JLH Chem ch 4
draw a Lewis structure for chloromethane, CH3CL
(Since C needs 4 e-) Place C in the center (H and Cl only need 1 e-) so place them on the periphery Count electrons (C has 4e-, H has 3e- because there is 3 Hs, Cl has 7e-, making a total 14 e-) Add the bonds & lone pairs
Electronegativity is a measure of an atom's attraction for electrons in a bond. The electronegativity of an atom is assigned a value from what to what?
0 to 4 the higher the value, the more electronegative an atom is & the more it is attracted to the electrons in a bond
Usually, a polar bond will be one in which the electronegativity difference between 2 atoms is what?
0.5 unit or greater
How many valence electrons does CN- have?
10 valence e-
A) How many tetrahedral carbons does dihydroxyacetone contain? B) How many trigonal planar carbons does it contain?
A) 2 B) 1
Lewis dot structure
An electron-dot structure for a molecule that shows the location of all valence electrons in the molecule, both the shared electrons in bonds and the nonbonded electron pairs.
While drawing a Lewis structure for a diatomic molecule with one bond is straightforward, drawing Lewis structures for compounds with 3 or more atoms is easier if you follow a general procedure. What is this general procedure?
Arrange the atoms next to each other that you think are bonded together. (always place hydrogens & halogens on the periphery) However many electrons needed to form an octet on an element is usually how many bonds that element will form. (N generally forms 3 bonds-place 3
How many covalent bonds are predicted for each atom: B N
B has 3 valence electrons. Thus, it is expected to form 3 bonds N has 5 valence electrons. Because it contains more than 4 valence electrons, it is expected to form 8-5=3 bonds.
Draw a Lewis structure for methanol (CH4O)
C needs 4 atoms around it O needs 2 atoms around it 14 total valence electrons
3 examples of covalent molecules in which each main group element is surrounded by 8 electrons.
CH4 (methane) NH3 (ammonia) H2O (water)
Name each compound: CS2 SO2 PCl5 BF3
CS2: carbon disulfide SO2: sulfur dioxide PCl5: phosphorus pentachloride BF3: boron trifluoride
Are these polar, nonpolar, or ionic? Cl2 HCl NaCl
Cl2: nonpolar HCl: polar covalent NaCl: ionic
Chlorate formula also give the other Cl formulas with less or more Os
ClO3- Hopochloite: ClO- Chlorite: ClO2- Perchlorate: ClO4-
When drawing a Lewis structure, if all valence electrons are used and an atom does not have an octet, what do you do?
Convert one lone pair to one bonding pair of electrons for each 2 electrons needed to complete an octet. This forms double or triple bonds.
In order to give each atom in CN- an octet, the two atoms must be joined by a triple bond, and both carbon and nitrogen have a lone pair. How do we know this?
Count the electrons (C=4e-; N=5e-; 1 more e- for the negative charge = total of 10e-) C can have up to 4 bonds; N can have up to 3 bonds To give each atom an octet = triple bond between C & N and one lone pair on both C and N
When do two atoms form covalent bonds rather than ionic bonds?
Covalent bonds are formed when two nonmetals combine or when a nonmetal and a metalloid bond.
resonance structures for tartrate C₄H₄O₆²⁻
C₄H₄O₆²⁻
How do we find the geometry of HCN (hydrogen cyanide)?
Draw Lewis structure count the groups around central atom (C). 2 groups, one H & one N surround the C. there are no lone pairs around central atom (C)
What are the 3 general rules for drawing Lewis structures?
Draw only the valence electrons 2. Give every main group element (except hydrogen) an octet of electrons 3. Give each hydrogen two electrons
Using the Lewis structure for acetylene determine the shape
Each C is surrounded by two atoms (one C & one H) and no lone pairs (two groups) linear with 180 deg
How do we determine whether CO2 is a polar or nonpolar molecule?
Each C-O bond is polar because the electronegativity difference between O (3.5) & C (2.5) is 1.0. since CO2 is linear, the 2 dipoles are equal & opposite in direction, so they cancel. No net dipole
How do we determine whether H2O is a polar or nonpolar molecule?
Each O-H bond is polar: electronegativity difference between O (3.5) & H(2.1) is 1.4. Since 2H is bent around O, the two dipoles reinforce (both point up), so H2O has a net dipole; that is, H2O is a polar molecule
Electronegativity increases across a row of the Periodic table as the _______ ______ increases (excluding the noble gases). Electronegativity decreases down a column of the periodic table as the ______ ______ increases, pushing the valence electrons farther from the nucleus.
Electronegativity increases across a row of the Periodic table as the nuclear charge increases (excluding the noble gases). Electronegativity decreases down a column of the periodic table as the atomic radius increases, pushing the valence electrons farther from the nucleus.
how many covalent bonds are predicted for each atom: F Si
F: 1 bond Si: 4 bonds
What covalent compound is a building block of proteins that compose heart muscle?
Glycine Hydrogen = White. Oxygen = Red. Chlorine = Green. Nitrogen = Blue. Carbon = Grey/Black. Sulphur = Yellow. Phosphorus = Orange
Draw an electron-dot symbol to show how a hydrogen atom and a chlorine atom form the diatomic molecule HCL.
H is surrounded by 2 electrons (same as He) Cl is surrounded by eight electrons (same as Ar)
Two examples of compounds that have an exception to the octet rule. (has P or S)
H3PO4 phosphoric acid (10 e-) H2SO4 sulfuric acid (12 e-)
What is an example of a diatomic molecule that has two different atoms?
Hydrogen fluoride (HF) called heteronuclear diatomic molecules. When atoms are not identical, the molecule forms by combining atomic orbitals of unequal energies. The result is a polar bond in which atomic orbitals contribute unevenly to each molecular orbital.
What is H2O2
Hydrogen peroxide
How do we determine the polarity of a molecule that has two or more polar bonds?
Identify all polar bonds based on electronegativity differences Determine the shape around individual atoms by counting groups Decide if individual dipoles cancel or reinforce
The molecular formula NH3 for ammonia shows that ammonia contains one nitrogen atom and three hydrogen atoms, but it does not tell us that ammonia has three covalent nitrogen-hydrogen bonds and that the N atom has a lone pair. What type of structure shows the connectivity between the atoms?
Lewis dot structure
Name each covalent molecule NO2 N2O4
NO2: nitrogen dioxide N2O4: dinitrogen tetroxide
Give the formula for diphosphorus pentoxide
P2O5
How do you arrange the electrons around the atoms in a Lewis structure?
Place one bond between every 2 atoms, giving 2 electrons to each H and no more than 8 to all other main group atoms Use all remaining electrons to fill octets with lone pairs, beginning with atoms on the periphery.
Is CH3Cl a polar or nonpolar molecule?
Polar (contains one polar bond between Cl and C)
Give the formula for silicon tetrafluoride
SiF4
How many electrons does a single covalent bond contain? double bond? triple bond?
Single bond: 2 e- Double bond: 4 e- Triple bond: 6 e-
What is CH4 (methane) geometry?
Tetrahedral (bond angle of 109°)
Are any of the bonds in H2O2 polar?
The O-O bond is nonpolar both O-H bonds are polar
Using electronegativity, how do we know what the central atom will be in a Lewis structure?
The atom with the least electronegativity (excluding Hydrogen)
What does a molecular formula show us?
The number and identity of all of the atoms in a covalent compound Does not tell us what atoms are bonded to each other.
With covalent compounds that have more than one polar bond, what determines the overall polarity?
The shape of the molecule determines the overall polarity when there is more than one polar bond.
What is the geometry of BF3?
Trigonal planar (bond angle= 120°)
resonance structure
Two Lewis structures having the same arrangement of atoms but a different arrangement of electrons.
When naming a covalent molecule we name the first by its element name and then add the prefix -ide to the second element name (oxide). What else needs to be done when there are more than one of the first or second non-metal?
Use a prefix (Mono, Di, Tri, Tetra, Penta)
Living organisms are a sea of covalent molecules. Name some covalent molecules in the body.
Water (major component of the body) Proteins that compose muscle Carbohydrates that are metabolized for energy stored fat DNA Hemoglobin
Sometimes it is not possible to give every main group element (except hydrogen) an octet of electrons by placing only single bonds in a molecule. For example, in drawing a Lewis structure for N2, each N has 5 valence electrons, so there are 10 electrons to place. If there is only one N-N bond, adding lone pairs gives one or both N's fewer than eight electrons. How do we fix this?
We must convert a lone pair to a bonding pair of electrons to form a multiple bond. Because we have 4 fewer electrons than needed, we must convert 2 lone pairs to 2 bonding pairs of electrons and form a triple bond.
When two hydrogen atoms with ___ electron each combine, they form a ________ bond that contains ___ electrons.
When two hydrogen atoms with one electron each combine, they form a covalent bond that contains two electrons.
dipole
a molecule that has two poles, or regions, with opposite charges
When we talk about groups around the central atom, what can be considered a group?
another atom or a lone pair of electrons
What are two ways the direction of polarity in a bond can be indicated?
arrow delta sign with + or -
What is the shape of H2S?
bent
When we represent the three-dimensional geometry of a tetrahedron on a two-dimensional piece of paper, we place two bonds in the plane of the paper, one bond in front, and one bond behind. What do we draw to represent: bonds in the plane bond in front of plane bond behind the plane
bonds in the plane: solid line bond in front of plane: wedge bond behind the plane: dashed wedge
Hybrid
composite of two or more resonance forms.
molecule
compound held together by covalent bonds
Most compounds that we come into contact with in our daily lives are what?
covalent compounds
A polar bond is said to be what?
dipole partial separation of charge
Draw a Lewis structure for BBr3
does not follow the octet rule (has only 6 electrons around it)
What symbol do we often use to show that 2 Lewis structures are resonance structures?
double-headed arrow ←→
How many bonds do atoms with 4 or more valence electrons form?
enough bonds to give an octet.
When the electronegativity difference is larger than 1.9 units, the electrons are essentially transferred from the less electronegative element to the more electronegative element and the bond is considered what?
ionic
According to the VSEPR theory, an atom surrounded by two groups is what?
linear, with a bond angle of 180°
What is a diatomic molecule?
molecules composed of only two atoms
Lewis Structures
more detailed. Adds lone pair electrons
Which structure, A or B, is an accurate representation for HCO3-?
neither The true structure is a composite of both resonance forms called a hybrid
What are unshared electron pairs called?
non-bonded electron pairs or lone pairs
Label the polar bonds in C2H6, and then decide if it is polar or nonpolar
nonpolar
Whenever two identical atoms are bonded together, each atom attracts the electrons in the bond to the same extent. The electrons are equally shared, and the bond is said to be what?
nonpolar
Is CH4 a polar or nonpolar molecule?
nonpolar (all of the C-H bonds are nonpolar) C has an electronegativity of 2.5 and H is 2.1 for a difference of 0.4.
Atoms with one, two, or three valence electrons generally form how many bonds?
one, two, or three bonds, respectively
Elements in group 3A of the periodic table, such as boron, do not have enough valence electrons to form an octet in a neutral molecule. What is the Lewis structure for BF3?
only 6 electrons around B
Ozone is formed in the upper atmosphere (stratosphere) by the reaction of what?
oxygen molecules (O2) with oxygen atoms (O)
Is HF polar, nonpolar, or ionic?
polar
If the individual bond dipoles do not cancel, is the molecule polar or nonpolar?
polar If the individual bond dipoles cancel, the molecule is nonpolar
Bonding between atoms of different electronegativity results in the unequal sharing of electrons. What is this called?
polar or polar covalent
What are the 2 factors that determine if an entire covalent molecule is polar or nonpolar?
polarity of the individual bonds overall shape
We say a molecule or ion that has two or more resonance structures is what?
resonance-stabilized
Molecular formula
shows how the atoms in a molecule are structurally connected
Empirical formula
shows identity & number of all atoms in a molecule
Resonance stabilizes a molecule by doing what?
spreading out lone pairs & electron pairs in multiple bonds over a larger region of space.
According to the VSEPR theory, an atom surrounded by four groups is what?
tetrahedral, with a bond angle of 109.5°
What is the +δ sign given to?
the less electronegative atom
What happens when there are two groups around an atom and the central atom has lone pairs (H2O)
the lone pair pushes the bonds away, making it bent
What is the δ- sign given to?
the more electronegative atom
net dipole movement
the overall direction the e- are being pulled towards
When two hydrogen atoms bond they form the hydrogen molecule, H2. This is an especially stable arrangement, because why?
the shared electrons (from each hydrogen) give each hydrogen atom the noble gas configuration of helium.
Another exception to the octet rule occurs with some elements located in the 3rd row & later. Explain-
these elements have empty d orbitals available to accept electrons, and thus they may have more than 8 electrons around them. Most common elements in this category are phosphorus & sulfur, which can have 10 or 12 e- around them.
Draw a Lewis structure for HCO3- and show both resonance structures
total e-, including elements valence electrons and the neg or positive (24 e- total)
According to the VSEPR theory, an atom surrounded by three groups is what?
trigonal planar, with a bond angle of 120°
What does NH3 look like?
trigonal pyramidal (one lone pair on N)
covalent bond
two electron bond in which the bonding atoms share valence electrons
VSEPR
valence shell electron pair repulsion