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Classifying Matter

Property – a characteristic or identifying feature of a substance.

Physical properties can be observed without any change in the actual substance while chemical properties require that a chemical change take place to be observed.

Intensive properties depend on the substance itself and its structure, while extensive properties depend on the amount of material and/or the effects of the physical environment (pressure,temperature…).

Matter is defined as anything that has inertia and takes up space. We measure these properties as mass and volume.

There are two main forms of matter: pure substances (element, compound) and mixtures (suspension, colloid, solution, mixture). An element is a substance composed of only one type of atom (details later). A compound is a chemical combination of 2+ elements and always has a definite ratio of those elements (2 H + 1 O = water). The elements in a compound can only be separated by a chemical change such as combustion, electrical separation, or other chemical reaction.

A mixture is a physical combination of 2+ pure substances. Some specific types of mixtures have definite identifying properties. A suspension is a heterogeneous mixture where some of the particles are large enough to see with the naked eye and will often settle out of the other substance(s) over time, they can also be filtered. In a solution, which is homogeneous, one of the substances (solute) is disassociated (pulled apart a little) in the other, solvent. This process is called dissolving. This makes the individual pieces so small they are invisible to the naked eye and cannot be filtered. A colloid is heterogeneous, and falls between a suspension and a solution, the particles can be seen by the naked eye, but they will not settle out over time nor can they be filtered in most cases. To determine whether or not particles are visible, simply use light. If you can see a beam of light as it passes through the substance, such as headlights in fog, the particles are visible. This is called the Tyndall Effect. A solution does not exhibit the Tyndall Effect.

A mixture can be separated by physical processes such as distillation, filtering, a centrifuge, magnetism, and paper chromatography.

As mentioned above, a solution is a homogeneous mixture. A solution results when a solute dissolves in a solvent. You are probably most familiar with solid solutes in liquid solvents, however, a solution can also result from two liquids, such as an alloy, or a gas in a liquid. Other combinations are much less likely due to the dissolving process itself. We will use solid in liquid as well as gas in liquid solutions since these are more familiar to us and are fairly easy to observe.

One simple example would be to put sugar in tea. A spoonful of sugar will dissolve in the tea. We commonly recognize this since the sugar slowly disappears. Basically the solute is “pulled apart” by the solvent. The sugar is in a crystal form and is composed of many molecules. The water is also composed of many molecules and the water being liquid can surround the surface of the sugar. The water molecule(s) will attract the sugar molecule(s) and pull the sugar as it moves along. This leaves more sugar molecules exposed to more water molecules, and on and on…eventually you cannot see the sugar molecules. But, when you taste the tea you know the sugar is still there. Carbonated beverages, CO2 in flavored water, as well as everyday water, oxygen for the marine animals and CO2 for the marine plants, are good examples of gas solutes in a liquid solvent. Three basic factors effect the rate at which a solute will dissolve in a solvent. (1) The rate increases with an increase in the solute's surface area, granules of sugar v. a sugar cube. (2) Increased agitation (stirring) of the mixture will speed the dissolving. (3) Temperature - for a solid solute an increase in temperature will speed the dissolving, while it slows the dissolving of a gas. Think of this last one in terms of average KE of the materials.

Solubility is a measure of the amount of solute that can dissolve in a certain amount of solvent. The solubility of most solutes changes as the temperature changes (much like (3) above) and is often represented on a solubility curve. A solubility curve is quite useful in determining whether a solution is saturated (the solvent has dissolved the maximum amount of solute), unsaturated ( the solvent has dissolved less than the maximum amount of solute), or supersaturated ( the solvent has dissolved more than the maximum amount of solute).

Dissolving a solute in a solvent also effects the boiling point and melting/freezing point of the solvent. It lowers the melting/freezing point and raises the boiling point. Two examples of the usefulness of this are antifreeze and water in a car and salt on ice in the winter.

Acids and bases

Acids and bases are often used in the classification of matter. A simple definition of acid and base: acids are chemicals that produce positively-charged hydrogen ions, H+, when dissolved in water, while bases are chemicals that produce negatively-charged hydroxide ions, OH-, in water.

Acids and bases have some general properties. Many acids have a sour taste, such as citric acid, found in oranges and lemons. Acids can conduct electricity. Many acids will react with metals (corrosive), producing metal containing compounds as well as a gas, often hydrogen gas. Bases usually have a bitter taste, like caffeine. Bases make solutions that are slippery. Bases are corrosive and can conduct electricity.

A useful classification of acids and bases is the pH scale which uses a range of colors to assign the acidity or basicity to a substance. The pH scale is based on the ability of acids and bases to change colors of certain chemicals known as indicators. Litmus has been used since it turns blue in the presence of bases and red in the presence of acids. Extracts made from red onions, red cabbage, and many other fruits and vegetables change colors in the presence of acids and bases. There are three ranges on the pH scale: 1 < pH < 7 is an acid,

7< pH <14 is a base, and a pH of 7 is neutral (pure water).

Chemically, acids and bases may be considered opposites of each other. Acids and bases react with each other in a reaction called neutralization. In a neutralization reaction, the hydrogen ion and the hydroxide ion react to form a molecule of water.
 
 

Atomic Structure – from Democritus to quanta

Democritus was a Greek philosopher (450 B.C.). Legend has it that he philosphized the idea of matter being composed of tiny indivisible particles – atomos – and that each substances atomos were different from one another. Our friend Aristotle did not agree. He followed the four elements (earth, wind, fire, and water) philosophy. That meant nobody paid any attention to other ideas until the early 1000’s A.D. (kind of like his ideas on motion)

Early alchemists – 1200-1500’s – Provided large amounts of recorded data/observations of many different physical and chemical reactions while attempting to change “base” metals into “precious” metals.

John Dalton (1808) – Used previously recorded data/observations to develop the first true atomic theory:

All matter is made up of indivisible and indestructible atoms

Atoms of the same element always have the same properties.

Atoms of one element have different properties from atoms of another element

Atoms of different elements combine chemically in simple whole number ratios to form compounds

Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions

Parts of Dalton’s theory have been modified, but the basic principles underlying it have not been disproved – we just kept finding smaller pieces.

J.J. Thomson (1898) – piece number one – The electron. During the 1800’s many devices were developed to study the behavior of electricity. Two of these devices assisted in the discovery of the electron as well as identifying its electrical charge, Crooke’s Tube and the cathode ray tube (CRT). Both are gas chambers nearly completely evacuated of air. Crooke’s tube contained a paddle wheel which moved when an electric current was applied to the ends of the tube. This movement of the wheel indicated that electricity must be composed of particles, electrons. The CRT was a glass tube that had not been completely evacuated, but had some traces of gas which illuminated due to the electric current applied to the ends of the tube. This allowed the beam of electrons to be seen and its behavior observed. This helped define the charge of the electron as negative, relative to the previously assigned positive cathode and negative anode. With this information Thomson developed a model of an atom called the plum pudding model. This model explained the atom as a solid positive sphere sprinkled with negative electrons, the atom was known to be electrically neutral.

Ernest Rutherford (1910) – Rutherford and his team were studying the effects of radioactivty on various metals. According to the current models of the atom the radioactive particles should pass through thin layers of metal in a nearly straight path. Shockingly some particles were deflected by the thin foils at every angle, even straight back where they came from. “It was like firing an 18 inch artillery shell at tissue paper and having it bounce back!” was reportedly Rutherford’s response. Based on this new information it was determined that the atom must be composed of a tiny positively charged nucleus surrounded by the electrons, most of the volume of the atom had to be empty space.

Niels Bohr – Planetary model – Bohr suggested that the atom’s structure was similar to the solar system. The sun was the nucleus and the planets were the electrons, which had specific orbits. Bohr stated that the electrons were in energy levels of specific quantities of energy, or quanta. According to Bohr and current theory, the electrons could only reside in specific energy levels.

Henry Mosely – Piece number two – The proton. Mosely was studying the effect of X-rays on different elements and noticed that each had a different Z number as he called it, this turned out to be the number of protons contained in each element. The Z number, or atomic number, is unique to each element. The proton is positively charged an 2000 times as massive as the electron. The protons are located in the tiny nucleus.

Irene Joliot-Curie, James Chadwick – 1930’s – As measuring methods became more precise it was noted that much of the mass in an atom was unaccounted for by just the protons and electrons, so there must have been something else in there. The neutron was discovered through the study of the effects of radioactivity on beryllium. The neutron is neutral in charge and has a mass similar to the proton.

The structure of the atom was fairly well set at this time – A nucleus containing protons (+) and neutrons (neutral) of nearly equal mass "orbited" by electrons (-) in specific energy levels and a mass 1/2000 of the proton. The current model of the atom differs due to the discovery of particles composing protons and neutrons, such as quarks, and a less specific model of the electron behavior known as the electron cloud model. The current atomic model and theory is described and studied through quantum theory, a mathematical description/explanation of what is believed to be the true nature of the atom.

Based on information we have about the currently known elements we can identify the numbers of each sub atomic particle as well as the mass of the atom (almost entirely from protons and neutrons). Refer to Atomic Structure worksheet #1.

The mass of an atom is usually given in terms of the atomic mass unit (amu). This is defined as 1/12th the mass of the carbon atom, which has a mass of 12 amu. The amu is simply an easier unit to use for quick calculations instead of masses in terms of 10-26 grams. Another helpful unit of measure in chemistry is the mole. It turns out that the gram mass of one mole of a substance is equal in number to the amu mass of one particle of that substance. A mole is similar to a pair (2), a dozen (12), or a trio (3) in that it simply represents a number, a large number: 6.02 x 1023

In addition to the typical atomic structure of an element, some atoms of an element may have a different number of neutrons. These are called isotopes of an element. The average atomic mass of an element is a weighted average of all the naturally occurring isotopes of that element. Elements may also change by gaining or losing electrons, this results in what are known as ions.
 
 

The Periodic Table

Before 1700 we knew of 14 elements, by 1800, we knew of 33 elements, by 1869 (Mendeleev’s time) we knew of 62 elements, by 1908 we knew of 85 elements (Mendeleev’s death) and since 1908 at least 30 elements have been discovered or synthesized. That gives us 115 at this time (3/6/2000)

Dmitri Medeleev developed a periodic chart of the known elements in 1869 while teaching chemistry at the University of St. Petersburg in Russia. He simply wanted an easier way for his students to learn the known elements and their properties. He did this by making flashcards of all the known elements that listed the known properties of each. He then arranged these, first by increasing atomic mass. As he did this he noticed some repeating patterns (trends) of many properties. He then further organized the elements into groups (columns) of similar properties and periods (rows) of trends of properties. In addition Mendeleev left empty spaces where he felt elements that had yet to be discovered should be located. This was a true scientific application of his chart and gives him credit for the first true “periodic” table of the elements.

Henry Moseley developed what is considered the Modern Periodic Table by arranging the elements according to the atomic number, which he had discovered. When this arrangement is used the periodic arrangement of the elements is a natural result. This is known as the periodic law: when elements are arranged in order of increasing atomic number, certain properties of the elements repeat in a regularly repeating pattern.

Glenn Seaborg is responsible for the lanthanide and actinide series being below the main body of the table. The valence electrons for these elements reside in the f sub-level. Groups IA and IIA: valence e- located in the s sub-level, Groups IIIA-VIIIA: valence e- located in the p sub-level, transition metals: valence e- located in the d sub-level.

Valence electrons are the electrons in the outermost energy level. You can determine the # of valence electrons many elements have by which group it is in: IA-VIIIA the group # is the # of valence electrons. The transition metals are less consistent; memorization or a reference chart is the ways to go.

The period number tells you the number of energy levels an atom contains.

There are different versions of element charts organized according to different properties and usefulness of information.