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Learning Goal •Draw Lewis structures for molecular compounds or polyatomic ions.•Draw resonance structures for certain polyatomic ions.•Predict molecular structures of molecule and ions.•Use electronegativity to determine the polarity of a bond or a molecule.•Describe the attractive forces among molecules.•Describe the changes of state between solids, liquids, and gases; calculate the energy involved.
Electron-dot formulas show• The sequence of bonded atoms in a molecule or polyatomic ion
• The bonding pairs of electrons shared between atoms
• The nonbonding or unshared (lone pairs) of electrons
• The central atom bonded to other atoms
The electron-dot formula for H2O contains:• 8 valence electrons (6 from O plus 1 from each hydrogen atom)• Two bonding pairs (between the H and O atoms) and two lone
In the valence shell electron repulsion theory (VSEPR), the three-dimensional shape of a molecule is determined by
1. Number of electron groups surrounding the central atom
2. Number of atoms bonded to the central atom
• Draw the electron-dot formula• Counting the number of electron groups (one or more electron pairs) around the central atom• Placing the electron groups as far apart as possible around the central atom
Central atoms with two electron groups (two double bonds) such as CO2 have a •linear electron-group geometry•linear shape with a bond angle of 180° to minimize repulsion
Central atoms with three electron groups (two single bonds, one double bond) such as H2CO have a •Trigonal planar electron-group geometry•Trigonal planar shape with a bond angle of 120° to minimize repulsion
Central atoms with three electron groups (a single bond, double bond, lone pair) such asSO2 have a •Trigonal planar electron-group geometry•Bent shape with a bond angle of 120° to minimize repulsion
Central atoms with four electron groups (four single bonds) such as CH4 have a •Tetrahedral electron-group geometry •Tetrahedral shape with a bond angle of 109° to minimize repulsion
The carbon-hydrogen dash indicates the bond is behind the plane of the paper,and the wedge indicates the carbon-hydrogen bond is in front of the paper plane.
Central atoms with four electron groups (three single bonds, one lone pair) such as NH3 have a •Tetrahedral electron-group geometry•Trigonal pyramidal shape with a bond angle of 109° to minimize repulsion
The wedge-dash notation represents the three-dimensional shape of the molecule.
Central atoms with four electron groups (two single bonds, two lone pairs) such as H2O have a •Tetrahedral electron-group geometry •Bent shape with a bond angle of 109° to minimize repulsion
The wedge-dash notation represents the three-dimensional shape of the molecule.
State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1) Tetrahedral (2) Trigonal pyramidal (3) Bent
State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1)tetrahedral (2) trigonal pyramidal (3) bent
State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1)tetrahedral (2) trigonal pyramidal (3) bent
We can learn more about the chemistry of compounds by understanding how electrons are shared in bonds.•Bonds formed by identical atoms share the bonding electrons equally.•Bonds formed between different atoms share the bonding electrons unequally.
Electronegativity•is the relative ability of atoms to attract shared electrons •is higher for nonmetals; fluorine has the highest with a value of 4.0 •is lower for metals; cesium and francium have the lowest value of 0.7
Figure 10.1 The electronegativity values of the representative elements in Group 1A (1) to Group 7A (17), which indicate the ability of atoms to attract shared electrons, increase going across a period from left to right and decrease going down a group.
Polarity of Covalent BondsTwo types of covalent bonds occur in molecules:•Polar covalent bonds; bonding electrons are shared unequally•Nonpolar covalent bonds; bonding electrons are shared equally
Example: Assign the polarity of the bonds:C-O N-O Cl-F
A polar molecule occurs when the dipoles from individual bonds do not cancel each other out.For molecules with two or more electron groups, the shape (such as bent or trigonal pyrimidal) determines whether or not the dipoles cancel.
Attractive forces between molecules and ions hold them close together in liquids and solids. Solids melt, and liquids boil when the attractive forces between molecules are broken. Attractive forces between molecules can be•dipole−dipole attractions•hydrogen bonding •dispersion forces
Polar molecules are attracted to each other by dipole-dipole attractions•when the positive end of one dipole is attracted to the negative end of a second dipole•such as the attractive forces between two molecules of H—Cl
Hydrogen bonds, especially strong dipole-dipole attractions, occur between•polar molecules containing hydrogen atoms bonded to very electronegative atoms such as fluorine (F), nitrogen (N), and oxygen (O) •a hydrogen atom with a partial positive charge attached to N, O, or F and a partial negative charge on N, O, or F
Dispersion forces, very weak attractive forces that occur between nonpolar molecules,•occur when movement induces a temporary distortion of the electrons in a molecule, creating a temporary dipole•make it possible for nonpolar molecules to exist as liquids and solids
Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between molecules of:
As the size and mass of similar types of molecules increase, •The attractive forces between the molecules also increase •There are more electrons available to produce stronger temporary dipolesThese increased attractive forces increase the boiling points of these molecules.
The melting point of a substance is related to the strength of the attractive forces between its particles. Molecules with•Weaker dispersion forces have lower melting points; it takes less energy to break the dispersion forces•Stronger dipole-dipole forces, or hydrogen bonds, require more energy to break the forces between them
The highest melting points occur in ionic compounds that have very strong attractive forces between positive and negative ions.Ionic compounds require large amounts of energy to break these forces and melt the substance.
Biological molecules such as proteins have many different functions. They are needed for•Structural components such as cartilage, muscles, hair, and nails•The formation of enzymes to regulate biological reactions•Transport of oxygen in blood and muscles
Hydrogen bonding inproteins also occursbetween the polarside chains on aminoacids on the outersurface of the protein,and the –OH and –Hof polar watermolecules.
In condensation water vapor is converted to a liquid as the water molecules lose kinetic energy and slow down.Condensation occurs at the same temperature as boiling, but heat is removed.
All the changes of state during the heating of a solid can be illustrated using a heating curve. Heating curves are a graph, with temperature on the vertical axis and addition of heat on the horizontal axis.Heating curves illustrate a change•of state, using a horizontal line•in temperature of a substance as energy is added or released by a sloped line
Use the cooling curve for H2O to answer each.A. Water condenses at a temperature of(1) 0 °C (2) 50 °C (3) 100 °C B. At a temperature of 0 °C, liquid water(1) freezes (2) melts (3) changes to a gasC. When water freezes, heat is(1) removed (2) added
Use the cooling curve for H2O to answer each.A. Water condenses at a temperature of(3) 100 °CB. At a temperature of 0 °C, liquid water(1) freezesC. When water freezes, heat is(1) removed