How the periodic table was built, metals and non-metals, and what groups and periods tell you about electrons.
What you will learn
- Explain why Mendeleev left gaps
- Find an element's group and period from its electrons
- Tell metals from non-metals by position
Mendeleev's gamble
Early tables ordered elements by atomic weight, and some ended up in the wrong groups.
Mendeleev swapped some pairs so that elements with similar properties lined up, and left gaps for elements not yet discovered, predicting their properties. When gallium and germanium were found, they fitted.
Ordering by atomic number (protons) and the discovery of isotopes later explained why his swaps were right.
Argon and potassium show why. Argon has a relative atomic mass of 39.9 and potassium 39.1, so ordering by mass would put potassium first, among the unreactive Group 0 gases. But argon has 18 protons and potassium 19: ordering by atomic number puts argon in Group 0 and potassium in Group 1, where their properties fit. Argon is heavier because more of its atoms are heavy isotopes.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Mendeleev's gamble
- Groups and periods
- Try it: Explore the table
9 practice questions, marked as you go; about 15 minutes in all.
Key terms
Atomic number, Period, Group, Noble gases
In this unit: Atomic structure and the periodic table
Atoms, formulae and equations, separating mixtures, the model of the atom, isotopes, electrons in shells and the groups of the periodic table.