1.1 The Basics of Matter
Chemistry is the study of the behavior of matter, particularly as different kinds of matter interact with each other on the very small scale of atoms. Matter is composed of atoms, which are generally referred to as indivisible particles and imagined as tiny billiard balls. Atoms are very small, being objects between 0.1 and 0.5 nm in diameter.
Atoms are, despite their assumed āindivisibility,ā composed of even smaller particles. (Atoms are in fact divisible, and the consequences of this fact are discussed in Chapter 11 of this book.) The particles that compose an atom are protons and neutrons, collectively known as nucleons, and electrons. Protons are positively charged particles and neutrons are electrically neutral particles and together carry the vast majority of the mass of the atom, whereas electrons are negatively charged particles and occupy most of the volume. Protons and neutrons have approximately the same mass while electrons are approximately 1/2000 the mass of either the proton or neutron.
Individual atoms are, in general, highly reactive and unstable, and so they tend to gather into groups known as molecules which lead to the formation of compounds. Compounds have chemical behaviors and physical properties that are distinct from the behavior of their component atoms. For example, ordinary table salt or sodium chloride has nothing in common with either sodium or chlorine, each of which is composed of a highly hazardous, reactive atom. Compounds are generally composed of simple whole number ratios of atoms combined into a new unit. The ways atoms form compounds can be broadly split into two categories, those being through covalent bonding, to make molecules where bonding electrons are shared, or ionic bonding, to make ionic compounds where electrons are effectively transferred from one atom to another. Other types of bonding and materials do exist, such as metallic bonding, necessary for the conduction of electrons in pure metals) and covalent network materials, (e.g. graphene and/or semi-conductors) but most of the concerns here will be covered by molecular and ionic compounds.
Perhaps, more to the point, chemistry is based on the premise that all matter is composed of some combination of 92 naturally occurring elements. Of these 92 elements, 11 are gases and 2 are known liquids at room temperature and pressure. Two others are solids but turn liquid at body temperature. The vast majority are solids and metals. The word ācombinationā is the key concept that gives chemistry its reputation of complexity. It may mean a mixture of elements, a mixture of compounds, or mixtures of elements with compounds. Furthermore, mixtures may be subdivided into homogeneous or heterogeneous. And to make matters even more complex, these systems of mixtures, if solutions, may exist as solids, liquids, or gases. Part of the challenge in chemistry, then, is to sort things out, organize, characterize, and identify them to understand their physical and chemical properties.
At the risk of repetition, chemistry is the branch of physical science that studies matter and the changes or āmolecular rearrangementsā it can undergo. These rearrangements, conventionally known as chemical reactions, involve the breaking of existing chemical bonds between atoms to form new bonds with other atoms. In the process, different molecules with different properties are formed. Electrons may be gained or lost, and energy changes generally accompany the reaction. Chemical reactions, as distinguished from nuclear reactions, involve the exchange of only electronsānever protons or neutrons. In a cosmic sense, chemistry is the recycling of atoms. Visible evidence of chemical reactions includes the following:
- Bubbling or fizzing, indicating the release of a gas
- Color change
- Temperature change (heat released or absorbed)
- Formation of a precipitate
- Emission of light (chemiluminescence) or sound
Examples of common chemical reactions include the following:
- Rusting of iron or corrosion of any metal
- Generation of a current by a battery
- Combustion of fuel to produce energy
- Neutralization of excess stomach acid by an antacid
- Hardening of concrete
Chemistry also studies the structure of matter, including chemical and physical properties, correlating properties on the microscopic scale with behavior observed on the macroscopic scale. Included are such properties as vapor pressure, osmotic pressure, solubility, boiling and melting points, and energy and its transformations. Many of these properties may dictate or influence the outcome of a reaction.
There are actually four, conventional, physical states in chemistry: solid (s), liquid (l), gas (g), and aqueous solution (aq). The first three are regarded as pure, consisting of only a single substance. For example, CaCl2(s) would be pure, solid calcium chloride, while CaCl2 (l) would be molten calcium chloride (at a very high temperature). The fourth physical state, solution, is a homogeneous mixture, i.e., of uniform and constant composition, where a solute and a solvent can be identified and distinguished. While attention to notation are may appear academic, it is essential and useful in understanding what products were actually formed in a chemical reaction. There are gaseous solutions like air (oxygen dissolved in nitrogen), liquid solutions like salt water (sodium chloride dissolved in water), and solid solutions like gemstones (iron or chromium atoms regularly spaced in an aluminum oxide crystal to form a sapphire or ruby, respectively). Diamond is composed of pure carbon atoms and is therefore not a solid solution. It is an allotrope of carbon; fullerene and graphite are also allotropes of carbon.
Often referred to as the universal solvent, water is a ubiquitous substance on the Earth and in the human body, and is the basis of all known life. Water-based or aqueous solutions are given the symbol (aq) inserted immediately after the chemical formula. Thus, an aqueous solution of calcium chloride would be written as CaC...
