periodic table groups and periods
The periodic table has 18 groups (vertical columns) and 7 periods (horizontal rows). That grid is not just visual organization: it reflects how electrons fill shells and orbitals, and that is what explains why elements in the same column react in similar ways.
The filter groups by chemical behavior; hydrogen is excluded because, despite sitting in the group 1 column, it is classified as a reactive nonmetal, not an alkali metal.
All six have a full valence shell, the trait that defines group 18; oganesson (Z=118), although in the same column, is marked as unsettled properties, because its chemical behavior has not yet been measured in a laboratory.
The 15 lanthanides occupy period 6, but appear in their own row below the main grid, the same layout solution used for the 15 actinides of period 7.
Atomic number is the number of protons in an atom's nucleus, and it's what defines the element's identity. Every atom with 6 protons is carbon, with 8 protons is oxygen, with 26 protons is iron, and so on. That's why the table is ordered by this number, from 1 (hydrogen) to 118 (oganesson).
It is a layout convention: the 15 lanthanides and 15 actinides belong to periods 6 and 7, but keeping them in the main grid would require widening the table to 32 columns instead of 18. That is why these two series appear in their own rows below the main table, still tied to the period they actually occupy.
In the main groups, 1, 2 and 13 through 18, yes, and that is why they react in similar ways, like the 6 alkali metals of group 1. In groups 3 through 12, the transition metals, filling of the d orbital breaks that regularity, so a direct valence-electron count per group does not hold the same way.
Across a period, nuclear charge grows without adding a shell, so the nucleus pulls the valence electrons in more tightly and electronegativity rises, peaking at fluorine, near 4.0 on the Pauling scale. Going down a group, each new period adds a whole shell, which moves the valence electrons farther from the nucleus and weakens that pull.
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