Page 54 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 3] ATOMS AND ATOMIC MASSES 43

Although the modern designation is seemingly simpler, it does not point up some of the relationships that
the older designations do. In this book, the classical system will be followed, with the modern number sometimes
included in parentheses. The designations are tabulated in Table 3-2. You should check to see which designation
your instructor or text uses, and use that designation yourself.
Table 3-2 Periodic Group Designations

Classical IA IIA IIIB ... VIII ∗ IB IIB IIIA ... VIIA 0
Amended IA IIA IIIA ... VIII ∗ IB IIB IIIB ... VIIB 0

Modern 1 2 3 ... 8910 11 12 13 ... 17 18

∗ Note that three columns are collected as one group (VIII) in the two older designations.

EXAMPLE 3.12. (a) What is the relationship between the numbers of the first eight columns in all three designations?
(b) of the last eight columns?

Ans. (a) They are all the same, 1 through 8. (b) The modern designation is 10 higher for each of these groups except the
last, where it is 18 higher.

Several important groups are given names. Group IA (1) metals (not including hydrogen) are called the
alkali metals. Group IIA (2) elements are known as the alkaline earth metals. Group VIIA (17) elements are
called the halogens. Group IB (11) metals are known as the coinage metals. Group 0 (18) elements are known
as the noble gases. These names lessen the need for using group numbers and thereby lessen the confusion from
the different systems.
The electrons in atoms are arranged in shells. (A more detailed account of electronic structure will be
presented in Chap. 4.) The maximum number of electrons that can occupy any shell n is given by
Maximum number in shell n = 2n 2
Since there are only a few more than 100 electrons total in even the biggest atoms, it can easily be seen that the
shells numbered 5 or higher never get filled with electrons. Another important limitation is that the outermost
shell, called the valence shell, can never have more than eight electrons in it. The number of electrons in the
valence shell is a periodic property.
Shell number 1 2 3 4 5 6 7
Maximum number of electrons 2 8 18 32 50 72 98
Maximum number of outermost shell 2 8 8 8 8 8 8

EXAMPLE 3.13. (a) How many electrons would fit in the first seven shells of an atom if the shells filled to their capacity
in numeric order? (b) Why does this not happen?
Ans. (a) A total of 280 (2 + 8 + 18 + 32 + 50 + 72 + 98) electrons could be held in this fictional atom. (b) Filling one
atom with 280 electrons does not happen because the shells do not fill to capacity before the next ones start to fill
and because even the biggest atom has only a few more than 100 electrons total.

EXAMPLE 3.14. (a) What is the maximum number of electrons in the third shell of an atom in which there are electrons
in the fourth shell? (b) What is the maximum number of electrons in the third shell of an atom in which there are no electrons
in the fourth shell?
Ans. (a) Eighteen is the maximum number in the third shell. (b) Eight is the maximum number if there are no electrons
in higher shells. (There can be no electrons in higher shells if there are none in the fourth shell.)

EXAMPLE 3.15. What is the maximum number of electrons in the first shell when it is the outermost shell? when it is not
the outermost shell?
Ans. The maximum number is 2. It does not matter if it is the outermost shell or not, 2 is the maximum number of electrons
in the first shell.