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Dmitri Mendeleev

  • Middle of the 19th century: Some 60 elements discovered.
  • Mendeleev organizes them by atomic mass.

An early periodic table.An early periodic table.

Dmitri Mendeleev.Dmitri Mendeleev.

Groups and periods

In a modern periodic table, the atoms are ordered by atomic number.

The groups of the periodic table are the vertical ↓ columns. The atoms in the same group contain the same number of valence electrons.

The periodic table.The periodic table.

The periods of the periodic table are the horizontal → rows. The atoms in the same period contain the same number of electron shells.

Trends in the periodic table

Trends in the periodic table. The arrows show an increase in the specified character.Trends in the periodic table. The arrows show an increase in the specified character.

Metals are generally to the left of the periodic table, nonmetals to the right.

The size (atomic radius) decreases as we move to the right of the periodic table

  • Increased nuclear charge ⇒ electrons tighter around the nucleus.

Reactivity increases downward in groups 1 and 2 because of lower ionization energy.

Reactivity increases upward in groups 16 and 17 because of higher electron affinity.

To learn by heart

Name Group Number of valence electrons
Alkali metals 1 1
Alkaline earth metals 2 2
Boron group 13 3
Carbon group 14 4
Nitrogen group 15 5
Oxygen group 16 6
Halogens 17 7
Noble gases 18 8 (He: 2) 
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