Chemical Bonding and Molecular Geometry

Chemical Bonding and Molecular Geometry

CHEMICAL BONDING AND MOLECULAR GEOMETRY Honors Chemistry Mr. Kinton Enloe High School CHEMICAL BONDS A strong attachment between 2 atoms or ions that is caused by the movement of electrons Chemical bonds are considered intra-particle forces

How the 2 atoms or ions interact with one another There are three different bond types that are significant to chemists CHEMICAL BONDS Ionic Bond: electrostatic force of attraction that exists between ions of opposite charge Formed from atoms by the transfer of one or more electrons from one atom to another

Covalent bond: electrostatic force of attraction between positive nuclei and negatively charged electrons Formed by atoms sharing 2 or more electrons Metallic Bond: electrostatic force of attraction between 2 metals Formed by positively charged metal atoms interacting with delocalized outer electrons.

IONIC BOND Typically composed of a metal and a non-metal More important a cation and an anion All ionic compounds are solids at room temperature (25o C) with high melting points All ionic compounds have high boiling points Ionic compounds form brittle solids, are not malleable and not ductile Aqueous and molten ionic compounds conduct electricty

COVALENT BONDS Can exist as solids, liquids, or gases at room temperature Covalent molecules have a wide range of melting points Most covalent molecules have a low boiling point Some covalent molecules form brittle solids Typically they are not malleable or ductile Molecules that are small and have a low mass (MW 100 amu) will be gases at room temperature

COVALENT NETWORK SOLIDS Type of solid that has a 3-D structure that is held together by covalent bonds All are solids at room temperature Quartz (SiO2), diamond (C), and graphite (C) Some form extremely hard solids Can be malleable or ductile METALLIC BOND

All metals except mercury (Hg), are solids at room temperature Most metals have extremely high boiling points Form hard solids that do not break easily Are malleable and ductile None of the metals exist as gases at room temperature Are conductors of electricity due to moving electrons Metals have luster (shiny) Malleability, ductility, luster, and electrical conductivity are all due to the free moving electrons

IDENTIFYING BOND TYPES Determine the type of bond in each of the following pairs: Na and F Cu and O H and F Si and Cl Ag and Ag Cu and Ni

LEWIS DOT DIAGRAMS A tool used to show the valence electrons for an element For each valance electron an element has, a dot is placed around the element symbol 1 dot is placed around each side before pairing electrons A maximum of 8 dots is allowed around an atom Octet rule: states that bonded atoms tend to have 8 valence electrons

LEWIS DOT DIAGRAMS Allow us to predict how many covalent bonds an atom is likely to form 7A elements like to form 1 covalent bond 6A elements like to form 2 covalent bonds 5A elements like to form 3 covalent bonds 4A elements like to form 4 covalent bonds Hydrogen and Fluorine will always form 1 covalent bond and will be terminal

LEWIS DOT DIAGRAMS Give the Lewis Dot diagram for each of the following: Se Cl Al C LEWIS DOT DIAGRAMS, LEWIS STRUCTURES AND COVALENT BONDS

From the Lewis Dot diagrams of 2 nonmetals a structure can be created to show how they are bonded together Lewis Structure: representation of covalent bonding in a molecule using Lewis symbols. Shared electrons are represented using lines connecting the 2 atoms Unshared/lone pairs/nonbonding electrons are shown as pairs of dots around the atom Only shows the valence electrons

COVALENT BONDS AND LEWIS STRUCTURES Bond Type Single Double Triple # of e-

2 4 6 1 2

3 Notation Bond Order Bond Energy Increases from single to triple Bond Length

Decreases from single to triple COVALENT BOND TERMINOLOGY Bond order: The difference between the number of bonding electron pairs and the antibonding electron pairs Bond energy/enthalpy: the energy required to break a bond when the substance is in the gas phase Bond length: distance between the centers of 2

bonded atoms HOW TO DRAW LEWIS STRUCTURES FOR COVALENT COMPOUNDS Method 1: Find the total number of valence shell electrons (vse) from all atoms Write the symbols for the atoms to show which atoms are attached to which and connect each atom with a single bond When a central atom has a group of other atoms bonded to it, the central atom is usually written first in the chemical formula

In oxyacids, the hydrogen is always bonded to an oxygen. The other atom is the central atom In some chemical formulas are written in the order the atoms are connected Hydrocarbons always have the carbon atoms attached to each other METHOD 1 Complete the octets of the atoms bonded to the central atom first Hydrogen only needs 2 electrons

Boron only needs 6 electrons Place leftover electrons onto the central atom If the central atom does not have an octet, form double and triple bonds using the appropriate atoms DRAWING LEWIS STRUCTURES FOR COVALENT COMPOUNDS Method 2 Count up the number of electrons needed to satisfy the octet

rule (Need) Count up the number of electrons available to satisfy the octet rule (Available) Determine the number of electrons that are shared by taking the difference between what you Need and what is Available (S = N A) (Shared). Then take S and divide by 2 Determine the difference between available and shared electrons (L = A S) (Left Over). Then take L and divide by 2 to get the number of lone pairs on your structure

LEWIS STRUCTURES Draw the Lewis Structures for the following molecules: H2 O2 C2H6 BH3 HCN HClO NH4+

LEWIS STRUCTURES FOR IONIC COMPOUNDS Write as many of each element symbol as indicated in the chemical formula. Include their most likely charge Draw the Lewis Dot diagram for each element. Use x for the electrons around the anion Show the transfer of electrons from the cation to the anion using arrows LEWIS STRUCTURES FOR IONIC COMPOUNDS Write the final Lewis structure for the compound

Use coefficients to indicate the number of each ion needed Place brackets around the anion with the charge inside the brackets Represent transferred electrons as dots and electrons present before transfer as x No electrons should be represented for the cation. LEWIS STRUCTURES Draw the Lewis Structure for the following ionic compounds:

LiF Na2O Al2O3 VALENCE SHELL ELECTRON PAIR REPULSION We know that like charges repel each other A similar thing happens when our atoms form bonds The electrons will repel each other if they get to close As a result our Lewis structures are rough estimates about

how our molecules are arranged VSEPR: model that accounts for the geometric arrangements of shared and unshared electrons around a central atom in terms of the repulsions between the electrons VSEPR VOCABULARY Bonding Domain: A location where electrons are being shared between 2 atoms Double and triple bonds count as only 1 bonding domain

Nonbonding Domain: Location around the central atom where lone pairs/nonbonding electrons are located Electron Domain Geometry: 3-D arrangement of the electron domains around an atom according to VSEPR Molecular Geometry: The arrangement in space of atoms in a molecule VSEPR To use VSEPR appropriately we must first count the total number of electron domains

This is the sum of the bonding and nonbonding domains around the central atom Then based on the number of domains we try to arrange the atoms attached to the central atoms as far apart from each other Chemists also look at the bond angles to maximize the distance between the atoms VSEPR alone will tell us the electron domain geometry

VSEPR AND LEWIS STRUCTURES As you can see, the VSEPR model gives us a 3-D structure however a Lewis Structure only gives us a 2D representation. To help correct that we can draw Lewis Structures using wedge-dash notation to help represent the 3-D features of our molecule A wedge represents an atom coming out towards you A dash represents an atom going in away from you MOLECULAR GEOMETRY AND BOND ANGLES

For linear molecules the bond angles do not change For trigonal planar molecules, the bond angle will change from 120o to less than 120o for bent structures For tetrahedral molecules, 2 changes can take place: If one lone pair is present, the bond angles are approximately 107.9o If 2 lone pairs are present, the bond angles are approximately 104.5o VSEPR AND REAL MOLECULES

VSEPR does a great job of producing general structures Sometimes what is predicted is not always what is observed This is because lone pairs of electrons on the central atom take up additional space and alter the shape of the molecule This also alters the bond angles around the molecule The real molecule will sometimes have a different Molecular geometry than what VSPER predicts

VSEPR Identify the electron domain geometry, molecular geometry and bond angles in the following molecules: H2 O2 BH3 HClO HCN C2H6

NH3 BOND POLARITY A measure of how equally the electrons are shared between 2 atoms in a chemical bond Ionic bond: complete transfer of electrons from one atom to another Covalent bond: sharing of electrons between atoms Nonpolar covalent: equal sharing of electrons in a bond Polar covalent: unequal sharing of electrons in a bond

This is due to differences in electronegativity BOND POLARITY Electronegativity Difference Difference Bond Type 0

Nonpolar 0-1.7 Polar 1.8 or greater Ionic

Location on the Periodic Table Look at the atoms that are bonded to each other: If 2 of the same nonmetals are bonded, it will be nonpolar If 2 different nonmetals are bonded, it will be polar

If a metal and a nonmetal are bonded, it will be ionic MOLECULE POLARITY A molecule that possesses a nonzero dipole moment The molecule has a positive and negative end (2 poles) Dipole: when 2 electrical charges of equal magnitude but opposite sign are separated by a distance Dipole moment: a quantitative measure of the

separation between the positive and negative charges This helps us determine and explain properties that we observe at a macroscopic level, in lab, and in life MOLECULE POLARITY To have a dipole moment the molecule most contain polar bonds and/or have lone pairs around the central atom As a result, polar molecules have separate centers of positive and negative charge

MOLECULAR POLARITY Polar Molecule Nonpolar Molecule HOW TO DETERMINE MOLECULAR POLARITY Symmetrical molecules with no lone pairs around the central atom and all the atoms around the central

atom are the same will be nonpolar Any tetrahedral, trigonal planar, or linear molecule satisfying these conditions will be nonpolar Any molecule that has one or more lone pairs around the central atom will be polar MOLECULAR POLARITY For each of the following molecules determine if they are polar or nonpolar:

H2 O3 HCN HClO C 2 H6 BH3 NH3

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