Electron shells and periods
An element’s period gives a first indication of the number of occupied electron shells in the simple model.
Start with the idea
An element’s period gives a first indication of the number of occupied electron shells in the simple model.
A concrete example
Lithium and sodium are in the same group, but sodium is one period lower and has one additional occupied shell.
Mendeleev’s tip
Do not start with a formula. Identify what changes, what stays constant and which quantity you are trying to explain or calculate.
Go one step deeper
For transition elements, talking only about shells quickly becomes insufficient. Subshells and orbitals become essential.
A trend or model is not an exceptionless law. Always check the quantity’s definition and the conditions being compared.
Check your understanding
Can you explain the phenomenon in your own words before looking at an equation?