Chapter 3

I would like you to read pages 48 - 54. Beginning on page 54 is a discussion of Dalton's Atomic Theory. I'll expect you to know the assumptions behind Dalton's Atomic Theory.

Here is a brief review of what we had discussed earlier about Dalton's Atomic Theory.

Atomic Theory and a Microscopic Model of Matter

It was a gentleman by the name of John Dalton who organized a collection of experimental observations into a theoretical framework. Dalton, who was a meteorologist, had considerable experience with air and wind and the effects of temperature on the volume of a gas. The results of his study of air suggests that his observations could be understood if matter consisted of tiny particles, a sort of submicroscopic billard ball. So Dalton proposed his atomic theory to explain his observations. Each element is made up of tiny, indivisible particles called atoms.

1. All atoms of a given element possess identical properties.

2. Atoms of different elements have different properties.

3. Chemical changes involve the combination, separation or rearrangement of atoms: atoms are neither destroyed, created or changed.

4. When atoms combine they do so in fixed ratios of whole numbers forming particles called molecules.

Click here to view a figure of this microscopic model.

What is the microscopic model of the three phases of matter?

Here is an atomic level exercise from the first Problem Set.

(Note: To run the atomic level exercise the version of Netscape Navigator must be accessorized with the MacroMedia ShockWave Plugin. If Netscape displays a broken icon after clicking on the link above, then you need to download the plugin and install it. To download the plugin go to the Shockwave site and follow the instructions.)

The textbook provides a section discussing some of the weaknesses of this theory on page 54. But fundamentally it is an excellent beginning point to understand the nature of matter from a mircoscopic view.

Read the section on Applying Atomic Theory to the Sturcture of Matter. We've talked about elements and we've even discussed allotropes (graphite and diamond in our Chapter 2 notes). On page 60 molecules and compounds are mentioned, again as a review. We've already defined molecules and compounds.

When talking about elements, molecules or compounds chemists like to use formulas. Formulas is where it's at! We've discussed formulas for the elements already. The formula for chlorine is Cl₂, and for zinc it is Zn. We've talked about the formula for water and for carbon dioxide. Water's is H₂O and carbon dioxide's is CO₂. But where do these formulas come form? How did we know the formula of water, of carbon dioxide? How do I determine the formula for some other compound?

To be able to answer those questions AND many more we need to do a few things first. I think the best place to begin is the periodic table. What do we know about the periodic table so far? We know is is a table of the known elements. All periodic tables list the elements in the same order. The periodic table also includes two numbers. The number in the upper right corner is called the atomic number. The number displayed below the symbol is called the atomic mass. What do these numbers mean...represent?

All matter contains atoms and atoms are composed of three fundamental elementary particles; the electron, the proton, and the neutron.

Electron

This was the first atomic particle discovered by J.J. Thomson in 1897. He characterized the properties of cathode rays, as a stream of negatively charged particles or electrons. Thomson found the particle to be negatively charged. He was also able to measure the charge-to-mass ratio of the cathode rays. The value he obtained was independent of the gas used in the cathode ray tube.

Proton

Thomson experimentally determined the existence of positively charged particles in the cathode ray tube, but he was unable to characterize these particles further. In 1919 Ernest Rutherford characterized the proton as a particle with a charge equal in magnitude to that of the electron but with the opposite sign. The mass was measured as 1.673 x 10^-27 kg.

It was not until 12 years later that Robert Millikan was able to determine the charge of an electron. He experimentally measured a value of -1.6022 x 10^-19 Coulombs. Using Thomson's charge-to-mass ratio the mass of an electron had a value of 9.109 x 10^-31 kg.

Neutron

The neutron was characterized by James Chadwick in 1932. The neutron has almost the same mass as the proton and no charge.

Structure of the Atom

Our current view of the structure of the atom was described as a result of experiments performed under the direction of Ernest Rutherford. In his experiment alpha particles (which he had characterized by 1908) were 'shot' at a thin piece of gold foil. The behavior of the scattered particles lead Rutherford to postulate a new model of the atom. His model, which we currently hold, locates almost all of the mass of the atom in the nucleus with the electron located outside the nucleus.

It turns out that the atomic number (the whole number in the upper right hand corner of the box containing the symbol of the element) is exactly equal to the number of protons in the element. For example carbon is atomic number six and has six protons. Chlorine has an atomic number of 17 and therefore has 17 protons. Chemists use a particular symbolism to identify the atomic number for a particular element. For carbon that symbolism is ₆C. For chlorine it is, ₁₇Cl.

For a neutral element the number of protons equals the number of electrons. But it is different if the element is charged. An element with a charge is called an ion. For example, Na⁺ and Cl^- are ions. Actually Na⁺ is an example of a cation and Cl^- is an example of an anion. So how many protons and how many electrons in Na⁺ and in Cl^-? Ions always indicate the sign and magnitude of the charge as a superscript on the right side of the element symbol. If no sign or number are in the upper right position the atom is neutral. So Na⁺ and Cl^- are ions, Na is a neutral atom.

For Na⁺ there are 11 protons since sodium has an atomic number of 11, and there are 10 electrons. How do we know there are ten electrons in Na⁺? Since protons each have a single positive charge, and there are 11 protons in a sodium atom, than the total positive charge is 11 from the protons. The symbol Na⁺ indicates this sodium atom has one more positive charge compared to negative charge. (Remember a neutral atom has equal numbers of positive protons and negative electrons.) So there must be 10 electrons.

For Cl^- there are 17 protons since chlorine has an atomic number of 17, and there are 18 electrons. How do we know there are 18 electrons in Cl^-? Since protons each have a single positive charge, and there are 17 protons in a chlorine atom, than the total positive charge is 17 from the protons. The symbol Cl^- indicates this chlorine atom has one more negative charge compared to positive charge. (Remember a neutral atom has equal numbers of positive protons and negative electrons.) So there must be 18 electrons.

For Al³⁺ there are 13 protons since aluminum has an atomic number of 13, and there are 10 electrons. How do we know there are ten electrons in Al³⁺? Since protons each have a single positive charge, and there are 13 protons in an aluminum atom, than the total positive charge is 13 from the protons. The symbol Al³⁺ indicates this aluminum atom has three more positive charge compared to negative charge. (Remember a neutral atom has equal numbers of positive protons and negative electrons.) So there must be 10 electrons.

Determine the number of protons and electrons in; K⁺, S^2-, Fe³⁺, and Br^-.

So now we know how to determine the number of protons and electrons. There is another atomic particle found in the nucleus, the neutron. How do we determine the number of neutrons in an atom? To do that we must be given the mass number. The mass number is equal to the sum of the protons and the neutrons. The mass number is located as a subscript to the left of the symbol of the element. For example, for am atom of carbon the symbol is ¹²C. The mass number is 12. So the number of protons plus the number of neutrons in this atom equal 12. Since the element symbol is carbon and the atomic number of carbon is 6, there are 6 protons. By subtracting the number of protons (atomic number) from the mass number we get the number of neutrons, in this example the number of neutrons is also equal to 6.

Another atom of carbon can be represented as ¹³C. How many protons and neutrons in ¹³C? The answer is 6 protons and 7 neutrons.

Another atom of carbon can be represented as ¹⁴C. How many protons and neutrons in ¹⁴C? The answer is 6 protons and 8 neutrons.

The three atoms; ¹²C, ¹³C and ¹⁴C are all examples of isotopes of the element carbon. That is, atoms with equal numbers of protons and different numbers of neutrons.

Determine the number of protons and neutrons in; ¹⁵N, ¹⁸S, ⁹⁶Mo, and ²³⁸U.

Now finally lets put all this together. Determine the number of protons, neutrons and electrons in ¹⁰⁹Ag⁺.

For even more fun complete all the blanks in the following table;