium, and the n of enn before nil.

Blocks

• Period = the principal quantum number of the outermost shell.
• s-block, groups 1 and 2: $ns^{1 \text{ or } 2}$. Helium is s-block but sits in group 18.
• p-block, groups 13 to 18: group $= 10$ + outer s and p electrons.
• d-block, groups 3 to 12: group = d electrons + outer s electrons. Transition metals.
• f-block: lanthanoids (4f, period 6) and actinoids (5f, period 7).
• Metals on the left and middle, non-metals at the top right, metalloids on the zigzag.

A worked example

Where is the element with $Z = 38$?

1. Configuration. After krypton (36), two electrons go into $5s$: $[\text{Kr}]\,5s^2$.
2. Period. The outer shell is $n = 5$: period 5.
3. Block and group. The last electron enters an s subshell, with 2 s electrons: s-block, group 2. It is strontium.

Practice with new numbers

A program to try

The program reads the period and the block from a few configurations written with a noble gas core.

Mistakes to avoid

• Ordering by mass. The modern table follows the atomic number.
• Group 15 or group 5? In the p-block add 10 to the outer s and p electrons.
• Forgetting the outer s electrons in the d-block. $3d^6\,4s^2$ is group 8.
• Calling helium a p-block element. It is $1s^2$.

Check yourself

1. Which two elements did Mendeleev's gaps predict that were found within twenty years?

2. What is the temporary name of element $107$?

3. An element is $[\text{Ne}]\,3s^2\,3p^1$. Give its period, group and block.

4. Why do periods 4 and 5 hold 18 elements each?