dorbital block, so we know that the 3dorbitals are completely filled. The electron configura-
tion of Ge is [Ar] 4s^23 d^104 p^2 or [Ar] 3d^104 s^24 p^2.
(c) Molybdenum, Mo, is in Group VIB, with the general configuration d^5 s^1 ; it is in Period 5,
which begins with 5sand is beyond the noble gas krypton. The electron configuration of Mo is
[Kr] 5s^14 d^5 or [Kr] 4d^55 s^1. The electron configuration of molybdenum is analogous to that
of chromium, Cr, the element just above it. The configuration of Cr was discussed in Section
5-17 as one of the exceptions to the Aufbau order of filling.
You should now work Exercise 106.
EXAMPLE 5-11 Unpaired Electrons
Determine the number of unpaired electrons in an atom of tellurium, Te.
Plan
Te is in Group VIA in the periodic table, which tells us that its configuration is s^2 p^4. All other
shells are completely filled, so they contain only paired electrons. We need only to find out
how many unpaired electrons are represented by s^2 p^4.
Solution
The notation s^2 p^4 is a short representation for shgphghh. This shows that an atom of Te
contains two unpaired electrons.
You should now work Exercises 100 and 102.
The periodic table has been described as “the chemist’s best friend.” Chemical reac-
tions involve loss, gain, or sharing of electrons. In this chapter, we have seen that the
fundamental basis of the periodic table is that it reflects similarities and trends in electron
configurations. It is easy to use the periodic table to determine many important aspects
of electron configurations of atoms. Practice until you can use the periodic table with
confidence to answer many questions about electron configurations. As we continue our
study, we will learn many other useful ways to interpret the periodic table. We should
always keep in mind that the many trends in chemical and physical properties that we cor-
relate with the periodic table are ultimately based on the trends in electron configurations.
Key Terms 225Key Terms
Absorption spectrum The spectrum associated with absorption
of electromagnetic radiation by atoms (or other species) result-
ing from transitions from lower to higher energy states.
Alpha () particle A helium ion with a 2charge; an assembly
of two protons and two neutrons.
amu See Atomic mass unit.
Angular momentum quantum number () The quantum
mechanical solution to a wave equation that designates the sub-
shell, or set of orbitals (s, p, d, f), within a given main shell in
which an electron resides.
Anode In a cathode-ray tube, the positive electrode.
Atomic mass unit An arbitrary mass unit defined to be exactly
one-twelfth the mass of the carbon-12 isotope.
Atomic number The integral number of protons in the nucleus;
defines the identity of an element.
Atomic orbital The region or volume in space in which the
probability of finding electrons is highest.
Aufbau (“building up”) Principle A guide for predicting the
order in which electrons fill subshells and shells in atoms.
Balmer-Rydberg equation An empirical equation that re-
lates wavelengths in the hydrogen emission spectrum to
integers.
Canal ray A stream of positively charged particles (cations) that
moves toward the negative electrode in a cathode-ray tube;
observed to pass through canals in the negative electrode.
Cathode In a cathode-ray tube, the negative electrode.