LECTURE 2 NOTES, January 14, 2003

1 - Atoms and Molecules - From PROPERTIES OF MATTER Website

  1. What are the constituents of atoms?
  2. What is the diameter of a typical atom?
  3. What is the diameter of the nucleus of a typical atom?
  4. How is an element defined? Roughly how many elements are there? How are elements related to atoms?
  5. What are molecules? How are molecules related to compounds?

2 - Atoms, Molecules, Elements, Compounds and Mixtures

We began with a relatively modern view of atoms. Now let's go back to the historical development of the subject.

The idea that all matter is constituted from indestructible, indivisible building blocks is dated back to Democritus (460-370 BC). He considered what would happen if you broke apart any material into smaller and smaller parts... would you come to a point where you could not divide any further? He concluded that would happen, and the building blocks, the smallest and most fundamental constituents, were the atoms. However, the more famous Aristotle (384-322 BC) didn't like this idea. What bothered him was that matter made of basic building blocks meant to him that there would be spaces between the atoms with "nothing," and he didn't believe there could be nothing. Aristotle favored the old view that all matter was constituted from earth, air, fire and water.

The alchemists may have been the first to develop the modern point of view. Perhaps they were the first, or among the first, to develop what we call the scientific method, i.e., doing experiments to confirm that ideas based on models of nature actually explain ones observations. They saw from their experiments that one could not construct all matter from earth, air, fire and water. They recognized the existence of elements and compounds.

One learns from experiments is that compounds are formed from combinations of elements in definite ratios. What is the connection between atoms, elements and compounds?

Copper is an element. If you subdivide copper into smaller and smaller pieces, you eventually break it down to its smallest constituent...a copper atom. Copper is normally in the form of a solid, so it is easy to picture cutting a solid copper wire into smaller and smaller pieces. Copper is an element made up of copper atoms. If it were made up of more than one kind of atom, it would be a compound.

Water is a compound because it is made up of water molecules. There is no such thing as water atoms. Water molecules are made of hydrogen and oxygen atoms, in the definite proportion of two hydrogens for one oxygen. Hence, a molecule is the smallest subunit of a compound while an atom is the smallest subunit of an element. Aggregates of single atoms make elements. Aggregates of molecules make compounds.

It is also possible to form mixtures of elements or compounds. Mixtures, unlike compounds, retain the properties of the original elements or compounds. A mixture is like a piece of cake made of, for example, strawberries, chocolate and almonds. Note that each of its constituents retain their properties. You can pick out a piece of almond, and it will still be an almond. But when you have water, you cannot separate a section of a glass of water and find that it behaves like hydrogen, and another part of the water behaves like oxygen...unless you actually break apart the molecules into their component atoms.

3 - Early Insights Into the Structure of Matter

It was not until Albert Einstein explained Brownian motion that it could be said atoms were proven to exist. Brownian Motion refers to an observation by Scottish botanist Robert Brown. He observed the jiggling of pollen grains on the surface of water with a microscope. Albert Einstein gave a quantitative interpretation of Brownian motion in 1900 and made the concept of atoms real by showing that the "jiggling" motion could be accounted for by collisions of water molecules with the pollen grains. Yet, even before Einstein, the clues were there.

The Law of Definite Proportions: 8 grams of the oxygen will combine with 1 gram of hydrogen to give the 9 grams of water, a ratio of 8 to 1. If one part of hydrogen combines with eight parts of oxygen all of the time, then it could be that an oxygen atom is eight times as massive as a hydrogen atom, and when the two come together they make a water molecule that is nine times as massive as hydrogen. This is incorrect! The correct picture is that two hydrogen atoms combine with one oxygen atom and that the mass of oxygen is 16 times as great as hydrogen. Then one has 2 units of hydrogen for one unit of oxygen, or a mass ratio of 2x1=2 compared with 1x16=16. The ratio of 2 to 16 is the same as 1 to 8. In other words, you need more information than the ratio of masses that combine to determine how many atoms of each are involved. The ratio of the masses doesn't tell you that two hydrogen atoms combine with one of oxygen to make water.

4 - Examples of Elements, Compounds and Mixtures

A parenthetic comment about water...a very common compound with familiar properties. Water can be in the form of a liquid, a gas (water vapor), or a solid (ice). Water molecules combine to form each of three discrete states: liquid, gas or solid. When either a liquid or a solid, the molecules are packed closely together, while as a gas, the distance between water molecules is large compared to their size. However, there is an important difference between the way molecule adhere in the liquid and solid states. When a liquid, the molecules are not rigidly connected to each other, and they move about with a more-or-less random orientation. However, in a solid, the molecules form a well-ordered (crystalline) structure. Indeed, there is more space between the water molecules as ice than as a liquid...and that is why ice floats in liquid water. It occupies a greater volume for a given number of molecules. More about this in a subsequent chapter.

Hence, hydrogen is an element made of an aggregate of hydrogen atoms while oxygen is an element made of oxygen atoms. However, water is a compound made of water molecules. Note that the molecules which form a compound are made of different types of atoms, but molecules can also consist of atoms of the same kind. In particular, hydrogen and oxygen are normally found as molecules formed by two atoms stuck together. Mixtures are different than compounds in that the properties of the constituents in a mixture don't change. Water has very different properties from hydrogen or oxygen. However, a MIXTURE of water and suger does not result in a substance with new properties. The sugar remains sugar and the water remains water.

A particularly interesting large molecule is formed solely by carbon atoms in a rigid structure that resembles the architectural structure invented by Buckminster Fuller, the famous architect. These, so-called, buckyballs were discovered in 1985 and in solid form in 1990. This 60-atom molecules is called a icosahedron (20 faces) consisting of pentagons and hexagons. Like its architectural analog, these are particularly strong molecules.

The molecules that constitute living things, the DNA, are large and complex. These intricate structures contain the code which controls who were are and how we function as human beings.

Note that atoms can combine in more than one way to form elements and compounds. Two elements made of carbon atoms that have very different properties: diamond and graphite. Two ways of combining carbon and oxygen to form gases with different properties:. One carbon atom combines with one oxygen atom to form carbon monoxide, one carbon atom combines with two oxygens forms carbon dioxide.

R.S. Panvini