ATOMIC STRUCTURE

3.4. ATOMIC STRUCTURE

From 50 to 100 years after Dalton proposed his theory, various discoveries were made that show that the atom is not indivisible, but really is composed of parts. Natural radioactivity and the interaction of electricity with matter are two different types of evidence for this subatomic structure. The most important subatomic particles are listed in Table 3-1, along with their most important properties. The protons and neutrons are found in a very tiny nucleus (plural, nuclei). The electrons are found outside the nucleus. The information in Table 3-1 must be memorized, but only the whole number part of the masses (1 amu, 1 amu, 0 amu) must be remembered.

Table 3-1 Subatomic Particles Charge (e)

Mass (amu)

In nucleus

Neutron

In nucleus

Electron

Outside nucleus

There are two types of electric charges that occur in nature—positive and negative. Charges of these two types are opposite one another, and cancel the effect of the other. Bodies with opposite charge types attract one another; those with the same charge type repel one another. If a body has equal numbers of charges of the two types, it has no net charge and is said to be neutral. The charge on the electron is a fundamental unit of electric

charge (equal to 1.6 × 10 − 19 Coulomb) and is given the symbol e.

EXAMPLE 3.4. Using the data of Table 3-1, find the charge on a nucleus that contains (a) 8 protons and 8 neutrons and (b) 8 protons and 10 neutrons.

Ans. (a) 8(+1) + 8(0) = +8 (b) 8(+1) + 10(0) = +8 Both nuclei have the same charge. Although the nuclei have different numbers of neutrons, the neutrons have no

charges, so they do not affect the charge on the nucleus.

EXAMPLE 3.5. To the nearest integer, calculate the mass (in amu) of a nucleus that contains (a) 17 protons and 18 neutrons

CHAP. 3]

ATOMS AND ATOMIC MASSES

Ans. (a) 17(1 amu) + 18(1 amu) = 35 amu (b) 17(1 amu) + 20(1 amu) = 37 amu

The nuclei differ in mass (but not in charge). Uncombined atoms as a whole are electrically neutral. EXAMPLE 3.6. Refer to Table 3-1 and deduce which two of the types of subatomic particles in an uncombined atom occur

in equal numbers. Ans.

The number of positive charges must equal the number of negative charges, since the atom has a net charge of zero. The number of positive charges, as shown in the table, is equal to the number of protons. The number of negative charges, also from the table, is equal to the number of electrons. Therefore, in an uncombined atom, the number of protons must equal the number of electrons.

The number of protons in the nucleus determines the chemical properties of the element. That number is called the atomic number of the element. Atomic number is symbolized Z. Each element has a different atomic number. An element may be identified by giving its name or its atomic number. Atomic numbers may be specified

by use of a subscript before the symbol of the element. For example, carbon may be designated 6 C. The subscript is really unnecessary, since all carbon atoms have atomic number 6 and all atoms with atomic number 6 are carbon atoms, but it is sometimes useful to include it. Atomic numbers are listed in the periodic table, page 350, and in the Table of Elements, page 349.

EXAMPLE 3.7. (a) What is the charge on a magnesium nucleus? (b) What is the charge on a magnesium atom? Ans.

(a) +12, equal to the atomic number of magnesium (from the Table of Elements, p. 349). (b) 0 (all uncombined atoms have a net charge of 0). Note that these questions sound very much alike, but are very different. You must read questions in chemistry very carefully.