6.1 How Compounds Form pp

Learning Goals By the end of this lesson, you should: Be able to explain the difference between an ionic bond and a covalent bond Be able to compare and contrast the properties of ionic and molecular compounds Be able to count how many atoms are in a chemical formula

Chemicals are everywhere Every kind of substance that you can think of is made of a type of chemical or mixture of chemicals. Water is a chemical, and the air you breathe is a mixture of chemicals.

Compounds A compound is a pure substance made up of two or more elements that are chemically combined. All compounds have properties that make them potentially useful as well as hazardous. Understanding the properties of compounds gives us the knowledge to make use of compounds safely and responsibly.

Chemical Formulae Chemical Formulae are combinations of chemical symbols used to represent compounds If only one atom of a particular element is included, no number (subscript) is written e.g. NaCl If more than one atom of a particular element is included, the number of atoms is written as a subscript following the symbol for that element. e.g. C 6 H 12 O 6

Chemical Formulae Subscripts can also be used with brackets To count atoms, you multiply the subscript outside the bracket by the one inside e.g. Mg(NO 3 ) 2 – there are 2 N’s and 6 O’s Lastly coefficients are used to tell you how many of the compound you have To count atoms, you multiply the coefficient by each subscript e.g. 2Al 2 O 3 – there are 4 Al’s and 6 O’s

Two Types of Compounds A small change in the way the atoms combine can make a big difference in the chemical and physical properties of compounds. Although millions of compounds have been discovered, almost all of them can be classified as one of two types: ionic or molecular.

Ionic Compounds Ionic compounds are formed by combining metals with non-metals in fixed proportions. An ionic compound is formed when one or more valence electrons are transferred from a metal atom to a non-metal atom.

Ionic Compounds This leaves the metal ion as a cation (positive ion) and the non-metal ion as a anion (negative ion). The two oppositely charged ions are attracted to each other by a force called a ionic bond.

Ionic Compounds Sodium chloride NaCl is a 1:1 ratio

Ionic Compounds Magnesium chloride MgCl 2 is a 1:2 ratio.

Ionic properties Ionic compounds are solids at SATP. In their solid form they form solid ionic crystals. These are more commonly known as salts

Ionic properties This structure provides ionic compounds with the general properties of high MP, BP and hardness. It takes a lot of energy to break the bonds between the ions. Any movement results in shift in where the ions are lined up and thus the like charges repel each other and the compound breaks. This explains why many ionic compounds are brittle.

Molecular Compounds When non-metals combine, a pure substance called a molecular compound is formed. In molecular compounds, the atoms share electrons to form covalent bonds. The atoms bonded together are called a molecule.

Molecular Compounds In ionic compounds metals can only bond to non-metals in so many ways. e.g. for every Ca we can only bond 2 F to it. Molecular compounds do not work that way. Molecular compounds can have various numbers of atoms bonded together to create various molecules. e.g. NO, NO 2, N 2 O 2, etc.

Molecular Compounds Due to this method of bonding there are thousands more molecular compounds than there are ionic compounds.

Molecular Compounds Most molecular compounds share the following properties: can be solids, liquids, or gases at room temperature usually good insulators but poor conductors of electricity have relatively low boiling points

Ways of Representing molecular compounds Bohr diagramball and- stick model Volume Filled model

Homework Read pp. 210 – 214 Answer p. 217 # 2,