Compounds and Molecules Section 4.1

What are compounds? Atoms which have combined with other atoms They are pure substances They can be written using a chemical formula Like H2O, CH4, C12H22O11, etc. They have their own physical and chemical properties, which may be different than the atoms that make them up A compound always has the same chemical formula

What holds compounds together? Chemical bond: The attractive force that holds atoms or ions together There are numerous types of bonds:

How strong are chemical bonds? Chemical structure: The arrangement of bonded atoms or ions within a substance Bond length: average distance between nuclei (nucleuses) of two bonded atoms Bond angle: the angle formed by two bonds to the same atom

Models of Compounds Models help you “see” the structure of a compound by showing how atoms are arranged. Structural Formula model Ball and Stick model Space filling model

What are the major types of chemical structures? Network structures Bonded ions Molecules

Network Structures Examples: All are strong solids SiO2 (silicon dioxide, quartz, sand) MgF2 (magnesium fluoride) NaCl (table salt) All are strong solids Lots of energy is needed to break the bonds, so all have high melting points SiO2: 1700˚C MgF2: 1261˚C NaCl: 801˚C

Bonded Ions Some networks are made of bonded ions Examples: NaCl (table salt) LiF (lithium fluoride) KI (potassium iodide) Oppositely charged ions form bonds that pack themselves close together

Molecules Examples: C12H22O11 (table sugar) H2O (water) H2S (dihydrogen sulfide) Much weaker attractions for other molecules than in bonded ions or network solids Have much lower melting temperatures because of that sugar : 186˚C water: 0˚C hydrogen sulfide : -85˚C

Summary Chemical Compounds Bonded Ions Network Structures Molecules