Q4. Iodine is [Kr] 4d10 5s2 5p5. Find its atomic number, period and group.
Z = 36 + 10 + 2 + 5 = 53. Highest n = 5, so period 5; seven electrons in 5s and 5p, one short of xenon, so group 17.
Q5. The atomic numbers of the alkali metals are 3, 11, 19, 37, 55 and 87. Explain the gaps between them.
The gaps, 8, 8, 18, 18 and 32, are the lengths of periods 2 to 6. Each alkali metal starts a new period, so the next one comes after all the elements of that period.
Q6. In which period does the first d-block element appear, and why not earlier?
In period 4 (scandium, Z = 21). The first d orbitals are 3d. Neutral K and Ca occupy 4s first, and scandium then begins the 3d series; this does not mean 4s is lower in energy for every atom or ion.
Q7. Write the outer electronic configuration common to all the noble gases except helium, and give helium’s.
ns2 np6 (a full outer s and p subshell); helium is 1s2, also a full outer shell. Helium’s 1s2 matches group 2 (ns2), yet it is placed in group 18 because its valence shell is full and it behaves as a noble gas (NCERT page 82).
Q8. Locate the element with Z = 50.
50 − 36 = 14 electrons beyond krypton: 5s2 4d10 5p2. Period 5. In periods 4 and 5 the group number is Z minus the previous noble gas’s Z, so group 14 (tin).