Chemical Bonding

Chemical Bonds A chemical bond is a mutual attraction between a nucleus of one atom and valence electrons of another atom. Classifications of chemical bonds  Ionic: attraction between cations and anions  Covalent: sharing of electron pairs Metal with a nonmetal : ionic Nonmetal with a nonmetal: covalent

Bond Energy Energy required to break a chemical bond to produce individual atoms The closer two atoms are the greater the bond energy needed to separate them

Chemical Bonds Electronegativity Difference Bond Type 0.0 to 0.4 Nonpolar covalent (equally shared electrons) 0.4 to 1.0 Moderately polar covalent (slightly unequal sharing of electrons) 1.0 to 2.0 Highly polar covalent (highly unequal sharing of electrons) > 2.0 Ionic (transfer of electron to more electronegative atom ) Bonds are classified by the difference in electro- negativity values between the two atoms.

Examples As H H O Na 1+ Cl

Properties of Ionic Compounds Salts (solids) conducts electricity when dissolved held together by ionic bonding (cation w/ anion) high melting points high boiling points

Properties of Covalent Bonds Tend to have low melting and boiling points Have a molecular formula which shows type and number of atoms in a molecule Not necessarily the lowest ratio: C 6 H 12 O 6 Formula doesn’t tell you about how atoms are arranged

Naming Ionic Compounds

Rules for naming ionic compounds Aluminum and oxygen Write the symbols for the ions side by side. Always write the positive ion first.  Al +3 O -2 Cross over the charges.  Al 2 O 3 Check to make sure you have the smallest possible whole ratio of ions.

Rules for naming ionic compounds When naming, the end of the anions name is dropped, and –ide is added to the end.  The name of Al 2 O 3 is Aluminum Oxide  Another ionic compound is MgBr 2. What is its name? Magnesium Bromide

Ionic Compounds Containing Polyatomic Ions Many polyatomic ions are composed of the same atoms, but in different ratios.  When naming these ions, the most common ion ends in –ate.  The ion with one less oxygen ends in –ite.  For example NO 3 -1 is named nitrate  And NO 2 -1 is named nitrite An example of a compound containing a polyatomic ion is Al 2 (SO 4 ) 3  The name of the compound is Aluminum Sulfate

Ionic Compounds with Transition Metals Some elements such as certain transition metals form two or more cations with different charges. They are named using the Roman Numeral system.  One example is Iron. It forms Fe +2 and Fe +3  Copper also forms different cations. Cu +1 and Cu +2  An example of a copper compound is CuCl 2. The name of this compound is Copper II Chloride  The subscript 2 denotes the charge of the copper cation.  Another example is CrF 3 which is named, Chromium III Fluoride.

Writing formulas for names Have to know which ions form from the names  Calcium hydroxide Ca +2 and OH -1  Then need to know how to make the compound neutral Ca(OH) 2  Important that subscripts go outside the parentheses of polyatomic ions to note that there are multiple of the entire entity

IDENTIFICATION AND NAMING Acids and Bases

Properties of Acids 1. Taste sour or tart and feels like water ex. Oranges, grapefruit, yogurt, sour milk, carbonated beverages, vinegar 2. React vigorously with many metals (produces hydrogen gas) 3. Electrolyte 4. Reacts with bases to give salt and water

Properties of Bases 1. Bitter taste 2. Feel smooth and slippery ex. Soap 3. Electrolyte 4. Reacts with acids to give salt and water

Indicators A substance that turns one color in an acidic solution and another color in a basic solution Litmus paper comes from a species of lichens red indicates an acid blue indicates a base Phenolphthalein clear indicator Clear in an acid Pink in a base Others include methyl red and thymol blue

Rules for Naming Acids

1. If the anion does not contain oxygen, the acid is named with the prefix hydro- and suffix –ic attached to the root of the element Example: HCl – hydrochloric acid HCN – hydrocyanic acid H 2 S – hydrosulfuric acid

Rules for naming Acids 2. When the anion contains oxygen, the acid name is formed from the root name of the central element of the anion or anion name with the suffix –ic or – ous 1. Anions ending in –ate change to –ic 1. H 2 SO 4 sulfate  sulfic acid 2. Anions ending in –ite change to –ous 1. H 2 SO 3 sulfite  sulfurous acid 2. HNO 2 nitrite  nitrous acid

Naming Covalent Compounds The prefix in the name of a binary molecular compound tells how many atoms of an element are present in each molecule of the compound. If just one atom of the first element is in the formula, omit the prefix mono- for that element. For example: CO 2 is named carbon dioxide not monocarbon dioxide. The compound name ends in –ide, just like ionic compounds. Prefix# of Atoms Mono-1 Di-2 Tri-3 Tetra-4 Penta-5 Hexa-6 Hepta-7 Octa-8 Nona-9 Deca-10

Lewis Structures

Predicts the results of sharing valence electrons in order to achieve Noble Gas Configuration Octet Rule: Atoms tend to gain, lose or share electrons in order to acquire a full set of valence electrons Most atoms would like 8 valence electrons

Ca K Ar Al Br C He O P H Give Lewis Structures for the following atoms

Formation Ionic compounds are formed when atoms “transfer” electron(s). The result of the electron transfer creates a cation (positive ion) and an anion (negative ion)

Formation: Electron dot K K S The arrows show the electron transfer. After the transfer, each K has 19 protons and 18 electrons. This gives each K a 1+ charge. After the transfer, the S has 16 protons and 18 electrons. This gives the S a 2- charge. Note the total charge on the compound is zero.

Ionic Bond Show how Na and Cl bond

Types of Covalent Bonds Single bond – sharing 1 pair of electrons (2 electrons) Double bond – sharing 2 pairs of electrons (4 electrons) only C, O, N, S Triple bond – sharing 3 pairs of electrons (6 electrons) only C, N

Example of Covalent Bonding Fluorine has 7 valence electrons A second element would also have 7 electrons By sharing one electron, each would have 8 electrons F F

Finding the bonds Sum up the total number of valence electrons Use a pair of electrons to form a bond between each atom Arrange the rest to fill the octet rule (8 electrons each) except for hydrogen that follows the duet rule (2 electrons only)

Representing Bonds Often use a line to indicate a bond Called a structural formula Each line is 2 valence electrons