### Learning check

Once you have watched the video, check your learning with this quiz.

## The periodic table

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

• Mendeleev organizes them by atomic mass.

Mendeleev's periodic table.

Dmitri Mendeleev.

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

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 hydrogen 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 helium 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

Model of a lithium atom.

### 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

All the electrons marked in red are valence electrons.

 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

### Example

Give the electron configuration for oxygen, 8O.

## 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.
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