Chemical Bonding

Chapter 23

Valence Electrons

• Chemical bonds result from the sharing or transfer of valence electrons

• Valence electrons are those in the outermost energy level (think Bohr model)

• http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/perlewis.html#c1

Central concept

• The interaction between atoms (bonding) = ONLY involves valence electrons

• Think of the inner electrons as being “locked” tightly in filled energy level

Electron Dot Structures

Represent valence electron configurations of atom/ions of an element

Electrons are represented by dots and arranged in pairs around the elemental symbol along the four sides of a square

Can be anywhere from 1 to 8 dots around an atom

Electron Dot Structures

Electrons are arranged unpaired until they must be paired (only if there is more than 4 valence electrons)**He is the only element that does not follow this rule!

Electron Dot Structures

• Why? Because we will use dot structures of atoms to show bonding.

• Try dot structures for these:Li KrMg GaSi S

Electron Dot Structures

Li Kr

Mg Ga

Si S

Class Work

Valence Bonding Theory explains

• How many atoms are connected• How these atoms are connected• Shape of the structure

– IONIC = geometry of the structure– MOLECULAR = shape of the molecule

• Strength of the bond (bond energy)– Ionic, nonpolar covalent, polar covalent

The Octet Rule

Atoms in compounds tend to gain or lose electrons (what’s this called?) to achieve the e- configurations of a noble gas

8 valence electrons!

Cation: atom with a + charge

• The LOSS of valence e- produces a cation

• produced from metals• lose 1, 2, or 3 electrons easily

• atoms become cations to satisfy the octet rule

Anion: atom with a - charge

• The GAIN of valence e- produces an anion

• produced from nonmetals• gains 1, 2, or 3 electrons readily

• atoms become anions in order to satisfy the octet rule

AGENDA May 13:

• Submit HW

• Notes – Ionic Compounds & Metals

• CW – Properties of Ionic Compounds WS

• HW – Types of Bonds Packet – highlight, complete all “Reading Checks”and “Picture This” in margins and 1& 2 on last page

Ionic bonds and properties of ionic

compounds• An ionic compound is the result of the transfer of electrons from one set of atoms to another and consists entirely of ions

• Ionic bond - forces of attraction that bind oppositely charged ions together

• THEY MUST BE NEUTRAL!

NaCl and ionic bonding

• http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

• http://www.visionlearning.com/library/module_viewer.php?mid=55

Electron Dot Structures and Ionic Bonding

Let’s determine the correct ratio of atoms that will come

together and form an ionic bond.

1) K & O2) Li & Cl3) Mg & N

Electron Dot Structures and Ionic Bonding

Then complete Ionic Bonding WS

Ionic bonds and properties of ionic

compounds

Properties of ionic bonds

Conduct electricitywhen dissolved in water

High MP due to crystalline structure

Net charge of zero

Salts

Composed of Formula units

Crystals at room temperature

Properties of ionic compounds

1. Conduct electricity when dissolved in water (melt)

• What needs to be present in order to conduct a current?

• when ionic solids dissolve in water, they break apart into their component ions

• these ions act as electrical conductors in water

Properties of ionic compounds

2. Salts - most ionic compounds are called salts because of their crystalline structure

• Crystal - a solid that contains atoms, ions or molecules in a regular repeating three dimensional pattern

Notice

• Each Li+ ion is surrounded by four F- ions

• Each F- ion is surrounded by four Li+ ions

Lithium Fluoride Again..

Properties of ionic compounds

3. High melting point due to crystalline structure

• it will take a lot of energy to break these highly organized bonds

Properties of ionic compounds

4. Net charge of zero• anions and cations connect in a ratio such that the overall charge of the ionic compound is zero

• Example: Al+3 + Cl-1 ==> AlCl3

Properties of ionic compounds

5. Composed of formula units• a formula unit represents a compound that is ionically bound

• The lowest whole number ratio of ions represented in an ionic compound ex. NaCl instead of Na2Cl2

Let’s do this together…

• Draw electron dot structures to illustrate the formation of the following two ionic compounds…– Magnesium Chloride– Aluminum Sulfide

Metallic Bonds

• Definition: Consist of the attraction of the free-floating valence electrons for the positively charged metal ions.

• Properties: Metallic bonds involve “moveable” electrons - this explains many of the properties of metals– conductivity– malleability– ductility

Metallic Bonds

• http://www.drkstreet.com/resources/metallic-bonding-animation.swf

AGENDA 25-APR:

• Submit HW

• Notes – Properties of Covalent Compounds

• CW – Properties of covalent compounds WS

• HW – pp. 406-408 KT #3, 7-9 RQ # 6, 17-22

Covalent bonds and properties of covalent

compounds• Covalent bond - forms when electrons are shared between two atoms

H + H ==> H2 (H H)

• Atoms can have SINGLE, DOUBLE, or TRIPLE covalent bonds

Covalent bonds

• Atoms share electrons in a covalent bond so that each atom has 8 valence electrons & more stability

Covalent bonds

http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Covalent bonds and their properties

Properties of covalent bonds

Share bonding electrons Low MP and BP

Not very solublein water; poor/nonconducting

Can be gas,liquid, or solidat room temperature

Composed of molecules

No charged particles here!

Covalent bonds can be...

1. SINGLE - one pair of e- is shared between two atoms - shown as a dash (-) instead of two dots

F - F

• dash is always used in covalent bonds to show shared electrons - NEVER for ionic bonds

Covalent bonds can be...

• Dots represent UNSHARED pairs of electrons– lone pairs (around the central bonding atom & affect molecular shape)

– nonbonding pairs (around the bonded atoms)

Covalent bonds can be...

2. DOUBLE - two pair of e- is shared between two atoms

• oxygen = common example– O2 - each O has 6 valence electrons so when the O2 bond is formed, it must share TWO pair of electrons to satisfy the Octet Rule

– O = O

Covalent bonds can be...

3. TRIPLE - three pair of e- is shared between two atoms

• nitrogen = common example– N2 - each nitrogen has 5 valence electrons so to fulfill the Octet Rule, three pairs of electrons must be shared

N N

AGENDA 28-APR:

• Submit HW

• Notes – Drawing Covalent Compounds

• CW – Covalent bonding WS

• HW – Covalent bonding WS; TEST NEXT MONDAY!

Drawing covalent bonds

• Some atoms form single, double, or triple bonds when forming compounds to achieve an octet of electrons

Drawing covalent bonds

HCl: H + Cl ==> H Cl = shared pair

Shared pairs can be represented with a DASH

H-Cl HCl has THREE unshared pairs of electrons

Drawing covalent bonds

H2O: 2H + O ==> O H or O H

H H

Water has two shared pairs and two lone pairs of electrons

Drawing covalent bonds

Draw the following covalent compounds in your notebooks now:

NH3

CH4

CO2

AGENDA 29-APR:

• Submit HW

• Notes - Polarity

• CW – Polarity of Molecules WS

• HW – pp. 406-408 KT #10-13, RQ # 23-28; TEST MONDAY!

Ionic vs. Covalent bonds

and electronegativity• What if one atom has a greater electronegativity than the other but not enough strength to pull the electron away all together (form an ionic bond)?

• We land somewhere in the middle with– polar covalent bonds– non polar covalent bonds

Polar vs. Nonpolar Covalent Bonds

• Electronegativtiy measures the nucleus’ ability to attract electrons

• When bonding atoms approach each other a tug of war begins

Example: HCl

Cl nucleus is stronger so bonding e- are closer to Cl

H Cl H :Cl

Non polar covalent bonds

• Forms when atoms share electrons equally (like in the H2 example or CH4)

• Each atom or BONDED atom has equivalent electronegativity values

Non polar covalent bonds

Polar vs. Non-polar

http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Hydrogen – non-polar

Water - polar

AGENDA 30-April:

• Submit HW

• Notes – Molecular Geometry

• CW – Shapes of Molecules WS #1-6

• HW – Shapes of Molecules #7-12; TEST MONDAY!

Three dimensional chemistry

• REMEMBER– electrons = same charge = repel each other

– Lone pair electrons also repel

• because electron pairs repel, molecules adjust their shape so that the valence electron pairs are as far apart as possible = VSEPR theory

Methane (tetrahedral, CH4)

Pyramidal (ammonia, NH4)

Bent (water, H2O)

AGENDA 1-April:

• Submit HW

• Molecular Geometry Activity

• HW – Review Study Guide; TEST MONDAY!

2-MAY:

• Questions from Study Guide?

• CW: Web Quest Review