Electronegativity, how strongly an atom pulls on shared electrons in a chemical bond, and ionisation energy, how much energy it takes to strip an electron away from an atom entirely, are two genuinely different chemical properties, but they follow the exact same trend across the periodic table: both increase moving left to right across a period, and both decrease moving down a group. That shared pattern isn't a coincidence; it comes from a single underlying cause, how tightly an atom's nucleus holds onto its outermost electrons at whatever distance those electrons happen to sit.
Moving across a period packs more nuclear charge into roughly the same electron shell
Moving left to right across a period, each successive element adds one more proton to the nucleus while its outermost electrons stay in roughly the same energy shell, meaning that growing positive nuclear charge pulls on those outer electrons from essentially the same distance. A more strongly charged nucleus holding electrons at a similar distance pulls harder on them, which is exactly why both electronegativity, the pull on shared bonding electrons, and ionisation energy, the energy needed to remove an electron entirely, both climb steadily as you move across a period.
Moving down a group pushes the outer electrons further from that same pull
Moving down a group instead adds a whole new electron shell further from the nucleus, and those additional inner shells actually shield much of the nucleus's charge from the outermost electrons, weakening the effective pull they feel even though the nucleus itself carries considerably more positive charge. That extra distance and shielding is exactly why both electronegativity and ionisation energy fall as you move down a group: the outermost electrons simply sit too far away, and too well shielded, for the growing nuclear charge underneath to hold onto them as tightly.
What we're still unsure about
That electronegativity and ionisation energy both track the same underlying nuclear-attraction pattern across the periodic table is well established, thoroughly confirmed inorganic chemistry taught consistently for well over a century. What's more genuinely a matter of ongoing subtlety is exactly how cleanly these trends hold at certain specific points on the table, since electron configuration quirks around fully or half-filled subshells create real, well-documented small exceptions to the otherwise smooth trend, and chemists continue to treat these exceptions as genuinely instructive rather than simply memorising past them, since they reveal real detail about electron shell structure the smooth overall trend alone doesn't capture.
This sits inside Periodic Trends (Electronegativity, Ionisation Energy), one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.