Atomic and molecular structure

Introduction

Chemistry is the study of matter and the reactions of matter. The use of chemistry creates new types of glass, plastics, paints, fabrics, drugs, flavors, metals, and many other materials. The molecular secrets of DNA, ceramics, lasers, medicines, cosmetics, etc. are areas of chemical study. And chemists continue to improve reactions that lead to fertilizers, rocket fuel, air bags, fuel cells, and more. Your study of chemistry is one that will allow you to understand the vocabulary that chemists use to identify different types of materials and reactions. You will also learn the reasons for different properties of matter such as boiling point and solubility. And, of course, you will study types of reactions from simple decomposition reactions to acid and base reactions.

Essential vocabulary

In previous science classes you were introduced to different classifications of matter and examined different properties of matter. The vocabulary from these earlier classes is necessary to explain and describe chemistry. Thus it is essential that you know definitions of the following words fully.

Matter is something that takes up space and has mass. All substances are matter. Matter is not energy, like heat or light, and it is not a force, like electrical force, magnetic force, or a push or pull.

Substance is a term used to identify a sample of matter that is being considered. A pure substance is either an element or a compound and is one type of matter with one uniform set of properties. Substances can either be pure substances or mixtures.

Atoms are the individual particles that make up the simplest forms of matter, which are elements. If you break apart an element over and over again you will finally get to the smallest particle that is still the same element: :, this is an atom. Atoms do have other pieces parts and can be broken, but if you break an atom the pieces are not the same as the element you started with.

Elements are the simplest form of matter. If you have a pure form of an element then you cannot break it down by any chemical reaction to make a new substance. An element is composed of a single type of atom.

Compounds are pure substances with one type of properties (that is they are not mixtures). While compounds are single types of matter, like water, carbon dioxide, sugar, and salt, they can be broken down into elements or simpler compounds with a chemical reaction. For example, electricity can be used to separate water, H2O, into the elements hydrogen, H2, and oxygen, O2.

A molecule is the smallest particle of a pure substance that still has the properties of the substance. A molecule of water would be one H2O, an oxygen with two hydrogens held together by chemical bonds. A molecule of hydrogen, H2, is two hydrogen atoms combined with a chemical bond.

Mixtures are combinations of different types of substances. Sugar water, paint, milk, air, brass, granola bars are examples of mixtures. Heterogeneous mixtures are chunky and look like mixtures with different, recognizable parts. Salads and chocolate chip granola bars are good examples. Homogeneous mixtures can easily be mistaken for compounds because they look like they have only one set of properties. But if a substance can be separated without a chemical change then it is a mixture. For example, sugar water can be separated by letting the water evaporated or air can be separated by using special filters and temperature changes that separate the nitrogen, oxygen, water, carbon dioxide, and argon.

A common way of showing the difference and similarities between the different types of matter is to use a flow chart.

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The periodic table is a list of all elements organized so that the atomic number increases from lowest number to highest number. The first 92 elements are created by natural events and the other elements (26 more at this time) are made by man. The list has rows called periods that stop when the next element has properties similar to the properties of the first column of elements. Each column is a group or a family.
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IUPAC Periodic Table, modern (image)

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metals and the nonmetals. Metals are on the left and extend from lithium, Li, down to francium, Fr, then across to the right to aluminum, Al, (not borium, B) to bismuth, Bi. The nonmetals are carbon, C, and the diagonal line to radon, Rn and all elements to the right of that diagonal line. Hydrogen is a nonmetal. This leaves a set of elements that have properties that are not fully metallic called semimetals (or metalloids): boron, B; silicon, Si; gallium, Ga; arsenic, As; antimony, Sb; tellurium, Te; and polonium, Po. Semimetals slightly conduct electricity and may be brittle and without luster.

Four groups on the periodic table have distinct names and distinct properties. The groups alkali metals, alkaline earth metals, halogens, and noble gases. When we understand the chemistry of one element in any of these groups then we know that the other members of the group have similar reactions and properties. (Recall that groups are columns of elements.)

The first column of the periodic table is the alkali metals: lithium, Li; sodium, Na; potassium, K; rubidium, Rb; cesium, Cs; and francium, Fr. Alkali metals are soft metals that can be easily cut, have low melting points, have low densities (lithium, sodium, and potassium float on water), react with oxygen and water quickly. They form oxygen compounds in a 2 to 1 ratio, e.g., Na2O and K2O, and form chlorine compounds in a 1 to 1 ratios, e.g., NaCl and KCl (Watch "the explosive results when water and alkali metals come together in this video).

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