Download 3 The Atom GOB Structures

4.3 The Atom
An atom is the smallest
particle of an element that
retains the characteristics
of that element.
Aluminum foil contains
atoms of aluminum.
Learning Goal Describe the electrical charge and location in
an atom for a proton, a neutron, and an electron.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Dalton’s Atomic Theory
In Dalton’s atomic theory, atoms
• are tiny particles of matter.
• of an element are similar to each other and different
from those of other elements.
• of two or more different elements combine to form
compounds.
• are rearranged to form new combinations in a
chemical reaction.
Atoms are never created or destroyed during a chemical
reaction.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Electrical Charges in an Atom
Atoms contain the following
subatomic particles:
• protons that have a positive
(+) charge
• electrons that have a negative
(–) charge
• neutrons that have no charge
(neutral)
Like charges repel and unlike charges
attract.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
J. J. Thomson’s Cathode Ray
From his experiment, J. J. Thomson realized that
• cathode rays contain negatively charged particles.
• particles (now called electrons) have a much smaller mass
than the atom.
Thomson proposed a “plum-pudding” model of the atom in
which protons and electrons were randomly distributed in a
positively charged cloud, like plums in a pudding.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
J. J. Thomson’s Cathode Ray
Thomson’s “plum-pudding” model had protons
and electrons scattered throughout the atom.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Rutherford’s Gold Foil
In Rutherford’s gold foil experiment, positively charged
particles
• were aimed at atoms of gold.
• mostly went straight through the atoms.
• were deflected only occasionally.
Rutherford’s experiment concluded that there must be a small,
dense, positively charged nucleus in the atom that deflects
positive particles that come close.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Rutherford’s Gold Foil
(a) Positive particles are aimed at a piece of gold foil. (b) Particles that
come close to the atomic nuclei are deflected from their straight path.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure of the Atom
An atom consists of
• a nucleus, located in the center of the atom, that
contains protons and neutrons and represents most of
the mass of an atom.
• electrons that occupy a large, empty space around the
nucleus.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Structure of the Atom
In an atom, the protons and neutrons that make up almost all the mass are
packed into the tiny volume of the nucleus. The rapidly moving electrons
(negative charge) surround the nucleus and account for the large volume
of the atom.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Mass of the Atom
Because the mass of subatomic particles is so small,
• chemists use a very small unit of mass called the atomic
mass unit (amu).
• 1 amu has a mass equal to 1/12 of the mass of the
carbon-12 atom that contains six protons and six neutrons.
• 1 amu = 1 Dalton (Da) in biology.
• Electrons have such a small mass that they are not
included in the mass of an atom.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Subatomic Particles in the Atom
Subatomic particles have a very small mass, and an
electron has a mass that is much less than that of a
proton or a neutron.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Which of the following subatomic particles fits each of the
descriptions below?
protons, neutrons, or electrons
A. found outside the nucleus
B. have a positive charge
C. have mass but no charge
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Which of the following subatomic particles fits each of the
descriptions below?
protons, neutrons, or electrons
A. found outside the nucleus
B. have a positive charge
C. have mass but no charge
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
electrons
protons
neutrons
© 2016 Pearson Education, Inc.