1. 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.

2. 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.

3. 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.

4. 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.

5. J. J. Thomson’s Cathode Ray
Thomson’s “plum-pudding” model had protons and electrons scattered throughout the atom.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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

13. Solution
Which of the following subatomic particles fits each of the descriptions below?
protons, neutrons, or electrons
A. found outside the nucleus electrons
B. have a positive charge protons
have mass but no charge neutrons