Section 2: Combinations of Atoms Preview Key Ideas Molecules Compounds Chemical Formulas Chemical Equations Balancing a Chemical Equation by Inspection Chemical Bonds Mixtures Maps in Action

Key Ideas Define compound and molecule. Interpret chemical formulas. Describe two ways that electrons form chemical bonds between atoms. Explain the differences between compounds and mixtures.

Molecules Elements rarely occur in pure form in Earth’s crust. They generally occur in combination with other elements. compound a substance made up of atoms of two or more different elements joined by chemical bonds The properties of a compound differ from the properties of the elements that make up the compound. molecule a group of atoms that are held together by chemical forces A molecule is the smallest unit of matter that can exist by itself and retain all of a substance’s chemical properties.

Compounds Click below to watch the Visual Concept.

H2O = 2 H (hydrogen atoms) + 1 O (oxygen atom) Chemical Formulas A chemical formula is a combination of letters and numbers that shows which elements make up a compound. A chemical formula shows the number of atoms of each element that are required to make a molecule of a compound. In a chemical formula, the subscript that appears after the symbol for an element shows the number of atoms of that element that are in a molecule. For example: H2O = 2 H (hydrogen atoms) + 1 O (oxygen atom)

Chemical Equations Elements and compounds often combine through chemical reactions to form new compounds. The reaction of these elements and compounds can be described in a formula called a chemical equation. Equation Structure In a chemical equation, the reactants (to the left of the arrow) form the products (to the right of the arrow) through chemical reactions. The arrow means “gives” or “yields.”

Chemical Equations, continued Equation Structure, continued In the following equation, one molecule of methane, CH4, reacts with two molecules of oxygen, O2, to yield one molecule of carbon dioxide, CO2, and two molecules of water, H2O. CH4 + 2O2  CO2 + 2H2O methane + oxygen yields carbon + water dioxide

Chemical Equations, continued The diagram below shows a chemical equation.

Chemical Equations, continued Balanced Equations A chemical equation must be balanced to be useful for showing the types and amounts of the products that could form from a particular set of reactants. An equation is balanced when the number of atoms of each element on the right side of the equation is equal to the number of atoms of the same element on the left side. To balance an equation, you must put numbers called coefficients in front of chemical formulas. A coefficient multiplies the subscripts in an equation.

Balancing a Chemical Equation by Inspection Click below to watch the Visual Concept.

Chemical Bonds The forces that hold together the atoms in molecules are called chemical bonds. Chemical bonds form because of the attraction between positive and negative charges. Atoms form chemical bonds by either sharing valence electrons or transferring them from one atom to another. Scientists can study interactions of atoms to predict which kinds of atoms will form chemical bonds together.

Chemical Bonds, continued Reading Check In what two ways do atoms form chemical bonds? Atoms form chemical bonds by transferring electrons or by sharing electrons.

Chemical Bonds, continued Ions When an electron is transferred from one atom to another, both atoms become charged. ion an atom or molecule that has gained or lost one or more electrons and has a negative or positive charge Ionic Bonds ionic bond the attractive force between oppositely charged ions, which form when electrons are transferred from one atom or molecule to another A compound that forms through the transfer of electrons is called an ionic compound.

Chemical Bonds, continued Covalent Bonds covalent bond a bond formed when atoms share one or more pairs of electrons A compound that forms through the sharing of electrons is called a covalent compound. Polar Covalent Bonds A covalent bond in which the bonded atoms have an unequal attraction for the shared electrons is called a polar covalent bond.

Chemical Bonds, continued The diagram below compares ionic bonds and covalent bonds.

Chemical Bonds, continued Reading Check Why do water molecules form from polar covalent bonds? The oxygen atom has a larger and more positively charged nucleus than the hydrogen atoms do. As a result, the oxygen nucleus pulls the electrons from the hydrogen atoms closer to it than the hydrogen nuclei pull the shared electrons from the oxygen. This unequal attraction forms a polar-covalent bond.

Mixtures mixture a combination of two or more substances that are not chemically combined Because the substances that make up a mixture keep their individual properties, a mixture can be separated into its parts by physical means. Heterogeneous Mixtures Mixtures in which two or more substances are not uniformly distributed are called heterogeneous mixtures.

Mixtures, continued Homogeneous Mixtures In chemistry, the word homogeneous means “having the same composition and properties throughout.” solution a homogeneous mixture throughout which two or more substances are uniformly dispersed Liquids, gases, and solids can all be solutions. An alloy is a solution composed of two or more metals, such as steel.

Maps in Action Element Resources in the United States