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Chemistry 114: Chapter 10
Terms in this set (47)
Models that predict how atoms bond together to form molecules
Named after G. N. Lewis; we represent electrons as dots and draw dot structures or "Lewis structures" to represent molecules
Atoms with 8 valence electrons -- which are particularly stable
Atoms with 2 paired electrons -- Hydrogen only wants 2 electrons to be like Helium (a duet)
The sharing or transfer of electrons to attain stable electron configurations for the bonding atoms
If the electrons are transferred, the bond is ionic (metal + nonmetal)
If the electrons are shared, the bond is covalent
In chemical bonding, atoms transfer or share electrons to obtain outer shells with eight electrons (there are exceptions: hydrogen and lithium want to achieve a duet not an octet)
How do you write the Lewis structure of an ionic compound such as MgO?
1. Draw the Lewis structures of magnesium and oxygen by drawing two dots around Mg and six dots around O
2. Next, because Mg loses 2 electrons to O, write Mg2+, and oxygen gains two electrons, so it can be drawn with eight dots surrounding it, with brackets and a 2- charge.
(For ionic compounds such as CaCl2, write one Cl in brackets on the left and one on the write of the Ca2+)
Which steps should you always follow when writing Lewis structures for covalent bonds?
1. Write the correct skeletal structure for the molecule.
(The most metallic element is generally central; Halogens and Hydrogen and generally terminal; Molecules tend to be symmetrical.)
2. Calculate the total # of valence electrons
(For ions, don't forget to add/subtract electrons!)
3. Place one pair of electrons between each pair of bonded atoms to form single bonds.
4. Add remaining electrons in pairs to complete the octets of all the atoms, duet if Hydrogen (complete the octets of the terminal atoms first, leaving central atom(s) until last; if the central atom has fewer than 8 electrons, move one or more lone pairs into a bonding pair to form multiple bonds)
What is the Lewis structure for CO2?
What is the Lewis structure for CCl4?
What is the Lewis structure for CH4?
What is the Lewis structure for NH3?
What is the Lewis structure for NH4+?
What is the Lewis structure for PCl3?
What is the Lewis structure for CN-?
What are the exceptions to the Octet rule?
1. Hydrogen, Lithium, Beryllium want to have duet, not octet.
2. Atoms with odd # of electrons, such as those in group 5 and 7, will not be able to fill a complete octet. NO is an example, with 11 electrons, it can have a double bond with oxygen and 3 other electrons around Nitrogen.
3. Boron - has 3 electrons, shares 3 more electrons (3 single bonds)
4. Expanded octets - Sulfur, Phosphorous may have more than 8 electrons around a central atom in their Lewis structures
What are resonance structures?
Two or more Lewis structures that are necessary to describe the bonding in a molecule or ion
Lone pairs, single bonds, or multiple bonds
(Valence shell electron pair repulsion theory); can be used to predict the shapes of molecules. Based on the idea that electron groups repel each other. The repulsion between the negative charges of electron groups on the central atom determines the geometry of the molecule.
The geometry of a CO2 molecule; the two electron groups get as far away from each other as possible, resulting in a bond angle of 180 degrees and a linear geometry
Trigonal planar geometry
The geometry of a H2CO molecule; this molecule has three electron groups around the central atom, which want to be as far apart from one another as possible, resulting in a bond angle of 120 degrees and a trigonal planar geometry
The geometry of a CH4 molecule; this molecule has four electron groups around the central atom, which want to be as far apart as possible, resulting in a tetrahedral shape with bond angles of 109.5 degrees
The geometrical arrangement of the electron groups
The geometrical arrangement of the atoms
Trigonal pyramidal geometry
The geometry of a NH3 molecule; this molecule has three single bonds with a lone pair of electrons attached to the Nitrogen, bond angles are 109.5 degrees (electron geometry is still tetrahedral)
Tetrahedral bent geometry
The geometry of a H2O molecule; with 2 single bonds and 2 lone pairs; angle between electron groups is also 109.5 degrees
Trigonal planar bent geometry
The geometry of a SO2 molecule; with 1 double bond, 1 single bond, and 1 pair of lone electrons; the angle between electron groups is 120 degrees
What do the hashed lines notate?
Bond projecting into the paper
What does the solid wedge notate?
Bond projecting out of the paper
What is the electron and molecular geometry of PCl3?
1. It has 26 electrons
2. There are 3 single bonds with 1 pair of lone electrons around the P atom.
3. It has a tetrahedral electron geometry and the shape of the molecule (molecular geometry) is trigonal pyramidal.
What is the electron and molecular geometry of the [NO3]- ion?
1. It has 24 electrons
2. Nitrogen has two single bonds and one double bond (3 electron groups)
3. It has trigonal planar electron geometry and trigonal planar molecular geometry.
The ability of an element to attract electrons within a covalent bond is called electronegativity
What is a dipole moment?
A separation of charge within the bond
Polar covalent bonds
Covalent bonds that have a dipole moment
Greater electronegativity difference =
Greater dipole moment, more polar the bond
Which is the most electronegative element?
Fluorine, with Oxygen second
Pure covalent bonds
Electrons are evenly shared, no dipole moment (nonpolar molecule)
Electron is completely transferred, metal + nonmetal
Electronegativity Difference of 0.0-0.4 =
Pure covalent (Cl2)
Electronegativity Difference of 0.4-2.0 =
Polar covalent (HF)
Electronegativity Difference of 2.0+
One with polar bonds that add together, rather than cancelling out, which forms a net dipole moment (CO2 has polar bonds but is nonpolar since the opposite pulls cancel each other out)
How do we represent dipole moments with vectors?
The arrow has a dash through it at the partial positive end, with the point of the arrow at the partial negative end
Is NH3 polar?
NH3 has three single bonds with hydrogen and one lone pair of electrons; resulting in trigonal pyramidal geometry; this results in a polar molecule
How does polarity relate to solubility?
Polar solvents dissolve polar molecules, nonpolar solvents dissolve nonpolar molecules
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