An atom's identity, which element it actually is, comes down to a single number: how many protons sit in its nucleus. The periodic table arranges every known element in order of that proton count, but the table's real power comes from a second layer built on top of that ordering: elements are arranged specifically so that ones with similar outer-electron arrangements, and so similar chemical behaviour, line up in the same vertical column, turning a simple list of atomic numbers into a genuinely predictive map of how each element actually behaves.
An atom's outer electrons, not its total electron count, drive its chemistry
An atom's electrons arrange themselves into distinct energy levels around the nucleus, and it's specifically the outermost electrons, the ones furthest from the nucleus and most available to interact with other atoms, that determine how that element actually behaves chemically, whether it readily forms bonds, how many bonds it typically forms, and with what kind of partner. Two elements can have wildly different total electron counts and still behave remarkably similarly if their outer electron arrangement happens to match, which is exactly the pattern the periodic table is built to expose.
Grouping elements by outer-electron pattern is what makes the table genuinely predictive
Because elements in the same column share a closely matching outer-electron arrangement, they tend to share real chemical properties, similar reactivity, similar typical bonding patterns, similar physical behaviour, which means a chemist can often predict a genuinely unfamiliar element's likely behaviour just from knowing which column it sits in, without having to observe that specific element directly first. This predictive power is exactly why the periodic table has remained chemistry's central organising tool since the nineteenth century: it isn't simply a convenient list, it's a structured map of chemical behaviour built directly out of the underlying pattern of electron arrangement.
What we're still unsure about
That the periodic table's column groupings reliably track shared outer-electron arrangements, and that this makes the table genuinely predictive of chemical behaviour, is extremely well established, confirmed chemistry going back more than a century and a half. What's more genuinely a subtler, ongoing area of study is exactly how well this simple electron-arrangement picture predicts behaviour for the heaviest, most complex elements near the bottom of the table, since relativistic effects on their innermost electrons, moving at meaningful fractions of the speed of light, start to noticeably distort the simple pattern that works cleanly for lighter elements, and chemists continue to study exactly where and how far those distortions push heavy-element behaviour away from what a straightforward reading of their column position would predict.
This sits inside Atomic Structure & the Periodic Table, one of eight topics in General Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.