Book Cover Chemistry 8th Edition / Chang
Student Study Guide

Chapter 2: Atoms, Molecules, and Ions

Index | 2.1 | 2.2 2.3 | 2.4 | 2.5 | 2.6 | 2.7 |



  1. Name and describe the three subatomic particles of most importance to chemistry and give their location within the atom.
  2. Write the symbol of an isotope having been given its mass number and element name.
  3. Determine the number of protons, neutrons, and electrons in an atom given its isotopic symbol.

Subatomic Particles. In the early 1900s, scientists learned that all atoms are constructed from the same three subatomic particles: the electron, proton, and neutron. Whereas atoms themselves are electrically neutral they were found to consist of electrically charged particles. The electron has a single unit of negative charge; the proton has a single unit of positive charge; and the neutron has no charge. The proton and neutron have essentially the same mass, 1.673 x 1024 g, and 1.675 x 1024 g, respectively. The electron mass is much smaller: 9.11 x 1028 g which is only 1/1836 the mass of a proton.

Rutherford's Experiment. Rutherford's experiments on alpha particle scattering in 1910 led the way to the nuclear model of the atom. Alpha particles are ejected from certain radioactive elements with very high kinetic energies. Remember that alpha particles have 2 units of positive charge, and a mass 4 times that of a proton. Being smaller than the atoms of most elements they can act as probes of the interior of atoms. Rutherford's experiments lead to the following conclusions and about the structure of atoms.

visual aid Rutherford's Experiment
Note: Please be patient. The file (347 KB) may take a moment to download.

Observations made by Rutherford when alpha particles were scattered by gold foil:

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Interpretations made by Rutherford about the nature of the atom:

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Atomic Number and Mass Number. An atom of one element is distinguished from an atom of another element by its number of protons. The atomic number, Z, of an element is the number of protons in the atomic nucleus. For example, the atomic number of oxygen is 8; therefore, all oxygen atoms contain eight protons and also eight electrons. You know this because atoms are neutral species and the positive charge must equal the negative charge.

The total mass of an atom is determined almost entirely by the number of protons and neutrons. In many cases the mass of the electrons can be neglected because it is so much smaller. The mass number, A, is the same as the total number of neutrons and protons present in the nucleus of an atom. The number of neutrons in an atom is A Z.

Isotopes. Atoms of a given element that differ in the number of neutrons, and consequently in mass are called isotopes. For example, there are two atoms of the element lithium, one with a mass number of 6 and another with a mass number of 7. Isotopes can be referred to by their mass numbers as in lithium-6 (pronounced lithium six) and lithium-7. The different mass numbers are the result of different numbers of neutrons per atom. An atom of the isotope lithium-6 contains three protons, three electrons, and three neutrons, whereas an atom of lithium-7 contains three protons, three electrons and four neutrons. Both atoms contain three protons, and so both are isotopes of the element lithium. In nature, elements are found as mixtures of isotopes.

Special symbols are used to designate specific isotopes.

The general symbol for an isotope is:

symbol of the element, or more commonly AX

The symbols for the isotopes of lithium are 6 Li and 7 Li.
3 3

Isotopes of an element have similar chemical properties, and form the same types of compounds. The chemical properties of an element depend on the number of protons and electrons in an atom, not the number of neutrons.

EXAMPLE Isotopic Symbols

The three isotopes of oxygen found in nature are oxygen-16, oxygen-17, and oxygen-18. Write their isotopic symbols.

O      O      O

EXAMPLE Isotopic Symbols

How many neutrons are present in the nucleus of each of the oxygen isotopes in the previous example?

For 16O, neutrons.
For 17O, neutrons.
For 18O, neutrons.


Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

  1. What are the three fundamental particles from which atoms are made? What are their electric charges. Which particles are in the atomic nucleus?

  2. What evidence did Rutherford find that supported his theory of the atomic nucleus?

  3. If the nucleus of an atom was the diameter of a nickel (about 2 cm), what would the diameter be of a such an atom?

  4. What is the mass number of a sodium atom that has 13 neutrons?

  5. How many neutrons are in an atom of Ag-109?

  6. Write the symbol for each of the following isotopes.
    1. An atom with Z = 30 and 37 neutrons.
    2. An atom with Z = 51 and 69 neutrons.


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