Modern Periodic Table — MHT-CET Chemistry

An element's period is the highest shell number in its configuration, its block is the subshell that receives the last electron, and its group follows from how many electrons sit in the outer s, p and (n−1)d subshells.

Across a period the nuclear charge grows and the shell stays the same, so atoms shrink and hold their electrons harder; down a group a new shell is added, so atoms grow and hold them less — every trend in the chapter follows from those two facts.