Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
45
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
5
Practice Problems
7. What feature of Bohr’s atomic model accounted for the fact that
electrons can have only certain energies?
8. Describe what is happening when an atom emits a photon.
9. How many electrons can the second principal energy level
hold? How many electrons can the third principal energy level
hold? Explain the difference in these numbers of electrons.
5.3 Electron Configurations
As you will learn, the number and arrangement of electrons around
the nucleus of an atom determines its chemical properties.
Therefore, it is useful to be able to determine and write out the electron arrangement, called an electron configuration, of an atom in
symbolic form.
Writing electron configurations The electron configuration of
an atom is written by stating the number of electrons in each energy
sublevel and writing the sublevels in order of increasing energy. The
number of electrons in an energy sublevel is indicated by a superscript integer. For example, the electron configuration of a hydrogen
atom, which has one electron, is written as 1s 1 . This indicates that
hydrogen’s one electron is in the 1s sublevel.
Helium has two electrons. Recall that an s sublevel consists of a
single orbital that can hold a maximum of two electrons. Therefore,
helium’s second electron fills the one available orbital in the 1s sublevel, and its electron configuration is written as 1s 2 .
A neutral lithium atom has three electrons. The first principal
energy level is filled with two electrons. Where does the third electron go? This electron is found in the second principal energy level,
which, like all energy levels, begins with an s sublevel. So, lithium
has two electrons in the 1s orbital and a third electron in the 2s
orbital, giving it the electron configuration 1s 2 2s 1 . With beryllium,
which has four electrons, the 2s orbital is filled with two electrons,
yielding a configuration of 1s 2 2s 2 .
▲
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
45
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
5
Practice Problems
7. What feature of Bohr’s atomic model accounted for the fact that
electrons can have only certain energies?
8. Describe what is happening when an atom emits a photon.
9. How many electrons can the second principal energy level
hold? How many electrons can the third principal energy level
hold? Explain the difference in these numbers of electrons.
5.3 Electron Configurations
As you will learn, the number and arrangement of electrons around
the nucleus of an atom determines its chemical properties.
Therefore, it is useful to be able to determine and write out the electron arrangement, called an electron configuration, of an atom in
symbolic form.
Writing electron configurations The electron configuration of
an atom is written by stating the number of electrons in each energy
sublevel and writing the sublevels in order of increasing energy. The
number of electrons in an energy sublevel is indicated by a superscript integer. For example, the electron configuration of a hydrogen
atom, which has one electron, is written as 1s 1 . This indicates that
hydrogen’s one electron is in the 1s sublevel.
Helium has two electrons. Recall that an s sublevel consists of a
single orbital that can hold a maximum of two electrons. Therefore,
helium’s second electron fills the one available orbital in the 1s sublevel, and its electron configuration is written as 1s 2 .
A neutral lithium atom has three electrons. The first principal
energy level is filled with two electrons. Where does the third electron go? This electron is found in the second principal energy level,
which, like all energy levels, begins with an s sublevel. So, lithium
has two electrons in the 1s orbital and a third electron in the 2s
orbital, giving it the electron configuration 1s 2 2s 1 . With beryllium,
which has four electrons, the 2s orbital is filled with two electrons,
yielding a configuration of 1s 2 2s 2 .
▲