Introduction to Matter


Chemistry is based upon the microscopic (atomic and molecular) understanding of the world around us and how these molecules behave and transform. Chemists desire to know nature and work within natural laws. Humankind's knowledge of Chemistry has a profound effect upon civilization. Chemical knowledge is required to live in the modern world and use the latest available technology.
Everyone has an example of how 'chemicals' have been used to improve or degrade the quality of their lives. Some of the most important issues regarding the survival of the Human race are a result of, and will be corrected by, the manipulation and control of chemical reactions.   Chemistry itself is neither Good nor Evil   The use of chemical knowledge by Humankind is no longer in debate. The understanding of Chemistry is needed to solve the Earth's problems and help protect you as an individual from errors of ignorance.

It is easy to 'see' macroscopic properties of matter. It is clear that the properties of materials are a consequence of their microscopic structure --- the structure and arrangement of the atoms and molecules that compose it.
It is not possible to see an atom. We must therefore go beyond the senses that are our birthright and learn about what we cannot see, hear, touch or smell.
We learn about things we cannot see all the time. I have never seen a gravity field, but I know how it affects my life. Precise measurements and predictive theory of the unseen gravity field are made (subconsciously) everytime we throw a ball, or ride a bike.
Measurement allows us to 'see' beyond our senses and develop new ones.
Science is an institution which refines, explains and communicates measurements to predict future behavior.
In short, Chemistry is the Science of Molecules (or perhaps the electrons in molecules).


Matter is a term to describe all the stuff around us that has mass an occupies space. Everything else is Energy. Matter, as we might have guessed already, is composed of very small 'particles' called molecules, which themselves are composed of atoms. The way the molecules in a substance move determines the phase of the matter, eg. molecules move freely in a gas. There are many kinds of matter, but only a few different phases of matter (what are they?) That means there are many different kinds of molecules. A substance that only has one kind of molecule in it is called a pure compound. A substance that has only one kind of atom in it is called an element. A substance that has more than one kind of molecule in it is called a mixture. Here is a cartoon of the molecules in a few example gases:
The classification of the type of matter that you have, involves knowing something about the physical (macroscopic) structure as well as the molecular (microscopic) structure of the the substance. This can sometimes be tricky, and is even sometimes open to a certain amount of interpretation. Often, a classification scheme like the following is used:
Lets discuss the questions in the blue boxes. The first question, 'Is it uniform?', really questions the macroscopic uniformity. If treated microscopically, nothing is uniform because even atoms themselves are lumpy. So, in answering this question we must examine the uniformity of the material over a distance that corresponds to many molecular diameters. So, a glass of water is uniform, but a piece of wood is not.
The second question, 'can it be separated?', also referes to processes that can be made on the bulk material, but by methods that can discriminate between individual molecules by some property. Examples of physical separation include crystallization, distillation, extraction, centrifugation, and all chromatographic methods.
The third question, 'can it be decomposed', now refers to chemical processes that can rearrange and perhaps separate the atoms within molecules. This is usually thought of as thermal decomposion, but remember that reactions with atmospheric gases (Oxygen, in particular) can occur even for a pure element and does not count as a decomposition. A pure (elemental) metal may oxidize in air at high temperature, but in a vacuum it just melts and then vaporizes (no chemical change)

In short, the classification of a substance as to mixture, compound, or element requires knowledge of what moleules are present and how they are arrange. To get that 'picture', you need to use your brain, not a flowchart.

The elemental composition of the matter around us is complicated, but does not involve an equal contribution from all the elements. Depending on what you are looking at, the abundance of the elements in a material will vary. Here are the elemental abundances, BY MASS, in the earths crust and in the Human Body:

If you cound the abundance of the elements in the Human Body by number of atoms and not by weight, then the most abundant element is by far HYDROGEN. In fact, if you consider the composition of the entire universe by number of atoms of each element, the universe is 91% H, 8.75% He, and 0.25% everything else. How can we convert between the percent by weight and percent by number of atoms? We need to know how much each atom weighs!

Here, with a total of 8 marbles, the number percent of yellow marbles is 3/8=37.5%. But the mass of the yellow marbles is 1.0 g/marble which is less than the average weight of the marbles in the box. The mass percent of yellow marbles is 3/16=18.75%. Can you find the number and mass percent of the other color marbles? answer
Measurement: The Eyes of Science

Because the science of chemistry needs to quantify very large and very small properties, we need a convenient way of expressing these properties in an undestandable and standard fashion. We desire to have convenient units for many different kinds of measurements, but allow these units to be interconcertable. We therefore choose a standard set of units as a 'base' for commonly measured things:

Base Units of the International System (SI)

The General Conference on Weights and Measures has replaced all but one of the definitions of its base (fundamental) units based on physical objects (such as standard meter sticks or standard kilogram bars) with physical descriptions of the units based on stable properties of the Universe.

For example, the second, the base unit of time, is now defined as that period of time in which the waves of radiation emitted by cesium atoms, under specified conditions, display exactly 9 192 631 770 cycles. The meter, the base unit of distance, is defined by stating that the speed of light, a universal physical constant, is exactly 299 792 458 meters per second. These physical definitions allow scientists to reconstruct meter standards or standard clocks anywhere in the world, or even on other planets, without referring to a physical object kept in a vault somewhere.

In fact, the kilogram is the only base unit still defined by a physical object. The International Bureau of Weights and Measures (BIPM) keeps the world's standard kilogram in Paris, and all other weight standards, such as those of Britain and the United States, are weighed against this standard kilogram.

This one physical standard is still used because scientists can weigh objects very accurately. Weight standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this. The 21st General Conference on Weights and Measures, meeting in October 1999, passed a resolution calling on national standards laboratories to press forward with research to "link the fundamental unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." The 22nd General Conference, in 2003, renewed this request. It is possible that the 24th General Conference, in 2007, will make a change in the definition.

Following are the official definitions of the seven base units, as given by BIPM. The links in the first column are to (possibly) less obscure definitions (Thanks: Rowlett at UNC).

meter (m)

distance

"The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."

kilogram (kg)

mass

"The kilogram is equal to the mass of the international prototype of the kilogram."

second (s)

time

"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

ampere (A)

electric current

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 10-7 newton per metre of length."

kelvin (K)

temperature

"The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."

mole (mol)

amount of substance

"The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."

candela (cd)

intensity of light

"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."

You are familiar with some of these units as they are used by most modern countries even by non-scientists.
One of the problems with discussing the properties of molecules (like we do in Chemistry) is that moleules are very tiny and there properties are very small and their numbers (count) are very large. It is for this reason that we invent the unit MOLE, which is like a 'bakers dozen' of atoms. So, instead of saying that 18 grams of water has 6.0 x 1023 molecules in it, we say it has 1.0 mole of molecules, where

1.00 mole of objects = 6.02 x 1023 objects
just like
1.00 dozen objects = 1.2 x 101 objects

Unfortunately, we also have to 'scale' all the other units for very small and very large measurements. We do this by putting a prefix on the unit base that conveys how many powers of ten we wish to multiply or divide the base unit by. Common prefixes are:

The units for length, volume, and mass for the SI system are quite cleverly interconverted. The unit of volume is defined to be 1 cubic decimeter. The unit of mass is 1 liter of WATER at 277 K. (water is common, and it has a maximum density at 4 oC. How the meter itself is defined is a long and historically boring story.

In general, the mass of a given volume of some substance is defined as the Density (or more precisely Mass Density) of that substance. So, we know the value of density for at least one sustance, water, in SI units; the density of water is 1.00 kg/l (or 1.00 g/cm3) at 277K.
Temperature, on the other hand, can be a bit of a mess. Even though the KELVIN temperature scale is supposed to be the standard, many scientists still report temperature in Celsius, and even worse, the weather channel report temperatures in degrees Fahrenheit. Luckily, the Kelvin and Celcius scales have the same size unit (a change of 1 K results in a change of 1 oC) and only differ by the ZERO of the temperature scale. BUT the zero of the Kelvin temperature scale is actually ABSOLUTE ZERO, so it is called an absolute temperature scale. Rational beings use only absolute temperature scales. If you always convert ALL of your temperatures to Kelvin at the beginning of every calculation, you cannot go wrong. Here is how the three most common temperature scales stack up.
What is the absolute zero of temperature on the Celsius scale? The Fahrenheit scale? answer
The Scientific Method
A complete philosophical dicussion of how science evaluates and understands nature through the test of hypothesis is clearly beyond the scope of this course. Remember, however, that science is an iterative process, and that occasionally, in the face of new evidence, we must abandon theories and concepts which seemed to serve us well...

Here are some interesting Lengths (a) Volumes (b) and Masses (c). Note the non-linear scale

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PJ Brucat || University of Florida