Earth Chemistry Section 2

Section 2: Combinations of Atoms

Preview• Objectives• Molecules• Compounds• Chemical Formulas• Chemical Equations• Balancing a Chemical Equation by Inspection• Chemical Bonds• Mixtures• Maps in Action

Earth Chemistry Section 2

Objectives

• Define compound and molecule.

• Interpret chemical formulas.

• Describe two ways that electrons form chemical bonds between atoms.

• Explain the differences between compounds and mixtures.

Earth Chemistry Section 2

Molecules

• Elements rarely occur in pure form in Earth’s crust. They generally occur in combination with other elements.

• compound *

• The properties of a compound differ from the properties of the elements that make up the compound.

• molecule *

Earth Chemistry Section 2

Compounds

Earth Chemistry Section 2

Chemical Formulas

• A chemical formula is a combination of letters and numbers that shows which elements make up a compound and 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)

Earth Chemistry Section 2

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.”

Earth Chemistry Section 2

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

Earth Chemistry Section 2

Chemical Equations, continued

The diagram below shows a chemical equation.

Earth Chemistry Section 2

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 from 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.

Earth Chemistry Section 2

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 or transferring electrons from one atom to another.

• Scientists can study interactions of atoms to predict which kinds of atoms will form chemical bonds together.

Earth Chemistry Section 2

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*

Ionic Bonds• ionic bond *• A compound that forms through the transfer of electrons

is called an ionic compound.

Earth Chemistry Section 2

Chemical Bonds, continued

Covalent Bonds

• covalent bond *

• A compound that forms through the sharing of electrons is called a covalent compound.

Polar Covalent Bonds

• A covalent bonds in which the bonded atoms have an unequal attraction for the shared electrons is called a polar covalent compound.

Earth Chemistry Section 2

Chemical Bonds, continued

The diagram below compares ionic bonds and covalent bonds.

Earth Chemistry Section 2

Mixtures

• mixture *

• 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.

Earth Chemistry Section 2

Mixtures, continued

Homogeneous Mixtures• In chemistry, the word homogeneous means “having the

same composition and properties throughout.”• solution a homogeneous mixture of two or more

substances that are uniformly dispersed throughout the mixture

• Liquids, gases, and solids can all be solutions. • An alloy is a solution composed of two or more metals,

such as steel.

Earth Chemistry Section 2

Maps in Action

Element Resources in the United States