Q4. An element has the outer configuration 3d5 4s1. Name it and give its period and group.
Chromium, Z = 24 (NCERT Table 4.1, page 90). Outer shell n = 4, so period 4; group = 5 + 1 = 6, in the 3d series (reasoned).
Q5. How many elements are in the d-block of periods 4 to 7, and how many of them are transition elements by NCERT’s definition?
Four series of ten: 40 d-block elements in NCERT’s layout. Its textbook classification excludes the four group-12 members, zinc, cadmium, mercury and copernicium, (n − 1)d10 ns2, giving 40 − 4 = 36 by that convention (NCERT page 91). For the heaviest synthetic elements these classifications rely partly on predicted configurations and analogy; they do not establish a complete experimental catalogue of ordinary ions.
Q6. Assertion: the 3d series lies in period 4. Reason: the 4s orbital fills before the 3d orbitals. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion. The period is set by the outermost shell: 4s, lower in energy, fills first in potassium and calcium, so by the time the 3d orbitals fill, the atoms already have n = 4 as their outer shell (NCERT Class 11 Chemistry, page 64; reasoned).
Q7. Why is the d-block ten columns wide while the f-block has fourteen?
A d subshell has five orbitals and holds 5 × 2 = 10 electrons; an f subshell has seven orbitals and holds 7 × 2 = 14. One element is added for each electron, so each d series has ten members and each f series fourteen (reasoned; NCERT page 89).
Q8. Gold, Z = 79, is [Xe] 4f14 5d10 6s1. Give its position. Is it a transition element, although its atom has a full 5d subshell?
Period 6, group 11 (10 + 1), in the 5d series. Yes. Gold(III) is a common oxidation state of gold, and Au(III) is a d8 ion (Wikipedia, “Gold”): removing the 6s electron and two 5d electrons leaves an incomplete 5d subshell. NCERT’s definition counts an incomplete d subshell in the atom or in its ions (NCERT page 89; reasoned, as for silver in NCERT Intext Question 4.1, page 92).