Chemical Bonds & Molecular Attractions Chapter 12

Student Learning Objectives Determine the type of chemical bond & the chemical formula Describe molecular attractions & provide examples

What is a chemical bond?  A chemical bond is an attraction between atoms.  The chemical bond puts each atom in a lower energy state. Water H 2 OIbuprofen C 13 H 18 O 2

What is conserved in chemical reactions?  Mass and charge are conserved in chemical reactions.  The formula mass of the product is equal to the formula mass of the reactants.  The relative mass of a substance is always the same. (law of definite proportions) H2OH2O

Practice Calculate the formula mass for each of these compounds, & determine the relative mass. 1. CO 2 2. HCl 3. NH 3 4. CH 4

How does an electron dot structure compare to the shell model?  The Lewis electron dot symbols represent the valence electrons of the main group elements. Shows unpaired e –  The electron dot structure is based on the octet rule.

Practice Draw the electron dot structures. How do the dot structures compare to the group number? 1. H 2. C 3. Ar 4. Na

What is an ionic bond?  Ionic bonds form between positive and negative ions.  Electric force establishes bond  Strongest chemical bond Outer ShellElectronsIon Mostly FullGainedNegative Mostly EmptyLostPositive

 Ion charges must balance to give a net charge of zero when atoms bond. Na + ClBe + N

Practice 1) Ionic bonds are usually formed between metals from the left side of the periodic table and nonmetals from the far right side of the periodic table. Why? 2) Determine the chemical formula, and then name the compound. 1. Mg + S 2. Ca + Br 3. Rb + Se

What is a metallic bond?  Metallic bonds form in metals as all electrons are shared among all the nuclei in the metal. ee e e ee ee e e e eee ee  Sharing of all electrons establishes bond  Results in conductivity and malleability of metals  Ions homogeneously distributed in alloys  Weakest chemical bond Cu Ni Cu

What is a covalent bond?  Covalent bonds are formed between atoms that share pairs of electrons in the inter-nuclear region.

Covalent Sharing of e —  Sharing of electron pairs establishes the covalent bond.  There can be 1, 2, or 3 pairs of electrons shared. H – H 2e single bond C  O 4e double bond N  N 6e triple bond

Polar Molecules  In general, the farther apart (left↔right) two atoms are on the periodic table, the more polar the bond will be.

Practice 1) Which are polar molecules? Which bond would be the most polar? a) CO b) CO 2 c) H 2 O d) NH 3 2) Polar molecules tend to have high boiling points. Why?

Molecular Attractions  Molecular attractions occur as a result of polarity.  Ion-Dipole – Sodium cation and the chloride anion attract to waters – Charged species as a rule dissolve readily in water – Ionic substances are very hydrophilic (water-loving)

Cl – AnionNa + Cation Ion-Dipole Attractions

 Dipole-Dipole – Water molecules are dipoles  Ion-Induced Dipole  Dipole-Induced dipole Soda Pop (Dipole-Induced Dipole) Intermolecular force between dipoles

 Induced Dipole-Induced Dipole Instantaneous Uneven Distribution of e – Carbon Dioxide Molecule Vibrates

Practice 1)Which of the molecular attractions is the strongest? Why? 2)List the molecular interactions in order from strongest to weakest. 3)What, specifically, causes the surface tension exhibited by water?

Hydrogen Bonding  Hydrogen bonding is a special form of a dipole- dipole molecular attraction.  This is not a chemical bond (no new substance produced)  H is part of the bond pair  Very uneven sharing of e −  Results in high L f and L v for water

A hydrogen bond is the attraction between the positively charged hydrogen and a negatively charged lone pair of electrons.

Practice 1)Why does it take a while for the bubbles to leave your soda? 2)Why does it take a relatively long time for water to boil?