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The periodic table

Radiation and the atom · level 3, lesson 3 of 6

A question to start

Chemists all over the world use the same chart of the elements, pinned up in almost every science classroom. How is it arranged?

The idea

The periodic table lists all the elements in order of their atomic number: hydrogen is 1, helium 2, lithium 3, and so on. (Level 4 shows what the number counts.)

  • Each element has a symbol of one or two letters: H for hydrogen, O for oxygen, Fe for iron.
  • The table has rows and columns. Elements in the same column behave in similar ways. The far right column holds gases like helium, neon and argon, which hardly ever join with anything.
  • Some symbols come from old names: Fe from the Latin ferrum (iron), Au from aurum (gold).

In 1869, Dmitri Mendeleev arranged the elements known then into a table like this, and left gaps for elements nobody had found yet. When they were found, they fitted his predictions.

HHeLiBeBCNOFNe elements in order: 1, 2, 3 ...gases that rarelyjoin with anything

Worked examples

1. Finding iron. Iron's symbol is Fe, from the Latin ferrum. Its atomic number is 26.

2. A column. Helium, neon and argon are all in the last column. They are all gases that almost never react.

3. A prediction. Mendeleev left a gap below aluminium and described the missing element. It was found in 1875 and named gallium.

Try it

  1. What order are the elements in?
  2. What is the symbol for oxygen?
  3. Why do some symbols not match the English name?
  4. Why was it clever of Mendeleev to leave gaps?
Answers
  1. Order of atomic number, starting with hydrogen, 1.
  2. O.
  3. They come from old names, often Latin, like ferrum for iron.
  4. He predicted elements nobody had found yet, and they were later found to fit.

Common mistake

"The periodic table is just a list." Its rows and columns group elements that behave alike, so it helps predict how an element will behave.

Where it leads