Radium (Ra)
Radium is the 88th element, a alkaline earth metal in the s-block. Its standard atomic weight is 226.
Radium
226
What the table records
- Atomic number
- 88
- Standard atomic weight
- 226
- Group
- 2
- Period
- 7
- Block
- s-block
- Category
- Alkaline earth metal
- [Rn] 7s2
- Valence electrons
- 2
- Typical valence
- 2
- Van der Waals radius
- 2 Å
- Covalent radius
- 2.2 Å
Molecules containing radium
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.