The arrangement of elements in the periodic table contains lots of clues about the characteristics of those elements, their families, and their relation to each other. The table is like a multi-facetted gemstone and the more you study it, the more you discover. It’s hard to think of a more meaningfully arranged two-dimensional table (although in some ways it’s three-dimensional). For any given element in the table, its location – column and row – tells you a lot about it, as do its neighbors – above, below, left side, right side.
A good place to start when introducing students to the periodic table is with the number that appears above each symbol. That number is three things simultaneously. I like to use the acronym A.P.E: Atomic number, number of Protons, number of Electrons. Take selenium (Se), element 34. The fact that 34 is its atomic number tells us that selenium has 34 protons in its nucleus, and 34 electrons orbiting that nucleus. This is a pretty good starting point in visualizing the structure of selenium – what we’d see if we had a really powerful microscope, one that could also snap a photo and freeze the electrons in motion.
The element to the left of selenium has one less proton and one less electron. And the element to the right of selenium has one more proton and one more electron. Simple as that.
The periodic table can get complicated, and there are many exceptions to its rules. So it’s important to start with those facts and trends which can be trusted to give consistently accurate and revealing information.