What do compounds consist of

Metals, typically found on the left side of the periodic table, are:. In contrast, non-metals, found on the right side of the periodic table to the right of the staircase , are:. Metalloids have some characteristics of metals and some characteristics of non-metals.

Silicon and arsenic are metalloids. As of November, , elements have been identified the most recently identified was ununseptium, in Of these known elements, only the first 98 are known to occur naturally on Earth. The elements that do not occur naturally on Earth are the synthetic products of man-made nuclear reactions. Hydrogen and helium are by far the most abundant elements in the universe.

The remainder is dark matter, a mysterious substance that is not composed of chemical elements. Dark matter lacks protons, neutrons, or electrons. Pure samples of isolated elements are uncommon in nature. Carbon is also commonly found in the form of coal, graphite, and diamonds. The noble gases e. Still, most of these elements are found in mixtures. When two distinct elements are chemically combined—i. Then identify the substance as either an organic compound or an inorganic compound.

B If the substance is an organic compound, arrange the elements in order beginning with carbon and hydrogen and then list the other elements alphabetically. If it is an inorganic compound, list the elements beginning with the one farthest left in the periodic table. List elements in the same group starting with the lower element and working up. C From the information given, add a subscript for each kind of atom to write the molecular formula.

Molecular formulas give only the elemental composition of molecules. In contrast, structural formulas show which atoms are bonded to one another and, in some cases, the approximate arrangement of the atoms in space. Knowing the structural formula of a compound enables chemists to create a three-dimensional model, which provides information about how that compound will behave physically and chemically.

The structural formula for H 2 can be drawn as H—H and that for I 2 as I—I, where the line indicates a single pair of shared electrons, a single bond. Carbon is unique in the extent to which it forms single, double, and triple bonds to itself and other elements.

The number of bonds formed by an atom in its covalent compounds is not arbitrary. Because the latter approximates the experimentally determined shape of the water molecule, it is more informative. Similarly, ammonia NH 3 and methane CH 4 are often written as planar molecules:. Many compounds—carbon compounds, in particular—have four bonded atoms arranged around a central atom to form a tetrahedron.

These representations differ greatly in their information content. The latter also called a wedge-and-dash representation is the easiest way to sketch the structure of a molecule in three dimensions. It shows which atoms are above and below the plane of the paper by using wedges and dashes, respectively; the central atom is always assumed to be in the plane of the paper.

Although a structural formula, a ball-and-stick model, a perspective drawing, and a space-filling model provide a significant amount of information about the structure of a molecule, each requires time and effort. Multiple groups attached to the same atom are shown in parentheses, followed by a subscript that indicates the number of such groups. For example, the condensed structural formula for methanol is CH 3 OH, which indicates that the molecule contains a CH 3 unit that looks like a fragment of methane CH 4.

Methanol can therefore be viewed either as a methane molecule in which one hydrogen atom has been replaced by an —OH group or as a water molecule in which one hydrogen atom has been replaced by a —CH 3 fragment. Because of their ease of use and information content, we use condensed structural formulas for molecules throughout this text. Ball-and-stick models are used when needed to illustrate the three-dimensional structure of molecules, and space-filling models are used only when it is necessary to visualize the relative sizes of atoms or molecules to understand an important point.

The substances described in the preceding discussion are composed of molecules that are electrically neutral; that is, the number of positively-charged protons in the nucleus is equal to the number of negatively-charged electrons. In contrast, ions are atoms or assemblies of atoms that have a net electrical charge. Ions that contain fewer electrons than protons have a net positive charge and are called cations.

Conversely, ions that contain more electrons than protons have a net negative charge and are called anions. Ionic compounds contain both cations and anions in a ratio that results in no net electrical charge. Ionic compounds contain both cations and anions in a ratio that results in zero electrical charge.

In covalent compounds, electrons are shared between bonded atoms and are simultaneously attracted to more than one nucleus. In contrast, ionic compounds contain cations and anions rather than discrete neutral molecules.

Ionic compounds are held together by the attractive electrostatic interactions between cations and anions. As shown in Equation 3. The electrostatic energy is negative only when the charges have opposite signs; that is, positively charged species are attracted to negatively charged species and vice versa. These energetic factors are discussed in greater quantitative detail later. If the electrostatic energy is positive, the particles repel each other; if the electrostatic energy is negative, the particles are attracted to each other.

One example of an ionic compound is sodium chloride NaCl , formed from sodium and chlorine. In forming chemical compounds, many elements have a tendency to gain or lose enough electrons to attain the same number of electrons as the noble gas closest to them in the periodic table.

They then have the same number of electrons as the nearest noble gas: neon. Ions such as these, which contain only a single atom, are called monatomic ions.

The charges of most monatomic ions derived from the main group elements can be predicted by simply looking at the periodic table and counting how many columns an element lies from the extreme left or right. Note that this method is ineffective for most of the transition metals, as discussed in Section 2. A Identify the group in the periodic table to which the element belongs. Based on its location in the periodic table, decide whether the element is a metal, which tends to lose electrons; a nonmetal, which tends to gain electrons; or a semimetal, which can do either.

B After locating the noble gas that is closest to the element, determine the number of electrons the element must gain or lose to have the same number of electrons as the nearest noble gas. In general, ionic and covalent compounds have different physical properties.

Ionic compounds form hard crystalline solids that melt at high temperatures and are resistant to evaporation. Covalent substances can be gases, liquids, or solids at room temperature and pressure, depending on the strength of the intermolecular interactions. Covalent molecular solids tend to form soft crystals that melt at low temperatures and evaporate easily. Some covalent substances, however, are not molecular but consist of infinite three-dimensional arrays of covalently bonded atoms and include some of the hardest materials known, such as diamond.

This topic will be addressed elsewhere. The covalent bonds that hold the atoms together in the molecules are unaffected when covalent substances melt or evaporate, so a liquid or vapor of independent molecules is formed. For example, at room temperature, methane, the major constituent of natural gas, is a gas that is composed of discrete CH 4 molecules.

When chemists synthesize a new compound, they may not yet know its molecular or structural formula. In such cases, they usually begin by determining its empirical formula, the relative numbers of atoms of the elements in a compound, reduced to the smallest whole numbers. Because the empirical formula is based on experimental measurements of the numbers of atoms in a sample of the compound, it shows only the ratios of the numbers of the elements present.

The difference between empirical and molecular formulas can be illustrated with butane, a covalent compound used as the fuel in disposable lighters. The molecular formula for butane is C 4 H The ratio of carbon atoms to hydrogen atoms in butane is , which can be reduced to The empirical formula for butane is therefore C 2 H 5.

The formula unit is the absolute grouping of atoms or ions represented by the empirical formula of a compound, either ionic or covalent. Butane has the empirical formula C 2 H 5 , but it contains two C 2 H 5 formula units, giving a molecular formula of C 4 H Because ionic compounds do not contain discrete molecules, empirical formulas are used to indicate their compositions. All compounds, whether ionic or covalent, must be electrically neutral.

Consequently, the positive and negative charges in a formula unit must exactly cancel each other. If the charges are not the same magnitude, then a cation:anion ratio other than is needed to produce a neutral compound. Ionic compounds do not contain discrete molecules, so empirical formulas are used to indicate their compositions.

An ionic compound that contains only two elements, one present as a cation and one as an anion, is called a binary ionic compound. One example is MgCl 2 , a coagulant used in the preparation of tofu from soybeans. For binary ionic compounds, the subscripts in the empirical formula can also be obtained by crossing charges: use the absolute value of the charge on one ion as the subscript for the other ion.

This method is shown schematically as follows:. Crossing charges. One method for obtaining subscripts in the empirical formula is by crossing charges.

When crossing charges, it is sometimes necessary to reduce the subscripts to their simplest ratio to write the empirical formula. Using the absolute values of the charges on the ions as subscripts gives the formula Mg 2 O 2 :. This simplifies to its correct empirical formula MgO. On the other hand, the components in a compound cannot be separated by physical means. Learn more about compounds and mixtures. Physical changes do not break down compounds.

Physical changes affect the size, shape, or state of the substance, but not the chemical properties. You can change the state of matter , but the compound does not change. If you leave an ice cube out in the sun it will melt into liquid water, but in either state it is still made of water molecules. You can apply a physical force to a solid glass and break it, but the molecules that make up glass will remain. Chemical changes in compounds happen when chemical bonds are created or destroyed.

Then the molecular structure changes; new molecules form and a new substance is created. Often heat is used to begin a chemical change, as when baking a cake. Another example of a chemical reaction is the rusting of a metal trash can. The rusting happens because the iron Fe in the metal combines with oxygen O 2 in the air.

Chemical bonds are created and destroyed to eventually make iron oxide Fe 2 O 3 , which we call rust. It is not easy to break chemical bonds, but it can be done in chemical reactions using energy to break the bonds.

For example, an electric current passed through water can cause a chemical change that breaks water down into hydrogen and oxygen. When a chemist mixes different compounds in a chemical reaction, the compounds may join together to make one compound or change into several new compounds. Some of the signs of a chemical reaction are a change in temperature, the formation of a gas, or a color change. Scientists have a specific way of naming compounds. There are some complex rules, but let's focus on the simple ones.

For molecules with two elements, the compound name has two words: the name of the first element, and the name of the second element changing its ending to "ide. If one of the elements has more than one atom, you add a prefix to the beginning of the name of the element, depending on the number of atoms. If there are two atoms, you add "di" at the beginning. If there are three, you add "tri" at the beginning. If there are four, you add "tetra. The compound of one atom of sodium and one atom of chlorine is named sodium chloride.

The compound of one atom of magnesium and one atom of sulfur MgS is named magnesium sulfide. The compound of one atom of carbon and two atoms of oxygen is named carbon dioxide.

The compound of one atom of carbon and four atoms of chlorine is named carbon tetrachloride. Compounds When two or more atoms join together, we call it a molecule. There are many other compounds that are already familiar to you: When one sodium atom Na combines with one chlorine atom Cl , it makes the compound NaCl, which we know as salt. Every time you breathe out, your breath contains CO 2 , a compound of one carbon atom C and two oxygen atoms O 2 that we call carbon dioxide.

Sometimes more than two elements make up a compound. A sugar molecule glucose is a compound of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, written as C 6 H 12 O 6. These specific atoms in these exact numbers make up a sugar molecule.