1. 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

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.

3. 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 *

4. Compounds

5. 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 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)

6. 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.”

7. 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.
CH O – CO H2O
methane oxygen yields carbon water
dioxide

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

9. 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.

10. 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.

11. 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.

12. 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.

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

14. 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.

15. 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.

16. Maps in Action
Element Resources in the United States