Chemistry · Periodic Classification Of Properties · NEET
The period number is simply the highest principal quantum number (n) that has electrons. NCERT says: 'the period number corresponds to the highest principal quantum number (n) of the elements in the period.' Look at the largest n in the config. Example: [Ne]3s1 has highest n = 3, so it is in Period 3. For [Rn]5f14 6d10 7s2 7p2 the highest n is 7 (the 7s and 7p), so Period 7. Ignore the fact that 5f and 6d have smaller n numbers - they are inner subshells, not the outermost shell.
For s-block: group = number of valence s-electrons. So ns1 is Group 1, ns2 is Group 2. For p-block: group = 10 + (s electrons + p electrons in the outer shell), OR simply group = 12 + number of p electrons. Example: ns2 np2 has 2+2 = 4 valence electrons, so group = 10 + 4 = 14 (carbon family). ns2 np5 gives 10 + 7 = 17 (halogens). This is why Z=114 with 7s2 7p2 is Group 14.
For d-block: group = number of (n-1)d electrons + number of ns electrons. Example: [Ar]3d3 4s2 has 3 + 2 = 5, so Group 5 (this is Vanadium). [Ar]3d5 4s1 (Cr) gives 5 + 1 = 6, Group 6. The rule adds the inner d electrons because those are the ones being filled in the d-block. Careful: this only works for d-block, not for s- or p-block.
Look at the subshell that received the LAST electron. If the last electron goes into an s-subshell it is s-block (Groups 1, 2). If into p, it is p-block (Groups 13-18). If into (n-1)d, it is d-block (Groups 3-12, transition metals). If into (n-2)f, it is f-block (lanthanoids and actinoids, inner-transition). NCERT: 'the block indicates the value of the azimuthal quantum number (l) for the last subshell that received electrons.'
Because they have the same valence shell electronic configuration - the same number and type of outer electrons. NCERT states: 'Elements having similar outer electronic configurations in their atoms are arranged in vertical columns.' For example every Group 1 element ends in ns1, so they all react the same way (lose one electron easily). Chemistry happens with the outer electrons, so same outer config = same behaviour.
Main group (representative) elements = s-block + p-block, general config ns1 to ns2 np6. Transition elements = d-block, general config (n-1)d 1-10 ns 0-2. So if the last subshell filled is s or p, it is main group; if it is (n-1)d, it is a transition element. Note: for NEET, the NCERT definition of 'main group' is strictly s-block + p-block.
The element Z = 114 has been discovered recently. It will belong to which of the following family/group and electronic configuration?
Which among the following electronic configurations belong to main group elements? A. [Ne]3s1 B. [Ar]3d3 4s2 C. [Kr]4d10 5s2 5p5 D. [Ar]3d10 4s1 E. [Rn]5f0 6d2 7s2
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. NCERT clearly states the period number equals the highest principal quantum number (n) present in the atom. Just find the largest n that has electrons and that is the period.
Because Groups 3 to 12 (the ten d-block columns) sit between Group 2 and Group 13. So a p-block element with 3 valence electrons is not Group 3, it is Group 13. Adding 10 accounts for those ten transition-metal columns.
No. Group and block are both decided by the electronic configuration, so a given valence configuration gives one fixed group in one fixed block. Same outer configuration means same group and same block.
Highest n = 4, so Period 4. Last electron enters (n-1)d, so d-block. Group = d electrons + s electrons = 2 + 2 = 4, so Group 4. This is Titanium, matching NCERT exercise 3.30.
For NEET, yes for position (period, group, block) and for family behaviour. NCERT says all physical and chemical properties are a manifestation of electronic configuration, so once you fix the config you can predict trends like radius, ionisation enthalpy and valence.
