ఆవర్తన పట్టిక లో మొత్తం ఎన్ని మూలకాలు ఉన్నాయి?
ఆవర్తన పట్టిక
Hydrogen and helium are often placed in different places than their electron configurations would indicate; hydrogen is usually placed above lithium, in accordance with its electron configuration, but is sometimes placed above fluorine,[110] or even carbon,[110] as it also behaves somewhat similarly to them. Hydrogen is also sometimes placed in its own group, as it does not behave similarly enough to any element to be placed in a group with another.[111] Helium is almost always placed above neon, as they are very similar chemically, although it is occasionally placed above beryllium on account of having a comparable electron shell configuration (helium: 1s2; beryllium: [He] 2s2).[21]