Page 74 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 4] ELECTRONIC CONFIGURATION OF THE ATOM 63

4.9. What is the maximum number of electrons that can occupy the first shell? The second shell?
Ans. The first shell can hold a maximum of two electrons. The second shell can hold a maximum of eight electrons.

4.10. What is the maximum number of electrons that can occupy the third shell?
Ans. n = 3; hence, the maximum number of electrons the shell can hold is
2
2
2n = 2(3) = 2 × 9 = 18
4.11. What is the maximum number of electrons that can occupy the third shell before the start of the fourth
shell?
Ans. The maximum number of electrons in any outermost shell (except the first shell) is eight. The fourth shell
starts before the d subshell of the third shell starts.

QUANTUM NUMBERS AND ENERGIES OF ELECTRONS
4.12. Arrange the electrons in the following list in order of increasing energy, lowest first:

n l m l m s n l m l m s

(a) 4 2 −1 1 (c) 4 1 1 − 1
2 2
(b) 5 0 0 − 1 (d) 4 1 −1 1
2 2

Ans. Electrons (c) and (d ) have the lowest value of n + l (4 + 1 = 5) and the lowest n, and so they are tied for
lowest in energy of the four electrons. Electron (b) also has an equal sum of n + l (5 + 0 = 5), but its n
value is higher than those of electrons (c) and (d). Electron (b) is therefore next in energy (despite the fact
that it has the highest n value). Electron (a) has the highest sum of n + l (4 + 2 = 6) and is highest in
energy.

4.13. If the n and l quantum numbers are the ones affecting the energy of the electron, why are the m l and m s
quantum numbers important?
Ans. Their permitted values tell us how many electrons there can be with that given energy. That is, they tell us
how many electrons are allowed in a given subshell.

4.14. How many electrons are permitted in the fifth shell? Explain why no atom in its ground state has that
many electrons in that shell.
2
Ans. For n = 5, there could be as many 2(5) = 50 electrons. However, there are only a few more than 100
electrons in even the biggest atom. By the time you put 2 electrons in the first shell, 8 in the second, 18 in
the third, and 32 in the fourth, you have already accounted for 60 electrons. Moreover, the fifth shell cannot
completely fill until several overlying shells start to fill. There are just not that many electrons in any actual
atom.

SHELLS, SUBSHELLS, AND ORBITALS
4.15. What is the difference between (a) the 2s subshell and a 2s orbital and (b) the 2p subshell and a 2p
orbital?

Ans. (a) Since the s subshell contains only one orbital, the 2s orbital is the 2s subshell. (b) The 2p subshell
contains three 2p orbitals—known as 2p x , 2p y , and 2p z .