Chapter D2, Section 1

Elements combine to form compounds.

Compounds have different properties than the elements that make them.• A compound is a substance made of atoms of 2 or

more different elements.• Most substances are compounds.• Atoms of different elements are held together in

compounds by chemical bonds.• Bonds help determine the properties of a

compound.• Properties of a compound determined by the atoms

in the compound and how they are arranged.

Cont’d.

• Properties of a compound are often very different that the properties of the elements that form them.

• Calcium (soft, silvery, metallic element)+ chorine (greenish-yellow, extremely reactive,

poisonous gas) = calcium chloride (nonpoisonous white solid)

Atoms combine in predictable numbers.

• A given compound always contains atoms of elements in a specific ratio.

• Ammonia = hydrogen + nitrogen (3) (1)

Ratio 3:1

Chemical Formulas

• A Chemical formula uses chemical symbols and subscripts to represent the atoms of elements and their ratios in a chemical compound.

• Subscripts are written to the right of an element’s symbol.

CO2 = 1 atom of carbon

(carbon dioxide) 2 atoms of oxygen

Chemical Formulas

• See steps on page D43• See Chemical Formulas chart on page D44.• Ratios are important because they determine

the compound’s identity.• The same elements can combine in different

proportions to form different compounds.