Answer
a. Second period.
b. Noble Gases (Group 8A (18))
c. Group 1A (1)
d. Second period.
Work Step by Step
See Figure 4.1, the Periodic Table of Elements for visual reference.
a. C, N and O are all on the second row of the periodic table. Since rows represent periods, the period that contains these three elements is the second period.
b. Helium (He) is the second element of the first row, and the first element of the 8A group, known as Noble Gases.
c. Alkali metals is a special name for the Group 1A (1).
d. Neon (Ne) is the second element of the Noble Gases (8A/18) group, and the last element of the second row. Thus, the second period ends with neon.
