Why the Periodic Table Has Its Shape
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Elements are ordered by atomic number, and each new period starts a new electron shell.
- Elements in a group have the same number of outer electrons, so they react alike.
- Reactivity changes predictably down a group, as the alkali metals and halogens show.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Mendeleev’s gaps
By the 1860s, chemists knew about sixty elements, but nobody had a good way to sort them. In 1869, a Russian chemist named Dmitri Mendeleev lined them up by their mass. He noticed that similar elements appeared again and again at regular steps. So he put them in columns of look-alikes. He also left gaps where no known element fitted. He said new elements would be found to fill them, and he predicted what they would be like. Gallium was found in 1875 and germanium in 1886, and both matched his predictions. Today the table is ordered by atomic number, the number of protons in an atom. Electrons sit in shells around the nucleus, and each new row starts a new shell. Elements in the same column have the same number of outer electrons, and outer electrons are the ones that react. That is why lithium, sodium and potassium behave alike. The noble gases have full outer shells, so they hardly react at all.
Trends down a group
Elements in a group behave like a family, but they are not all the same. The differences follow a pattern. Group 1 metals show this in a demonstration done by a teacher behind a safety screen. A small piece of lithium in water fizzes steadily. Sodium reacts faster. It melts into a shiny ball that zooms around on the water. Potassium reacts so fast that the gas it makes catches fire with a lilac flame. So the metals get more reactive as you go down the group. Why? Each step down adds a shell of electrons. The single outer electron is farther from the nucleus, so it is held less tightly and lost more easily. Losing it is how these metals react. Group 17, the halogens, does the opposite. They react by gaining an electron, and small atoms grab electrons hardest. So fluorine, at the top, is the most reactive. Patterns like these let chemists predict how an element will behave.
Curriculum links: NGSS HS-PS1-1 · NGSS HS-PS1-2