What is chemistry, what do chemists do?

Chemistry is a science that describes the physical properties, composition, structure and reactions of matter.

To understand this definition more clearly we need to define some terms.

Matter – Anything that has mass and occupies space.

Atom – The smallest building block of matter.^*

Molecule – Groups of atoms held together in a specific connectivity and shape.

* In reality atoms are made up of smaller building blocks called protons, neutrons and electrons, (we will learn about these particles in chapter 2).

Matter that contains only one type of atom is called an element, i.e. oxygen, nitrogen, carbon, gold, uranium, etc. All known elements are contained in the periodic table, and each is represented by a symbol.

You should memorize the elements and their symbols found in table 1.2 (p. 10).

Matter that contains combinations of elements in specific ratios is called a compound, for example:

• Water (H₂O) - contains two hydrogen atoms for every oxygen atom
• Ethanol (C₂H₆O) - contains two carbon atoms and six hydrogen atoms for every oxygen atom
• Sodium chloride (NaCl) - contains equal numbers of sodium and chlorine atoms

Compounds can be described by their composition and structure.

Composition tells us the types of atoms that are present in a compound and the ratio of these atoms (for example H₂O, C₂H₆O, etc.).

One property of pure compounds is that they follow the law of constant composition, that is they always have the same ratio of elements regardless of how the compound was made or where it came from. The elemental ratio maintained by pure compounds originates from the number of each type of atom contained in a molecule of that compound.

Structure tells us which atoms are connected (bonded) to each other, how far apart they are, and the shape of the molecule.

Physical properties are the identifying characteristics of matter. Some properties can be readily measured with our senses, such as odor and color, instruments are needed to measure other properties, such as electrical resistivity, compressibility, hardness, melting point, etc.

Chemical properties describe the reactivity of a substance. Examples include: "ethanol burns in air", "sodium reacts vigorously with water", corrosion of metal parts (rust), the physiological effect of a drug, etc.

A less rigorous, but perhaps more descriptive picture of chemistry can be obtained by comparison with architecture.

An architect sets out to design and build a building that will serve a certain purpose, for example:

• a personal residence
• a grocery store
• an art museum
• a sports arena

A chemist sets out to design and synthesize a molecule that will have desirable properties, for example:

• conduct electricity with no resistance (Nb₃Sn)

critical component in high field magnets, MRI devices

• high strength and flexibility (Kelvar – C₁₄H₁₀O₂N₂)

high strength ropes and cables, bulletproof vests, skis, etc.

• possess a bright orange color (b-carotene – C₄₀H₅₆)

provides the orange color in some foods, including carrots

• decompose explosively (trinitroglycerin – C₆H₅(NO₂)₃ )

TNT/dynamite

• dissolve oils and dirts (alkylbenzene sulfonate – NaC₁₈H₂₉SO₃)

detergent, used in soaps and laundry detergents

The first three examples illustrate physical properties of molecules, while the last two involve chemical properties.

Just as changes in the size, shape and/or building materials can affect the way in which a building is used, changes in the structure and composition of a molecule can alter its properties. To illustrate this consider the following examples.

To illustrate how subtle changes in structure and composition affect the properties of molecules consider the following opiates

Morphine - Analgesic extracted from the opium plant. Reacts with receptors in the brain, and reduces the sensation of pain.

• Use – Pain releiver
• Side Effects – Highly addictive

Heroin - Illegal drug with effects similar to morphine. Considerably more addictive than morphine.

Codeine - Also used as a pain reliever, but with 1/10 the potency of morphine.

Thebaine – Similar to codeine, with one exception. It causes convulsions if ingested.

Here we see that very small changes in the composition and structure drastically influence the chemical properties of the morphine molecule.

To illustrate what I mean by the terms properties, composition and structure consider a liquid known widely on college campuses - Everclear

Composition ~ 95% ethanol (C₂H₆O) and 5% water (H₂O)

Structure

Ethanol

Water

Physical properties ® colorless liquid, which gives off a fairly strong odor reminiscent of vodka, low viscosity. Instruments are needed to measure other properties.

Chemical properties ® reacts rapidly in air when exposed to an open flame, leads to rapid intoxication upon introduction to the human digestive system, etc.

If I now consider just the ethanol molecule and change the structure slightly, by adding one oxygen atom to the molecule. I get ethylene glycol, which has radically different properties and uses.