Chapter 3 Chemical Bonding

transcript

WEEK 3CHEMICAL BONDING

What is Ion

• Ion is an electrically charged atom or group of atoms.

• Cation is a positively charged ion• Anion is a negatively charged ion.• Ionization energy is the energy required to

remove one electron from a single atom in a gaseous state.

• Electron affinity is the energy released on adding an electron to a single atom in the gaseous state.

• Ionization energyThe ease with which an atom loses an electron to form a positively charged cation. Eg, Alkali metal

• Electron AffinityThe ease with which an atom gains an electron to form a negatively charged anion. Eg, Halogen

Types of Chemical Bonds

1. Ionic Bonds – Gain/Lose electrons2. Metallic Bonds – “sea of electron”3. Covalent Bonds – “sharing”

Why do atoms form bonds?

• Chemical bonds allow atoms to exist at lower energy states that are more stable.– Na and Cl – two very reactive and dangerous

substances,…» Together they make salt (NaCl)

Chemical Bonding

• Whether atoms will interact depends almost entirely on;1. The arrangement of their outer energy level

electrons (valence electrons)2. Strength of attractive force between protons and

electrons.

Outer Energy Level Electrons

• Valence electrons– Goal : get 8 valenve electrons

– Octet Rule (Rule of eight)

– Makes atoms more stable• Only s and p sublevel electrons will be valence

electrons– Core electrons – those in the lower energy levels.

What are Electron Dot Diagrams?(Lewis Dot Diagrams)

• A way of keeping track of valence electrons.• How to write them;– Write symbol– Determine number of valence electrons– Put one dot for each valence electron– Don’t pair up until they have to

Electron (Lewis) Dot Diagrams

Valence Electrons• Electrons in the highest unfilled energy level

(these are the electrons that make chemical bonds)

• Example

Lithium (Li)

1s22s1

What is the highest energy level that has electrons?

How many valence electrons does lithium have?

3 ways to determine valence electrons

1. Given a Bohr model, count the electrons in the outer energy level

2. Given an Electron Configuration, count the electrons in the highest energy level.

3. Use the periodic table

1. Given a Bohr model, count the electron in the outer energy level

• How many electrons are in carbon’s outer energy level?

• What does the electron dot diagram look like?

2. Given an electron configuration, count the electrons in the highest energy level

• Carbon’s electron configuration is:1s22s22p2

• What is the highest ENERGY LEVEL in carbon?

• How many electrons are in carbon’s highest energy level?

• What does the electron dot diagram look like?

2 + 2=4

3. Use the periodic table

Example : what column is carbon in?• Use the following table to determine how many

valence electrons that column has.

Column 1 2 13 14 15 16 17 18Valence

Electrons1 2 3 4 5 6 7 8

3 4 5 6 10987 11 12

13 14 15 16 17

2 3 4 5 6 7

Valence Electron

Column

Write the electron dot diagram for the following:

• Arsenic• Iodine• Silicon• Sulfur

Chemical bonding

• Whether atoms will interact depends almost entirely on;

1. The arrangement of their outer energy level electrons• Octet Rule

2. Strength of attractive force between protons and electrons.

Attractive Force – Protons & Electrons

• Electronegativity – the tendency of an atom to attract electrons– Helps determine the type of bond formed

between atoms.• Ionic, polar covalent, nonpolar covalent

Pair- Share

• Arrange the following from the least to greatest in electronegativity:

Sr, S, F, Fr

Ionic Bonds

• Electrons are transferred between atoms.• Metal and NonMetal– One atom gains electrons– Other atom loses electrons

• Each atom becomes an ion.

Ionic Bonds

Note : for elements in ionic compounds, show the charges on the electron dot diagrams

Why are ionic Bonds so strong?

• Anions and cations are held together by opposite charges.– Electrostatic attraction– Occurs between all the ions

Properties of Ionic Compounds

• Solid crystal at room temperature– Crystal Lattice– Ions are strongly bonded.

• High melting points• Soluble in water• Conducts electricity– When melted or dissolved– Electrolyte-compounds that conduct electricity

Practice

• Ionic compound consisting of Aluminum & Sulfur

1. Write the formula;Alluminium Sulfite

2. Draw the Lewis dot structure of the compound.

1. Ionic Bonds – Gain/Lose electrons2. Metallic Bonds – “sea of elcetron”3. Covalent Bonds – “sharing”

Metallic Bonds• Metal – Metal compound• Metals hold onto their valence electron very

weakly• Positive ions surrounded by a sea of electrons.

Properties of Metals

• Solid at room temperature• Conduct electricity• Malleable – can be hammered into a shape

(Bendable)• Ductile – drawn into wires

• What is the different between an ionic bond and metallic bond?

• What is the different in properties between ionic compounds and metallic compounds?

IONIC BONDS METALLIC BONDS• Solid crystal at room

temperature:- Crystal Lattice

• Conducts electricity- When melted or dissolved

• Brittle• High melting points• Soluble in Water

• Solid at room temperature

• Conducts electricity• Malleable• Ductile

Covalent Bonds

• Electrons are shared between atoms– Atoms that do not lose electrons easily usually

form covalent bonds• The attraction between both nuclei and he

shared electrons hold the atoms together in a covalent bond

Covalent bonding

• Fluorine has seven valence electrons• A second atom also has seven• By sharing electrons• Both end with full orbitals

Properties of Covalent Bonds

• Gases, Liquid or solids.• Low melting points• Poor electrical conductors• Not soluble in water, but soluble in nonpolar

liquids.

Electron Dot Diagram for covalent compounds

• First let’s review how to do electron dot diagrams for ionic compounds:– H2O

– CO2

Steps for Electron Dot Diagrams (Lewis structure) for Covalent Compounds

1. Count the number of valence electrons2. Write the symbols and place a bond between

each.– A single bond requires 2 share electron

3. Place a total of 8 electrons around each atom.4. If come up short, then try putting a double or

triple bond between the atoms.Draw the electron dot diagram for : Br2,NF3,CO2

Octet Rule – double & Triple Bonds

• Some atoms share more than one pair of electrons

• Double bond – two pairs of electrons are shared by the bonding atoms– Ex : CO2

• Triple bond – three pair of electrons are shared– Ex : N2

Carbon dioxide

• The only solution is to share more • Requires two double bonds• Each atom gets to count all

the atoms in the bond

8 valence electrons

Shared Electron/Bond Shortcut

• Can use a line to indicate a bond– Not for one pair electrons

• Each line is 2 valence electrons• Called a structural formula

Shared Electron / Bond Shortcut

Coordinate Covalent Bond

• When one atom donates both electrons in a covalent bond.

• Carbon monoxide (CO)

Coordinate Covalent Bonds

• When one atom donates both electrons in a covalent bond

• Carbon monoxide (CO)• SO4

Resonance

• Resonant structure : when more than one electron dot structure is possible– Ex : sulfur trioxide – SO3

– In a resonant structure, the molecule is said to exist in a hybrid state between the two.• Bond strength is greater than a single bond, but less

than a double bond

Octet Rule Exceptions• Molecules with an odd number of electron

(NO)• Elements without octets (BF3 and BeF2)

• Elements with more than an octet (SF6)

2 or more atoms chemically combined

HOW IS BOND FORMED

Attraction between positive nucleus of 1 atom to negative electrons of another (covalent bond sharing) or attraction between positive and

negative ions (ionic bonds)

WHY ATOMS BOND

To fill outermost s&p sublevels (location of valance electron)

PROCESS OF BOND

Get 8 electrons to fill them completely because you need 8 electrons to follow the octet rule because the elements wish to be like noble

COVALENT BONDS

Electrons between atoms and nuclei between 2 or more non-metal atoms sharing of valence

electrons leads to proper positioning

IONIC BOND

between metal and non-metal, transfer of valence electrons which causes attraction

between cation and anion

CRYSTAL LATTICE

where cations and anions attract to one another/basic structural unit of ionic

compounds

WHY IS 8 MOST STABLE

because when bonds are formed, energy from chemical potential account is released and when

it decreases, they become more stable

ELECTRONEGATIVITY DIFFERENCE

compares electronegativity between 2 atoms

ELECTRONEGATIVITY

Measure of how strong an atom in a molecule can attract electrons in a chemical bond

MOST ELECTRONEGATIVE

F, O, Cl, N because non-metals like to gain electrons and more reactive as you go up a

THE GREATER THE DIFFERENCE IN ELECTRONEGATIVITY

the more the electrons will be attracted to the element with the greater electronegativity

WHAT HAPPENS DURING METALLIC BONDING

metals conduct electricity as solid, mobile valance electrons(charge that can move and

conduct) --> empty overlapping orbitals especially in transition

METALLIC BONDING

attraction between delocalized electrons and metals atoms

COVALENT BONDS VARY IN STRENGTH

metallic strongest --> ionic --> covalent weakest

Chapter 3 Chemical Bonding

Science