## The periodic table

Middle of the 19th century: Some 60 elements discovered.

• Mendeleev organizes them by atomic mass.
 Dmitri Mendeleev. Mendeleev's periodic table.

A modern periodic table.

### The order in the periodic table

Atoms are ordered by atomic number.

The electrons in the atoms are ordered in electron shells, so they have the lowest possible energy.

• The closer to the nucleus, the lower the energy.

## Models of some atoms

 Model of a hydrogen atom.The electron configuration of hydrogen Atomic number: 1 Number of p+ in the nucleus: 1 Number of n in the nucleus: 0 Mass number: 1 Number of e– around the nucleus: 1 Model of a helium atom.The electron configuration of helium Atomic number: 2 Number of p+ in the nucleus: 2 Number of n in the nucleus: 2 Mass number: 4 Number of e– around the nucleus: 2 Model of a lithium atom.The electron configuration of lithium Atomic number: 3 Number of p+ in the nucleus: 3 Number of n in the nucleus: 4 Mass number: 7 Number of e– around the nucleus: 3

### How many e– are there room for in the different shells?

 Shell 1 (K) 2 (L) 3 (M) 4 (N) $$n$$ Max. number of e– 2 8 18 32 $$2n^2$$

### Electron configurations for some atoms

 Element Atomic # K L M N Hydrogen, H 1 1 Helium, He 2 2 Litium, Li 3 2 1 Beryllium, Be 4 2 2 Fluorine, F 9 2 7 Neon, Ne 10 2 8 Sodium, Na 11 2 8 1 Sulfur, S 16 2 8 6 Argon, Ar 18 2 8 8 Potassium, K 19 2 8 8 1 Zinc, Zn 30 2 8 18 2 Krypton, Kr 36 2 8 18 8

All the electrons marked in red are valence electrons.

### Example

Give the electron configuration for oxygen, 8O.

#### Solution

Oxygen has 8p+ and 8e

 K L M 8p+ 2e– 6e– 0

## Valence electrons

Valence electrons decide much of an element's properties.

• These properties were what Mendeleev noticed!
• The farther away from the nucleus, the easier they may be released.
• The fewer they are, the easier they may be released.
• Eight valence electrons are particularly stable!

Rule of octets – noble gas configuration

• All the noble gases except helium have eight valence electrons.
• Eight valence electrons (or two, for helium) is called noble gas configuration.
• In chemical reactions: The atoms strive to attain noble gas configuration.