chemical bonding chapter 23. valence electrons chemical bonds result from the sharing or transfer of...
TRANSCRIPT
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
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
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
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
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 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
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
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