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What is a compound? What is the difference between a compound and a molecule? What is an element? How many elements are there? How are compounds different than mixtures? What molecules are not compounds? How are compounds classified? Is neon (Ne) gas a compound? Is clay (HAlSiO4) a binary or ternary compound? What are ionic compounds? How do we name ionic compounds? How are compounds different than mixtures? What molecules are not compounds? How are compounds classified? Who was the first to classify materials as "compounds"?
Simple compounds:
Can a compound be classified as ionic or molecular from its formula alone?What properties distinguish ionic compounds from covalent compounds? What is a binary compound? Hydrates and addition compoundsHow can I find the formulas for common hydrates of an ionic compound?Why do some compounds absorb water from air? MoleculesWhat holds atoms together in molecules?How can I estimate the volume of a molecule? Names and formulasWhich member of a family of oxanions takes the -ate ending?Where can I look up the formula for a compound, given its name? Where can I find the structure of a compound from its name or formula? Why is the anion suffix -ide used to name molecular compounds? Where can I find the name for a specific compound, given only its molecular formula? What do you call a compound that contains a metal and an acid anion? Can you predict the formulas for binary covalent compounds? Polyatomic ionsWhat is a carbonate, and what is it used for?What is thiosulfate ion, and what is it used for? Properties of compoundsWhere do I find compound properties on the Web? |
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What is a compound? Compounds are pure substances made of atoms of two or more elements chemically combined in fixed ratios. Compounds must meet all of these criteria: * Compounds are pure substances and not mixtures. For example, table salt is not a compound because it is a mixture of sodium chloride, dextrose and calcium silicate (to keep it from clumping in high humidity) and potassium iodide (an important nutrient necessary for proper thyroid function). But if these components are separated physically, each can be obtained in pure form and each meets all the criteria for compounds. * Compounds are made of atoms of two or more elements. For example, O2 and O3 are considered forms of the element oxygen, and not compounds of oxygen. * Compounds are chemically combined elements. Mixing 2.0 grams of H2 with 70.9 g of Cl2 in the dark results in a homogenous mixture of H2 and Cl2. Mix the gases in the exact same proportions in the light and there is a violent explosion. The gas is now 72.9 g of HCl, a compound. Components in a homogenous mixture may influence each other's chemical behavior slightly, but there is not radical alteration in chemical and physical properties. Compounds are completely different chemically and physically from the elements that form them. * Atoms of elements combine in fixed ratios to form compounds. Carbon dioxide is always composed of molecules with two atoms of oxygen for every one atom of carbon. Carbon monoxide has only one oxygen atom per atom of carbon- and it is a completely different substance than carbon monoxide. Examples include water (H2O) and carbon dioxide (CO2). However, not all molecules are compounds. Compounds are pure substances that contain two or more elements combined in a definite fixed proportion. A diatomic molecule of hydrogen, though, represented by H2, is made of atoms of only one element and therefore is not regarded as a compound. A molecule is what you get when any atoms join together. A compound is what you get when atoms of two or more different elements join together. All compounds are molecules, but not all molecules are compounds. Water is a molecule because it is made from atoms that have been chemically combined. It is also a compound because the atoms that make water are not all the same - some are oxygen and some are hydrogen. Oxygen in the atmosphere is a molecule because it is made from two atoms of oxygen. It is not a compound because it is made from atoms of only one element - oxygen. This type of molecule is called a diatomic molecule, a molecule made from two atoms of the same type. What is the difference between a compound and a molecule? A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds. Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are not compounds because each is composed of a single element. Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element. The smallest bit of each of these substances would be referred to as a molecule. For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen while a single molecule of water is made from two atoms of hydrogen and one atom of oxygen. What is an element? How many elements are there? An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing a single proton and a single electron. If you change the number of protons an atom has, you change the type of element it is. If you had very, very good eyes and could look at the atoms in a sample of hydrogen, you would notice that most of the hydrogen atoms would have no neutrons, some of them would have one neutron and a few of them would have two neutrons. These different versions of hydrogen are called isotopes. All isotopes of a particular element have the same number of protons, but have a different number of neutrons. If you change the number of neutrons an atom has, you make an isotope of that element. As of October 16, 2006, scientists know of 117 different elements. Some, like gold, silver, copper and carbon, have been known for thousands of years. Others, such as meitnerium, darmstadtium and ununquadium, have only recently been created by scientists. * How are compounds different than mixtures? * What molecules are not compounds? * How are compounds classified? Compound different than a mixture Compounds are the chemical bonding of two or more different elements into a molecule. They are different than mixtures, which is a combination of two or more different materials that are not in chemical combination. Mixtures can be separated by mechanical means, while compounds can't be separated that way. Another way a compound is different than a mixture is that an individual compound has the same proportion of each element in all of its molecules. For example, the water molecule H2O is a compound that always is made up of two atoms of hydrogen and one atom of oxygen. Examples of other compounds include: Carbon monoxide: CO Carbon dioxide: CO2 Acetone: (CH3)2CO Zinc sulfide: ZnS Magnesium chloride: MgCl2 Molecules that are not compounds There are a number of molecules that are a combination of the same element. Although they can be involved in chemical reactions, they are not considered compounds. Common examples of such molecules include: Oxygen molecule: O2 Ozone: O3 Hydrogen molecule: H2 Nitrogen molecule: N2 Chlorine molecule: Cl2 Types of compounds Compounds can be classified according to the number of different elements in its molecule. The most common are the binary compound, which consists of two elements, and the ternary compound, consisting of three elements. Binary compounds have two elements Examples of binary compounds include: Table salt or sodium chloride: NaCl Iron sulfide: FeS Water: H2O Ternary compounds have three elements Examples of ternary compounds include: Sodium hydroxide: NaOH Perchloric acid: HClO4 Sulfuric acid: H2SO Since sand is silicon dioxide (SiO2), is it a compound or a mixture? Sand is a mixture, because the grains can be easily separated Pure sand (SiO2) is a compound Sand is a form of gravel, thus is has no classification Correct answer:Pure sand (SiO2) is a compound Is neon (Ne) gas a compound? Yes, because it consists of N + e It depends on what element neon combines with No, because it is a single atom Correct answer:No, because it is a single atom Is clay (HAlSiO4) a binary or ternary compound? Ternary because it has more than two elements Binary because it can be split in two Neither, since it consists of four elements Correct answer:Neither, since it consists of four elements Elements combine to form chemical compounds that are often divided into two categories. Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules. 2Na+Cl2 ------ 2NaCl Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules. 2H2+O2----------------2H2O Mixtures are heterogeneous forms of matter. Mixtures are composed of variable proportions of molecules and atoms. Compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions (1:1 depicted here). Examples of mixtures: * soil * ocean water and other solutions * air * the cytosol of a cell Examples of compounds: * water (H2O) * table salt (NaCl) * table sugar (C12H22O11) Properties of Mixtures * The composition of a mixture is variable . * Each of its components retains its characteristic properties. * Its components are easily separated. Properties of Compounds * The relative proportions of the elements in a compound are fixed. * The components of a compound do not retain their individual properties. Both sodium and chlorine are poisonous; their compound, table salt - NaCl - is absolutely essential to life. * It takes large inputs of energy to separate the components of a compound. Separating the Components of a Mixture Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative * size * density * solubility * electrical charge Dialysis Dialysis is the separation of small solute molecules or ions (e.g., glucose, Na+, Cl-) from macromolecules (e.g., starch) by virtue of their differing rates of diffusion through a differentially permeable membrane. An example: Cellophane is perforated with tiny pores that permit ions and small molecules to pass through but exclude molecules with molecular weights greater than about 12,000. If we fill a piece of cellophane tubing with a mixture of starch and sugar and place it in pure water, the sugar molecules (red dots) will diffuse out into the water until equilibrium is reached; that is, until their concentration is equal on both sides of the membrane. Because of their large size, all the starch (blue disks) will be retained within the tubing. Chromatography Chromatography is the term used for several techniques for separating the components of a mixture. Follow the links below for examples. Link to a description of paper chromatography, where the molecules are separated by size and solubility Link to a description of exclusion chromatography, where the molecules in a mixture are separated by size. Link to a description of affinity chromatography, where molecules are separated on the basis of their attraction to material in the chromatographic column. Electrophoresis Electrophoresis uses a direct electric current to separate the components of a mixture by the differing electrical charge. Link to a description of how the proteins in blood serum are separated by electrophoresis. Pure Substances v Some of the pure substances isolated from mixtures cannot be further broken down. Oxygen (O2) is an example. It is one of the elements; the fundamental building blocks of matter. Link to discussion of elements. Most pure substances are compounds. Table salt, sodium chloride (NaCl), is an example; water (H2O) is another. If we pass an electrical current through molten NaCl, two new substances will be formed: * sodium, a shiny metal so reactive that it must be stored out of contact with the air * chlorine, a yellowish poisonous gas. In this operation, a compound has been decomposed into its constitutive elements. Note the differences between separating the components of a mixture and those of a compound. * The decomposition of NaCl required a large input of energy. This is because of the strength of the ionic bonds holding the Na and Cl atoms together. * The ratio of the weights of the two products are always 23 parts of sodium to 35.5 parts of chlorine. This reflects: o the invariance of the ratio (1:1 in this case) of the number of atoms in a compound Link to discussion of valence. o the relative weights (23:35.5) of the atoms in table salt. * The properties of the components of the compound are not the same as those of the compound itself. Both sodium and chlorine are hazardous to life; their compound, sodium chloride, is a vital ingredient of all animal diets. |