Erbium (Er)
Erbium is the 68th element, a lanthanide in the f-block. Its standard atomic weight is 167.26.
Erbium
167.26
What the table records
- Atomic number
- 68
- Standard atomic weight
- 167.26
- Group
- Period
- 6
- Block
- f-block
- Category
- Lanthanide
- [Xe] 4f12 6s2
- Valence electrons
- 12
- Typical valence
- Van der Waals radius
- 2.32 Å
- Covalent radius
- 1.89 Å
Molecules containing erbium
How to read these numbers
The standard atomic weight is an average over the isotopes as they occur naturally, which is why it is rarely a whole number. It is the figure a molar mass calculation uses. Work in mass spectrometry wants the monoisotopic mass of a single isotope instead, which is a different number.
The two radii are not the same measurement. The covalent radius is half the distance between two bonded atoms of the element; the van der Waals radius is how close a non-bonded atom gets before repulsion takes over, and is always the larger. Space-filling models are drawn with the second, ball-and-stick models with the first — which is why the same molecule looks so different in the two.