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Chapter 4 Lecture
Basic Chemistry
Fourth Edition
Chapter 4
Atoms and Elements
4.3 The Atom
Learning Goal Describe the electrical
charge and location in an atom for a
proton, a neutron and an electron.
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The Atom
All elements listed on the periodic table are
made up of atoms. An atom is the smallest
particle of an element.
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Dalton's Atomic Theory
The idea of atoms did not become scientific
theory until 1808.
John Dalton (1766–1844) developed an atomic
theory proposing that atoms were responsible for
the combinations of elements in compounds.
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Dalton's Atomic Theory
1. All matter is made up of tiny particles
called atoms.
2. All atoms of a given element are identical to
one another and different from atoms of other
elements.
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Dalton's Atomic Theory
3. Atoms of two or more different elements
combine to form compounds. A particular
compound is always made up of the same
kinds of atoms and the same number of each
kind of atom.
4. A chemical reaction involves the
rearrangement, separation, or combination of
atoms. Atoms are never created or destroyed
in a chemical reaction.
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Atoms
Atoms are
• the building blocks of everything around us
• too small to see with the naked eye
A small sample of nickel
contains many, many
nickel atoms.
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Electrical Charges in an Atom
By the end of the 1880s,
• experiments with electricity showed that atoms
were composed of tiny particles, called
subatomic particles including protons,
neutrons, and electrons
• it was shown that some subatomic particles in
an atom have charge
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Electrical Charges in an Atom
Electrical charges can be
positive or negative as when
• two positive charges
repel each other
• two negative charges
repel each other
• unlike charges attract
each other
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Structure of an Atom
In 1897, J. J. Thomson
• discovered that cathode rays were streams of
small negatively charged particles called
electrons
• proposed the “plum pudding” model of an
atom in which protons and electrons were
distributed throughout the atom
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Structure of an Atom
J. J. Thomson’s “plum pudding” model had protons
and electrons scattered throughout the atom.
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Structure of an Atom
In 1911, Ernest Rutherford worked with
J. J. Thomson and developed a new structure
for the atom based on Rutherford’s gold foil
experiments.
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The Structure of an Atom
From the gold foil experiments, Rutherford
realized that atoms contained
• a small region in the center with positive
charge called the nucleus
• a region of space around the center of the
atoms occupied by electrons
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The Structure of an Atom
The positively charged particles are called protons,
and the negatively charged particles are called
electrons.
If an atom were the size of a football stadium, the
size of the nucleus would be the size of a golf ball
in the center of the field.
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The Structure of an Atom
In 1932, James Chadwick discovered that the
nucleus of the atom also contained neutral particles
called neutrons.
In an atom, the protons and neutrons that make up almost all the mass of the atom 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.
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Mass of the Atom
• The mass of the atom is due to the protons and
neutrons in the nucleus. Electrons have a much
smaller mass.
• Chemists use a unit called atomic mass unit
(amu), defined as one-twelfth of the mass of
the carbon atom with 6 protons and 6 neutrons.
• The mass of all elements in the periodic table is
compared to the mass of this carbon atom.
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Mass of an Atom
On the amu scale, the mass of a proton and a
neutron have a mass of about 1 amu.
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Learning Check
Is each of the following statements true or false?
A. The mass of an electron is greater than the mass
of a proton.
B. Protons have a positive charge, and electrons
have a negative charge.
C. The nucleus of an atom contains only the
protons and neutrons.
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Solution
Is each of the following statements true or false?
A. The mass of an electron is greater than the mass
of a proton.
False
B. Protons have a positive charge, and electrons
have a negative charge.
True
C. The nucleus of an atom contains only the
protons and neutrons.
True
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