The Structure of an Atom

1.1 The Structure of an Atom

An atom consists of a tiny dense nucleus surrounded by electrons that are spread

throughout a relatively large volume of space around the nucleus. The nucleus contains

positively charged protons and neutral neutrons, so it is positively charged. The

electrons are negatively charged. Because the amount of positive charge on a proton

equals the amount of negative charge on an electron, a neutral atom has an equal number

of protons and electrons. Atoms can gain electrons and thereby become negatively

charged, or they can lose electrons and become positively charged. However, the number

of protons in an atom does not change.

Protons and neutrons have approximately the same mass and are about 1800 times

more massive than an electron. This means that most of the mass of an atom is in its

nucleus. However, most of the volume of an atom is occupied by its electrons, and that

is where our focus will be because it is the electrons that form chemical bonds.

Li Be B C

the second row of the periodic table

N O F

4 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

Louis Victor Pierre Raymond duc

de Broglie (1892–1987) was born in

France and studied history at the

Sorbonne. During World War I, he

was stationed in the Eiffel Tower as a

radio engineer. Intrigued by his exposure

to radio communications, he returned

to school after the war, earned

a Ph.D. in physics, and became a

professor of theoretical physics at the

Faculté des Sciences at the Sorbonne.

He received the Nobel Prize in

physics in 1929, five years after obtaining

his degree, for his work that

showed electrons to have properties

of both particles and waves. In 1945,

he became an adviser to the French

Atomic Energy Commissariat.

Erwin Schrödinger (1887–1961)

was teaching physics at the University

of Berlin when Hitler rose to

power. Although not Jewish,

Schrödinger left Germany to return

to his native Austria, only to see it

taken over later by the Nazis. He

moved to the School for Advanced

Studies in Dublin and then to Oxford

University. In 1933, he shared the

Nobel Prize in physics with Paul

Dirac, a professor of physics at Cambridge

University, for mathematical

work on quantum mechanics.

The atomic number of an atom equals the number of protons in its nucleus. The

atomic number is also the number of electrons that surround the nucleus of a neutral

atom. For example, the atomic number of carbon is 6, which means that a neutral carbon

atom has six protons and six electrons. Because the number of protons in an atom

does not change, the atomic number of a particular element is always the same—all

carbon atoms have an atomic number of 6.

The mass number of an atom is the sum of its protons and neutrons. Not all carbon

atoms have the same mass number, because, even though they all have the same number

of protons, they do not all have the same number of neutrons. For example,

98.89% of naturally occurring carbon atoms have six neutrons—giving them a mass

number of 12—and 1.11% have seven neutrons—giving them a mass number of 13.

These two different kinds of carbon atoms and are called isotopes. Isotopes

have the same atomic number (i.e., the same number of protons), but different mass

numbers because they have different numbers of neutrons. The chemical properties of

isotopes of a given element are nearly identical.

Naturally occurring carbon also contains a trace amount of which has six protons

and eight neutrons. This isotope of carbon is radioactive, decaying with a half-life

of 5730 years. (The half-life is the time it takes for one-half of the nuclei to decay.) As

long as a plant or animal is alive, it takes in as much as it excretes or exhales.

When it dies, it no longer takes in so the in the organism slowly decreases.

Therefore, the age of an organic substance can be determined by its content.

The atomic weight of a naturally occurring element is the average weighted

mass of its atoms. Because an atomic mass unit (amu) is defined as exactly

of the mass of the atomic mass of is 12.0000 amu; the atomic

mass of is 13.0034 amu. Therefore, the atomic weight of carbon is 12.011 amu

The molecular weight is the

sum of the atomic weights of all the atoms in the molecule.

PROBLEM 1

Oxygen has three isotopes with mass numbers of 16, 17, and 18. The atomic number of

oxygen is eight. How many protons and neutrons does each of the isotopes have?

1.2 The Distribution of Electrons in an Atom

Electrons are moving continuously. Like anything that moves, electrons have kinetic

energy, and this energy is what counters the attractive force of the positively charged

protons that would otherwise pull the negatively charged electrons into the nucleus.

For a long time, electrons were perceived to be particles—infinitesimal “planets” orbiting

the nucleus of an atom. In 1924, however, a French physicist named Louis de

Broglie showed that electrons also have wavelike properties. He did this by combining

a formula developed by Einstein that relates mass and energy with a formula developed

by Planck relating frequency and energy. The realization that electrons have

wavelike properties spurred physicists to propose a mathematical concept known as

quantum mechanics.

Quantum mechanics uses the same mathematical equations that describe the wave

motion of a guitar string to characterize the motion of an electron around a nucleus.

The version of quantum mechanics most useful to chemists was proposed by Erwin

Schrödinger in 1926. According to Schrödinger, the behavior of each electron in an

atom or a molecule can be described by a wave equation. The solutions to the

Schrödinger equation are called wave functions or orbitals. They tell us the energy of

the electron and the volume of space around the nucleus where an electron is most

likely to be found.

According to quantum mechanics, the electrons in an atom can be thought of as occupying

a set of concentric shells that surround the nucleus. The first shell is the one

10.9889 * 12.0000 + 0.0111 * 13.0034 = 12.0112.

13C

1>12 12C, 12C

...