Chapter 4 Lecture

Basic ChemistryFourth Edition

Chapter 4 Atoms and Elements4.3 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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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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The Atom

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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.

Dalton's Atomic Theory

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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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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.

Dalton's Atomic Theory

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Atoms are• the building blocks of everything around us• too small to see with the naked eye

A small sample of nickel contains many, manynickel atoms.

Atoms

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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

Electrical Charges in an Atom

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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

Electrical Charges in an Atom

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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

Structure of an Atom

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

Structure of an Atom

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In 1911, Ernest Rutherford worked with J. J. Thomson and developed a new structure for the atom based on Rutherford’s gold foil experiments.

Structure of an Atom

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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

The Structure of an Atom

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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.

The Structure of an Atom

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In 1932, James Chadwick discovered that the nucleus of the atom also contained neutral particles called neutrons.

The Structure of an Atom

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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On the amu scale, the mass of a proton and a neutron have a mass of about 1 amu.

Mass of an Atom

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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.

Learning Check

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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

Solution