Lesson 5: Ions & Isotopes

Chemists have developed a system of symbols for indicating atoms, ions, and isotopes. You’re already familiar with the symbols for the atoms: they’re just the one- and two-letter symbols that appear in the periodic table. These symbols indicate atoms with equal numbers of protons and electrons, in other words, neutral atoms.

Isotopic and ionic symbols are based on these atomic symbols with numbers and signs added to show mass number, charge, and sometimes atomic number.

Recall that the atomic number determines the element, so giving the symbol for the element makes it unnecessary to show the atomic number, but sometimes it is convenient to do so anyway. For this lesson, when you work on the practice problems or your problem set to turn in, you should show the atomic number when you write the isotopic symbols.

Ions | Isotopes | Practice

Ions

When an atom gains or loses electrons, it becomes a charged particle known as an ion. When an atom gains extra electrons, it becomes a negatively charged ion. If it has only one extra electron, its charge is -1. If it gains two extra electrons, its charge is -2, and so on. These charges are indicated by superscripts that follow the symbol of the atom.
Note the conventions illustrated here. A charge of -1 is indicated with the minus sign only. The number 1 is omitted. Charges of -2, -3, and so on, are indicated by the number followed by the minus sign: 2-, 3-, etc. This is a relatively new convention and you may see symbols for ions written with the minus sign in front of the number instead of behind it.

Example 10b in your workbook shows how the charge of the chloride ion is related to the number of protons and electrons it has. Atoms never gain or lose protons. An ion is negative because it has gained extra electrons, not because it has lost protons. A negative ion is called an anion (pronounced AN-eye-on).

Atoms that lose electrons become positively charged. If they lose only one electron, their charge becomes +1. If they lose two electrons, the charge becomes +2, and so on. As with negative ions, the charge is indicated by a superscript that follows the symbol. The conventions for writing the symbols for positive ions are similar to those for writing the symbols of negative ions. A charge of +1 is indicated by a + sign. Larger positive charges have the positive charges have the positive sign follow the number: 2+, 3+, and so on.

Example 10a in your workbook shows how the positive charge on the beryllium ion is related to the number of protons and electrons it has. Atoms never gain protons; they become positively charge only by losing electrons. A positive ion is called a cation (pronounced: CAT-eye-on).

You may have notice that the number of neutrons in each of these ions was not specified. Since neutrons are neutral, they don’t affect the charge on an atom or ion. They do, however, have mass, and so affect the mass number.
The number of neutrons that an atom can have in its nucleus is limited. If an atom has too many or too few neutrons, the nucleus becomes unstable and will spontaneously break apart. Atoms with unstable nuclei do form but are radioactive.

The three forms of zinc shown here are the three most common types of zinc atoms. Two other types also have stable nuclei, but are less common. One has 35 neutrons and the other has 40 neutrons. Ten other zinc atoms, all with unstable nuclei, have been found.

These atoms are all zinc:	Atom #1 has 30 protons and 34 neutrons
	Atom #2 has 30 protons and 36 neutrons
	Atom #3 has 30 protons and 38 neutrons
Neutral zinc atoms have 30 electrons. Zinc can also form +2 ions, with 28 electrons.

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Isotopes

You may recall that atoms with the same atomic number but different mass numbers are called isotopes. Because they have the same atomic number, they must be atoms of the same element, but they have different masses because they have different numbers of neutrons. To indicate a specific isotope, we place the mass number as a superscript before the symbol.

Mass numbers, as we have indicated before, are not the actual mass of the atom, but they are close. The mass number is the total number of protons and neutrons in the nucleus. Each proton and each neutron weighs very close to, but not exactly, 1 amu, hence the mass number is close to the actual mass.

Since this superscript is not the number of neutrons, but the mass number (the number of neutrons plus protons), you determine the number of neutrons by subtracting the atomic number (the number of protons) from the mass number.

The number of protons (the atomic number) is the same for all atoms of a given element, and can be found on any periodic table. All beryllium atoms, for example, have 4 protons.

A complete symbol can also show the atomic number. This is done largely for convenience, since knowing what an element is tells you its atomic number. The atomic number is placed as a subscript in front of the symbol.
These symbols are taken from Example 11 in your workbook.

Had these been symbols for charged ions, the charge would have been included in the usual location – as a superscript following the symbol.

Practice

Exercise 12 in your workbook will give you some practice writing and interpreting symbols for atoms, ions, and isotopes. Be sure and complete this exercise before you move on.

Answers to Ex. 12: