The Periodic Table Layout and Electronic Structure
The periodic table is organised into blocks based on where an atom's outermost electrons live. Groups (the vertical columns) contain elements with the same number of electrons in their outer shell, which is why they behave similarly.
The s-block includes Groups 1 and 2, where the outermost electrons are in s subshells. The p-block covers Groups 3-8 (though helium is an exception), and the d-block contains the transition metals. Each subshell can hold a maximum number of electrons: s holds 2, p holds 6, and d holds 10.
To work out an element's position, just count its electrons. Calcium has 20 electrons arranged as 2.8.8.2 - since it has 2 electrons in its outer shell, it sits in Group 2. The electronic structure literally determines where each element belongs on the table.
Quick Tip: Remember that periods are horizontal rows and groups are vertical columns - this will save you marks in exams!
