Chemistry · Periodic Classification Of Properties · NEET
A period is a horizontal ROW (going left to right). A group is a vertical COLUMN (going top to bottom). There are 7 periods and 18 groups. Elements in the same group have similar chemical properties because they have the same number of valence (outer) electrons. Elements in the same period do NOT have similar properties, but their period number equals their highest shell number (principal quantum number n).
It has 7 rows (periods) and 18 columns (groups). This is why it is also called the 18-column periodic table. Mendeleev's old table had only 8 columns; the long form spreads them out into 18 so that each block (s, p, d, f) is clearly separated.
Each new period starts filling a new outer shell. In period 1, the outermost electrons go into the n=1 shell (1s). In period 2, they go into n=2 (2s, 2p), and so on. So the period number tells you the highest value of n (the valence shell number) for elements in that period. Example: sodium (Na) is in period 3, so its valence shell is n=3.
The 14 lanthanoids (Ce to Lu) and 14 actinoids (Th to Lr) are the f-block elements. If they were inserted into the main body, the table would become 32 columns wide and very hard to print and read. To keep the table compact AND keep elements with similar properties in the same column, these two rows are pulled out and shown separately below. They actually belong in period 6 and period 7.
Period 1 has 2 elements, periods 2 and 3 have 8 each, periods 4 and 5 have 18 each, and period 6 has 32. Period 7 is theoretically also 32 but is now complete up to element 118. The rule: number of elements in a period = twice the number of orbitals being filled in that energy level.
The long form table arranges elements by increasing atomic number (not atomic mass), which fixes the anomalous pairs in Mendeleev's table. It also separates elements into clear s, p, d, and f blocks based on which orbital the last electron enters. This makes it easier to predict electronic configuration just from an element's position.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The long form table is based on the Modern Periodic Law given after Henry Moseley (1913) showed that atomic number, not atomic mass, is the fundamental property. It is the improved, widely used version of the table developed after Mendeleev and Lothar Meyer.
There are 18 groups (vertical columns) and 7 periods (horizontal rows).
The s-block (groups 1 and 2), the p-block (groups 13 to 18), the d-block (groups 3 to 12), and the f-block (lanthanoids and actinoids shown at the bottom). Each block is named after the orbital that receives the last electron.
Position in the table lets you quickly predict an element's electronic configuration, valence electrons, block, and its trends in size, ionization enthalpy and electronegativity. NEET regularly asks you to place a given element (by atomic number) into the correct group and block.
