Last time we talked, we never got to why people believe in atoms. We saw that scanning tunneling microscopy can be used to image individual atoms. But this is relatively recent technology- why did people believe in the existence of atoms the previous 200 years?

Modern Atomic Theory

Law of Conservation of Mass (LaVoisier) - Matter is neither created nor destroyed by reactions.

Law of Constant Composition (Proust) - Pure samples of a given compound  always have the same composition

Proust - contemporary of LaVoisier; interested in minerals and their composition - he found that no matter what the sample of mineral was, if it was pure, was always the same. It was not clear before this that all limestone had the same composition - we take this for granted.

e.g., magnesium oxide- 60% magnesium, 40% oxygen (by mass), always. It is never 50/50%, or 39.5/61.5%.

water- 88.8% oxygen, 11.2% hydrogen (by mass), always.

Aristotle thought that you could take any form of matter and change the makeup around.

These two laws lead to atomic theory. Proust and Le Voisier’s theories lead to John Dalton, an English Quaker school teacher, proposing the atomic theory (1803). Introduced the first modern atomic theory - it was not just an idea, it was based on physical observation. The chemists began using this theory earlier that the physicists (some of whom remained unconvinced until Einstein's explanation of Brownian Motion in the early 1900s.

Modern atomic theories’ four points:

1) All matter consists of tiny particles (atoms, from the Greek word, atomos, for indivisible).

2) All atoms of a given element are identical; atoms of different elements are different - that explains the law of constant composition. (Sidebar - Dalton was unaware of isotopes, atoms of the same element that have slightly different masses)

3) Compounds (these are different than elements) exist as combinations of atoms in specific ratios. This explains the Law of Constant Composition. For example, in the case of mercuric oxide, HgO, the compound exists as a 1:1 ratio of mercury and oxygen atoms. Since all atoms of mercury and all atoms of oxygen have the same mass, any random sampling of HgO will have the same mass ratio of mercury and oxygen.

4) Chemical reactions involve the rearrangement of atoms in compounds; but atoms are not destroyed or created. This explains the Law of Conservation of Mass.

Elements - purest form of matter. 90 "naturally" occur. Uranium, element #92, is the "heaviest" of the naturally occuring elements. The two "light" artificial elements are technetium (#43) and promethium (#61) - (Prometheus in Greek mythology- gave humans fire)

The periodic table of elements was developed by Dmitri Mendeleev in the late 1860s. The table is organized according to observed patterns of properties and reactivities (for example, on the far right of the table is a column consisting of helium, neon, argon, krypton, xenon and radon - they are all gases and all are very unreactive - the so-called "Noble Gases") 

Each element has a symbol on the periodic table. Most are based on English, except for a few that have Latin origins (Cu is copper (cuprium), Au is gold (aurum), Ag is silver (argentum), mercury is Hg (hydroargentum- hydro is water, argentum is silver, "liquid silver").

Compounds exist when elements react chemically. (not a simple physical mixture). There are approximately 20,000,000 compounds known. There are infinite number of possible compounds, BUT, not all combinations of atoms are possible,

e.g., CO (carbon monoxide) exists, CO₂ (carbon dioxide) exists, but CO₃ does not exist.

Compounds & Chemical Reactions

Types of compounds:

1) molecular compounds: exist as discreet molecules.

e.g.- H₂O (the number subscripts represent how atoms of the listed element there are in the molecule; if no subscript is written, there is one atom of the element indicated; so there are two hydrogen atoms and one oxygen atom in a water molecule).

Balancing chemical reactions:

According to the Law of Conservation of Mass, the number of atoms of each element must be the same in the reactants (on the left) and products (on the right). So the reaction above is not balanced - there are two oxygen atoms on the left and only one on the right.

To balance equations, use the following guidelines:

• Never, EVER mess with the subscripts; this changes the compounds involved. (CO does not equal CO₂)
• Change only the coefficients in front of each species.

a)

These coeffiecents balance the oxygen atoms (four on each side) and hydrogen atoms (four on each side). There is only one carbon atom involved.

b)

These coeffiecents balance the oxygen atoms (two on each side), hydrogen atoms (four on each side) and chlorine atoms (four on each side). There is only one titanium atom involved.