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 |
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| Helium, He | 2 | 2 |
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| Litium, Li | 3 | 2 | 1 |
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| Beryllium, Be | 4 | 2 | 2 |
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| Fluorine, F | 9 | 2 | 7 |
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| Neon, Ne | 10 | 2 | 8 |
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| Sodium, Na | 11 | 2 | 8 | 1 |
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| Sulfur, S | 16 | 2 | 8 | 6 |
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| Argon, Ar | 18 | 2 | 8 | 8 |
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| 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.
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.
