The modern periodic law and naming new elements

A puzzle in Mendeleev's table

Mendeleev placed tellurium before iodine, although tellurium is heavier. Argon ($39.95$ u) and potassium ($39.10$ u) show the same problem. If atomic mass were the true basis of the table, such pairs would sit in the wrong order. Something more basic than mass had to decide the order.

Moseley's X-rays

In 1913 Henry Moseley, a young English physicist, fired electrons at targets made of different elements. Each element gave out X-rays of its own frequency $\nu$. When he plotted $\sqrt{\nu}$ against the atomic number $Z$, the points fell on a straight line. Against the atomic mass, they did not.

For the strongest X-ray line, $\sqrt{\nu} = a(Z - b)$, with $a$ and $b$ constants. The atomic number, the charge on the nucleus, is the true identity of an element.

Move $Z$ by one step at a time. The square root of the frequency goes up by the same amount each time. That is what a straight line means.

The modern periodic law

The physical and chemical properties of the elements are a periodic function of their atomic numbers.

Ordering by $Z$ removes the old puzzles. Tellurium has $Z = 52$ and iodine $Z = 53$, so tellurium comes first, as Mendeleev had guessed. The atomic number counts the protons, and in a neutral atom it also counts the electrons. The electrons decide the chemistry, which is why $Z$ gives the right order.

The long form of the table

Today's table, from IUPAC, is the long form.

• The horizontal rows are periods. There are 7. They hold $2, 8, 8, 18, 18, 32$ and $32$ elements.
• The vertical columns are groups or families. There are 18, numbered 1 to 18.
• Fourteen elements of period 6 and fourteen of period 7 are placed in two rows at the bottom: the lanthanoids and the actinoids. Keeping them there stops the table from becoming too wide.

Naming elements above 100

When a heavy element is first made, the discovery must be checked before it gets a permanent name. Until then, IUPAC gives a temporary name built from the digits of $Z$.

The rules:

1. Join the roots of the digits in order and add ium.
2. The symbol is the first letter of each root, with the first one capital.
3. The i of bi and tri is dropped before ium: "bium", "trium".
4. The n of enn is dropped before nil: "ennil".

So element $101$ is unnilunium, Unu, now called mendelevium, Md. Element $105$ is unnilpentium, Unp, now dubnium, Db. Element $112$ is ununbium, Uub, now copernicium, Cn.

Practise with new numbers

Try it in Python

The program builds the temporary name and symbol of any element from its digits, with both spelling rules.

Common mistakes

• Plotting against the atomic mass. Moseley's straight line is against $Z$.
• Forgetting the dropped letters. It is "ununbium", not "ununbiium".
• Mixing up groups and periods. Groups go down, periods go across.
• Thinking the lanthanoids are a separate table. They belong in group 3 of period 6, and are only drawn below to save space.

Remember these

• Moseley: $\sqrt{\nu}$ against $Z$ is a straight line. $Z$ is the true basis of the table.
• The modern periodic law: properties are a periodic function of the atomic number.
• The long form: 7 periods, 18 groups, lanthanoids and actinoids drawn below.
• Temporary names: roots nil, un, bi, tri, quad, pent, hex, sept, oct, enn, plus ium.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Argon is heavier than potassium. Why does argon still come first?

2. What is the temporary name of element $114$?

3. How many elements are in period 4, and why that many?

4. In Moseley's graph, what changes by the same amount each time $Z$ goes up by one?

5. Why does an element get only a temporary name when it is first made?