New Atomic Theory Basics

Now we have a good working model of the atom. It consists of a nucleus with a diameter of about 10^-13 cm and electrons that move about the nucleus at an average distance of about 10^-8 cm. (Marble of diameter 0.5 inch --> Dome with a radius of about 547 yards -- check this calculation yourself). As we will see later, the chemistry of atoms is mainly due to the electrons and therefore we can make do with a crude model of the nucleus (in nuclear chemistry this will not suffice) which is that the nucleus consists of protons (positive charges) and neutrons (no charge):

particle Mass Charge

Electron 9.11 x 10^-31kg -1

Proton 1.67 x 10^-27kg +1

Neutron 1.67 x 10^-27kg None

particle	Mass	Charge
Electron	9.11 x 10^-31kg	-1
Proton	1.67 x 10^-27kg	+1
Neutron	1.67 x 10^-27kg	None

From our Dome model we see that the atom is mostly space with the electrons occupying most of that volume. It is the "intermingling" of the electrons which causes the binding that occurs when atoms combine to form molecules. Since elements can contain very different numbers of electrons then we expect there to be significant differences in the chemical reactivity of atoms. We indicate the number of electrons and protons in an atom with a symbol:

^A_ZX

where X is the symbol for the element (C for carbon, O for oxygen, etc), A is called the mass number (the total number of protons and neutrons in the nucleus), and Z is called the atomic number (the number of electrons which is also equal to the number of protons). Some examples follow.

¹⁴₇N This is the symbol for nitrogen showing that it has 7 electrons, 7 protons, and 7 neutrons (14-7).

⁵⁶₂₆Fe This is the symbol for Fe showing that iron has 26 electrons, 26 protons, and 30 neutrons (56-26).

When we get to carbon we find experimentally that two possible symbols exist: ¹²₆C and ¹³₆C. We see that each have 6 electrons and 6 protons, but one has 6 neutrons and one has 7 neutrons. Experimentally we also see that about 91% of all carbon atoms have the first set and about 9% have the second set. Since each has the same number of electrons, each will behave the same chemically. This observation leads to the definition of isotopes :atoms of the same element which have the same number of electrons but differ in the number of neutrons .

Most isotopes are referred to simply by their element and their mass number such as U-235 (uranium 235) instead of ²³⁵₉₂U. Since the symbol tells us the atomic number (from the periodic table -- see later), we can omit the atomic number and just use the mass number: ²³⁵U. However some isotopes have specific names such as ¹H is hydrogen, ²H is deuterium, and ³H is tritium. When water is made from deuterium rather than hydrogen, it is referred to as "heavy water".

In the early 1900's Niels Bohr developed a theory for the atom which was based upon the assumption that the electrons circled the nucleus in orbits much like the planets revolve about the sun. This led to further understanding of some concepts, but the basic assumption was incorrect. The current model of the atom was developed independently by Schrodinger and Heisenberg in 1927 and is now called Quantum Mechanics. We will discuss these theories later. For now just know that the electrons determine the chemical properties of the atoms.

MOLECULES AND IONS

The chemical bond is the forces that hold atoms together in a molecule. These bonds can be classified as to one of two types depending upon the type of forces that are involved in holding the atoms together.

I. Covalent Bond In this type of bonding the atoms share the electrons in forming the bonds. We can represent this bonding as a chemical formula such as CO₂ or H₂ or as a structural formula which shows a bit more about the actual structure as in O=C=O which shows that the molecule is linear and the type of bonds are the same ("double bonds" -- we will see what that means later). We also show the structural formula of water as


   O
  /  \
 H    H

which shows that the molecule is planar (three points define a plane) but not linear and that the bonds are all the same ("single bonds" -- later). With computer modeling, we can show the structures even better by using space-filling models.

II. Ionic Bond

An ion is an atom or group of atoms with a net charge such as the following;

Na --> Na⁺¹ + e^-1 This ion is positive and called a cation.

Cl + e^-1 --> Cl^-1 This ion is negative and called an anion.

When these ions combine (one has a +1 charge and one has a -1 charge) we get the neutral ionic compound NaCl. It is important to remember that ionic compounds do not really exist as individual molecules as covalent compounds do. Ionic compounds exist as a huge collection of cations and anions such as NaCl. However in some of the work that we do we will treat them as if they were individual molecules.

Polyatomic ionic compounds also exist such as NH₄NO₃ which is composed of the cation NH₄⁺¹ and the anion NO₃^-1.

Now take a practice quiz to help you understand if you understand the basic concepts.

You must use your real name when it asks for a name.

The test will only submit when you have answers all of the questions correctly.

If you are not taking this course for credit please do not answer all the questions correctly for I don't want to be flooded with email answers to the tests.