The idea
Mendeleev arranged the elements by mass and noticed properties repeating. The modern table is arranged by atomic number, and the repetition turns out to come from electron shells.
What position tells you
- Period (row) = number of occupied electron shells
- Group (column) = number of valence electrons, for the main groups
- Elements in a group behave alike because they have the same number of outer electrons
| Group | Name | Valence electrons | Behaviour |
|---|---|---|---|
| 1 | Alkali metals | 1 | Extremely reactive; react with water |
| 2 | Alkaline earth metals | 2 | Reactive, less so than group 1 |
| 17 | Halogens | 7 | Extremely reactive non-metals |
| 18 | Noble gases | 8 (full) | Almost entirely unreactive |
A prediction that made the table famous
Mendeleev left gaps, and predicted the properties of elements nobody had found. When gallium was isolated in 1875 and matched his predicted density and melting point, the table stopped being a filing system and became a theory.
Trends worth knowing
- Atomic radius grows down a group (more shells), shrinks across a period (stronger nuclear pull on the same shell).
- Reactivity increases down group 1, and up group 17.
Investigate these yourself in Finding Patterns in the Periodic Table.
Curriculum connection
C2.4
explain the relationship between the position of an element in the periodic table and the structure of its atoms, using models
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C2.5
investigate the physical and chemical properties of elements, and use their findings to relate these properties to the organization of the periodic table, classify elements, and identify patterns in the periodic table
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