The periodic table's main group elements, made up of its s-block and p-block columns, include the elements most people would actually recognise from everyday chemistry: hydrogen, oxygen, carbon, nitrogen, the alkali and alkaline earth metals, the halogens, the noble gases. Unlike the transition metals sitting in the table's central block, whose properties vary in more complicated, less immediately predictable ways, main group elements show strikingly clear, consistent trends in their properties as you move across a row or down a column of the periodic table.
Moving across a row steadily changes an element's electron-attracting pull
Moving left to right across a row of main group elements, electronegativity, an element's tendency to attract shared electrons toward itself in a chemical bond, generally increases steadily, alongside a corresponding steady increase in ionisation energy, the energy needed to remove an electron from the atom. This pattern tracks the steadily increasing number of protons in the nucleus across a row, which pulls more strongly on the atom's outer electrons even as the number of electron shells stays the same, giving main group elements within a single row a genuinely predictable, gradually shifting chemical character from the row's metallic left side to its non-metallic right side.
Moving down a column changes an element's size and how loosely it holds electrons
Moving down a single column, atomic radius generally increases, since each successive element down the column has an additional electron shell, and that added distance from the nucleus generally makes outer electrons easier to remove, so ionisation energy generally decreases moving down a column, the reverse trend from moving across a row. These two trends, crossing a row and descending a column, together give the main group elements their characteristic, strikingly predictable grid of chemical behaviour, letting a chemist reasonably anticipate an unfamiliar main group element's rough chemical character just from its position on the table.
What we're still unsure about
The basic periodic trends in electronegativity, ionisation energy and atomic radius across main group elements are precisely measured, well established, foundational inorganic chemistry. What's more genuinely a matter of ongoing refinement is predicting exactly how these trends interact in more complex real compounds involving several main group elements at once, since a compound's actual observed properties often depend on a more intricate combination of these individual atomic trends than a simple row-and-column rule alone fully captures — computational and experimental inorganic chemists continue refining more detailed models for these more complicated multi-element cases, without every such prediction being fully solved from first principles alone.
This sits inside Main Group Chemistry, one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.