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Practical class 9.

Theme: Atomic structure. Dalton’s atomic theory.

Atomos, the Greek root of the word atom, means “indivisible.” It was originally believed that the atom was the ultimate indivisible particle of which all matter was composed. Lord Rutherford showed in 1911 that the atom is not homogeneous, but rather has a dense, positively charged center surrounded by electrons. Subsequently, scientists have learned that the nucleus of the atom can be subdivided into particles called neutrons and protons. In fact, in the past two decades it has become apparent that even the protons and neutrons are composed of smaller particles called quarks.

For most purposes, the nucleus can be regarded as a collection of nucleons (neutrons and protons), and the internal structures of these particles can be ignored. The number of protons in a particular nucleus is called the atomic number (Z), and the sum of the neutrons and protons is the mass number (A). Atoms that have identical atomic numbers but different mass number values are called isotopes. However, we usually do not use the singular form isotope to refer to a particular member of a group of isotopes. Rather, we use the term nuclide.

In 1803, John Dalton (1766–1844) proposed his atomic theory, including the following postulates:

1. Matter is made up of very tiny, indivisible particles called atoms.

2. The atoms of each element has mass, but the mass of the atoms of one element is different from the mass of the atoms of every other element.

3. Atoms combine to form molecules. When they do so, they combine in small, whole-number ratios.

4. Atoms of some pairs of elements can combine with each other in different small, whole-number ratios to form different compounds.

5. If atoms of two elements can combine to form more than one compound, the most stable compound has the atoms in a 1: 1 ratio. (This postulate was quickly shown to be incorrect.)

The first three postulates have had to be amended, and the fifth was quickly abandoned altogether. But the postulates explained the laws of chemical combination known at the time, and they caused great activity among chemists, which led to more generalizations and further advances in chemistry.

The postulates of Dalton’s atomic theory explained the laws of chemical combination very readily.

1. The law of conservation of mass is explained as follows: Because atoms only exchange “partners” during a chemical reaction and are not created or destroyed, their mass is also neither created nor destroyed. This, mass is conserved during a chemical reaction.

2. The law of definite proportions is explained as follows: Because atoms react in definite integral ratios (postulate 3), and atoms of each element have a definite mass (postulate 2), the mass ratio of one element to the other(s) must also be definite.

3. The law of multiple proportions is explained as follows: Because atoms combine in different ratios of small whole numbers (postulate 4), for a given number of atoms of one element, the number of atoms of the other element is in a small, whole-number ratio. A given number of atoms of the first element implies a given mass of that element, and a small, whole-number ratio for the atoms of the second element (each of the same mass) implies a small, whole-number ratio of masses of the second element.