How Compounds Form

Lesson 13 October 21 st , 2010

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.

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

and the non-metal ion as a

anion

. The two oppositely charged ions are attracted to each other by a force called a

ionic bond

. The smallest amount of substance that has the composition given by its chemical formula is the

formula

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.

The Cross over rule

• This rule allows you to figure out how many atoms you will need of each element for bonding to occur without the need to draw Bohr diagrams

The Cross over rule

• • Step 1. Write the symbols, with the metal first (the element with the positive charge)

Mg I

The Cross over rule

• Step 2 Write the Ionic charge above each symbol to indicate the stable ion that each element

2+ + Mg I

The Cross over rule

• Step 3 Draw an arrow from the metals charge to the non-metal and an arrow from the non metal charge to the metal. (Cross over the arrows)

2+ Mg + I

The Cross over rule

• • • Step 4 Fill in the number of atoms from each element will have by following the arrows. If need be reduce to lowest terms (in other words, if they are the same number, you don’t write those numbers down because you could divide the whole molecule by that number which would = 1)

MgI 2

(if the number crossed is a 1, the 1 is not shown)

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

molecules

.

Molecular Compounds

• Ionic compounds can only bond to non metals in so many ways. Ex 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. • Ex. 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 diagram ball and- stick model Volume Filled model

Questions- Home work

hand in • • • • • •

1.

What is the main difference between a compound and an element?

I (1) 2.

What two kinds of elements join together to form an ionic compound?

K (1) 3.

What is an ionic bond?

I (1) 4.

How must atoms change so that they can join to form an ionic compound?

I (1) 5.

What physical property or properties could you use to identify an ionic compound?

K (1)

Building Molecular Models lab

• Chemists use models to gain information about the shape of a molecule. The shape of a molecule is a good predictor of its properties. In this activity, you will build ball-and-stick models of simple molecules using a molecular model kit.

Building Molecular Models lab

• •

Purpose

To represent the molecules of some common Substances