Chemistry - A Molecular Science

ten electrons have a 1s

2 2s

2 2p

6 configuration, which is sometimes abbreviated as [Ne],

while the 1s

2 2s

2 2p

6 3s

2 3p

6 configuration of Ar is abbrev

iated as [Ar]. The symbol of the

preceding noble gas in a square bracket is often used to abbreviate electron configurations. K has 19 electrons, so its configuration is 1s

2 2s

2 2p

6 3s

2 3p

6 4s

1 , but it is often written

[Ar]4s

1. Similarly, the configuration of the 34 electrons of selenium is [Ar]4s

2 3d

10

4p

4. To

the right of each sublevel in Figure 2.15 is a list of those elements whose highest energy electrons are in that sublevel. Thus, the elements aluminum (Z = 13) through argon (Z = 18) are filling the 3p sublevel while scandium (Z = 21) through zinc (Z = 30) are filling the 3d sublevel.

2.8

QUANTUM THEORY AND THE PERIODIC TABLE

The periodic table contains four distinct bloc

ks (Figure 2.16): two columns on the left; six

columns on the right; ten columns in the middle; and fourteen columns inserted in the bottom. One of the major accomplishments of

modern quantum theory is that electron

configurations explain this arra

ngement of the periodic table.

Ba

H

He

B

C

N

O

FNe

Al

Si

P

SCl

Ar

Li

Be

Na MgK

Ca Sc

Ti

Cs

YZr

V Nb

Cr Mo

Rb Fr Ra

Sr

La

Hf

Ta W

Ac

Rf

Db

Sg

Mn Fe

Co

Cu

Zn

Ga Ge

As

Se

Br Kr

In Tl

Sn Sb

Te

I

Xe

Pb

Bi

Po At Rn

Tc Ru Rh Pd Ag

Ni

Cd

Re

Os

Ir Pt Au Hg

Bh Hs Mt UunUuu

Uub

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Dy Ho

Tb

Er

Tm

Yb

Lu

Th

Pa U

Np

Pu Am Cm Bk

Cf

Es

Fm Md No Lr

l=0ns

l=2(n-1)d

l=3(n-2)f

l=1np

n^1234567

1

2

1 23 4 56 78 910

12345

6

Figure 2.16 Quantum numbers and the periodic table The number at the top of

each group is the number of electrons in

the highest energy sublevel for that group. Thus, the ‘6’ above Fe indicates that Fe has

six 3d electrons (3d

6 ).

(^) •
The column specifies the
l quantum number of the sublevel of the highest energy electrons in
an atom. The highest energy electrons in elements
in the first two columns are in s sublevels,
those in the last six are in p sublevels, thos
e in the 10 columns in the middle (transition
elements) are in d sublevels, and those in the
14 columns at the bottom are in f sublevels.
(^) •
The period (row) number equals the
principle quantum number of the
outermost shell
(n). The
principle quantum number s and p sublevels equals n,
but it is n-1 for the d sublevels and n-2
for the f sublevels.
Thus, the first two columns (Groups 1A and 2A) are called the
s block
, last 6 columns on
the right (Groups 3A through 8A) comprise the
p block
, the 10 columns in the middle (the
B groups or transition metals) form the
d block
, and the 14 columns at the bottom make
up the
f block

. The position of the element in a block equals the number of electrons that

element has in that sublevel. Example

2.6 shows how to determine the electron

configuration of an atom from its

position in the periodic table.

Example 2.6

Use the periodic table to determine the electron configurations of Ti and Bi. Ti is the second element in the d block of the 4th row. For the d block, n = row - 1 = 4 - 1 = 3, so it has two electrons in the 3d sublevel

. The previous noble gas is Ar, and only the 4s

block (K and Ca) lies between Ar and the 3d sublevel. Thus, Ti is [Ar] 4s

2 3d

2.

Chapter 2 Quantum Theory

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