In Chemical Reactions,Elements combine to

form compounds

Reactions that affect an Atom

• Nuclear reactions – these type reactions change the composition of an atom’s nucleus.

• Chemical reactions – the most common one where atoms interact only through their valence electrons; the nuclei remain unchanged.

Lewis Dot Diagrams• G.N. Lewis used ‘dots’ to represent the valence

electrons around an atom in his teaching of chemical bonding.

• An electron dot diagram is a model of an atom in which each dot represents a valence electron. The chemical symbol in the middle represents the nucleus and all the other electrons in the atom.

• It is those valence electrons that are lost, gained or shared to in a chemical reaction between atoms.

Remember this?• A compound is a substance made

from two or more different atoms chemically bonded together.

• For example, water(H2O) and salt (NaCl) are compounds.

• A compound always contains two or more elements joined in fixed amounts.

Just like the Alphabet

• How many words can be formed from the 26 letters of the alphabet?

• The elements in the periodic table can likewise form millions of compounds.

• So when you think of all the substances around you, they can’t all be elements can they?

• Most substances, therefore, are compounds.

More on Compounds• The properties of a compound depend on:

– Which atoms the compound contains– How the atoms are arranged

• Atoms are not changed by chemical reactions, just rearranged into different compounds.

• Atoms are neither created nor destroyed by chemical reactions.

Chemical Formulas• A chemical formula represents the

atoms of elements and their ratios (fixed amounts) in a chemical compound.

• Example:Carbon Dioxide is a compound of 1 atom of

carbon bonded to 2 atoms of oxygen.

To indicate the chemical formula we use C and O with a subscript of 2 beside oxygen to show there are 2 atoms.

This is how we get CO2

More Chemical FormulasCompound

NameAtoms Chemical

Formula

Hydrogen Chloride

H & Cl HCl

Water H & O H2O

Hydrogen Peroxide

H & O H2O2

Ammonia N & H NH3

Methane C & H CH4

Propane C & H C3H8

Chemical bonds• Chemical bonds are the ‘glue’ that

holds the atoms of elements together in compounds.

• Chemical bonds form when the valence electrons in the electron cloud around the atoms interact.

• When atoms exchange their electrons to form compounds, the number of electrons given must equal the number taken. Therefore, all compounds are neutral.

Types of Chemical Bonds• We will learn about two types of chemical bonds – Ionic

and Covalent bonds.

• Ions are formed when atoms lose or gain electrons:– Gaining e- makes a negative ion or anion– Losing e- makes a positive ion or cation

• An element’s location on the periodic table gives clues about the type of bond an element might form with another atom.

Ionic bonds• Always involve metals• Metals bonding with non-metals or metalloids.

Ionic Bonds• What happens when an atom of an element from Group 1, like Na,

meets an atom of an element from Group 17, like Cl?

• Na loses an electron and forms a positive ion and Cl gains that electron to form a negative ion.

• What happens to particles with opposite charges?

• They are drawn toward each other by electrical attraction.

• This force of attraction between positive and negative ions is called an ionic bond.

• The overall charge on the compound formed is neutral or zero; the number of electrons lost by one atom must equal the number of electrons gained by the other atom. Also, the outer shells of each atom will be complete.

Naming Ionic Compounds• The name of an ionic compound is based on the names of the ions

that made it.

• The positive ion uses the name of the atom which formed it.

• The name of the negative ion is formed by dropping the last part of the name of the atom and adding the suffix – ide.

• To name an ionic compound, the name of the positive ion is placed first, followed by the name of the negative ion.

• Therefore, in our example the ionic compound is NaCl – Sodium Chloride.

• So, how would you name the compound with the chemical formula of BaI2 - KBr - MgF2?

Covalent Bonds• Another way in which atoms can bond together is by sharing

electrons.

• A pair of shared electrons between two atoms is called a covalent bond.

• ‘co’ means partner and ‘valent’ refers to valence.

• In a covalent bond, neither atom gains or loses an electron, therefore, no ions are formed.

• They share in pairs Each atom in the compound formed has a share in the number of electrons required to complete the outer valence shell.

• Compounds that contain only covalent bonds are called molecular compounds or molecules.

Covalent Bonds

• Nonmetals reacting with another nonmetal will usually form a covalent bond.

• A metalloid reacting with a nonmetal will usually form a covalent bond.

Covalent Bonds

Covalent BondsChemical Formula

When writing the chemical formula for a covalent bond, the elements are written in order of increasing group number. (NO for nitric oxide; not ON)

Summary Ionic vs. Covalent • Ionic Bonds – atoms lose or gain electrons to fill their

outermost energy level. (form ions)– Usually metals reacting with nonmetals will form an ionic bond– Ionic bonds are very strong – They are usually solids at room temperature.– Ionic bonds are good conductors of electricity– Ionic bonds have high melting points– Ionic bonds dissolve in water

• Covalent Bonds – atoms share electrons to fill their outermost energy level.

– Nonmetals reacting with another nonmetal will usually form a covalent bond

– They tend to be gases or liquids at room temperature– They are very poor conductors of electricity– They have low melting points and low boiling points.– Covalent bonds are weaker than ionic bonds.