Unit 4: Bonding and Chemical Formulas Two ions are talking to each other in solution. One says: "Are you a cation or an anion?" The other replys, "Oh, I'm a cation." The first asks, "Are you sure?" The reply, "I'm positive." If you don’t get it yet…you will at the end of the notes!

Electronegativity and Bonding Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to cesium and francium which are the least electronegative at 0.7

Electronegativity and Bonding

No electronegativity difference (0) between two atoms leads to a pure non-polar covalent bond. A small electronegativity difference ( ) leads to a polar covalent bond. A large electronegativity difference (1.7 or more) leads to an ionic bond.

Electronegativity and Bonding

Which type of bond will be formed by Ca and Cl?___________ Which type of bond will be formed by O and S? ____________ GENERAL RULE: ___________________________________ Ionic Covalent Metal + nonmetal = ionic Nonmetal + nonmetal = covalent

Ionic Bonds  Atoms want to be stable like the noble gases!!!  Remember that the noble gases are unreactive because they have full energy levels (ns2 np6).  This full set of 8 electrons is called an OCTET. Ahh, stability…

Ionic Bonds THIS IS WHY ATOMS FORM COMPOUNDS! (To be more stable) ***There are some exceptions to the octet rule (like transition metals).

Ionic Bonds Atoms are electrically neutral (the protons equal the electrons). When an atom loses or gains electrons, it becomes charged. When atoms are charged, they are called ions.

Ionic Bonds Positive ions are attracted to negative ions. THIS ATTRACTION IS WHAT FORMS IONIC BONDS!

Ion Formation

OXIDATION NUMBERS OF CATIONS  Metals have low electronegativity values. They also have a low number of electrons in their valence shell.  Therefore, metals typically transfer 1-3 electrons to nonmetals in ionic bonds.

Lets look at Sodium Sodium has 11 protons; 1 valence electron It is easier for Sodium to lose an electron to “be happy.” 11 p + 11 e - neutral 11 p + 10 e - cation 0+1

Practice: Aluminum is in group 13. How many valence electrons does aluminum have? ______ Will aluminum lose 3 electrons, or gain 5 electrons? ___________________________ What oxidation number will aluminum have? _____ Write the symbol for an aluminum ion: __________ Write the name for an aluminum ion: _____________

OXIDATION NUMBERS OF CATIONS LABEL YOUR PERIODIC TABLE! Group 1  loses 1 e - and has an oxidation # of 1 + Group 2  loses 2 e - and has an oxidation # of 2 + Group 13  loses 3 e - and has an oxidation # of 3 +  The transition metals are “weird” and the charges will vary. However they will ALWAYS lose electrons to form cations. For example, iron can lose 2 or 3 electrons to form two different cations: Fe +2 or Fe +3.

OXIDATION NUMBERS OF ANIONS  Nonmetals have high electronegativity values. They also have a high number of valence electrons in their valence shell.  Therefore, nonmetals typically gain electrons from metals.

Lets look at Oxygen Oxygen has 8 protons; 6 valence electron It is easier for oxygen to gain two electrons to “be happy.” 8 p + 8 e - 0 neutral 8 p + 10 e - anion O 2-

Practice: Fluorine is in group 17. How many valence electrons does fluorine have? ______ Will fluorine lose 7 electrons, or gain 1 electron? ___________________________ What charge will fluorine have? ____________ Write the symbol for a fluorine ion: ____________ ***When describing anions, an -IDE is added to the end of the element name. Write the name of the ion formed by fluorine: ____________________

OXIDATION NUMBERS OF ANIONS LABEL YOUR PERIODIC TABLE! Group 15  gains 3 e - and has an oxidation # of 3 - Group 16  gains 2 e - and has an oxidation # of 2 - Group 17  gains 1 e - and has an oxidation # of 1 -

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Method 1 – Make a neutral compound  When metals bond with nonmetals, the charge of the compound formed is 0 (neutral). The formula for the compound should contain enough of each atom to create a compound that is neutral.

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Method 1 – Make a neutral compound We Do: Write the formulas for these compounds: a. Magnesium bromide (magnesium and bromine) Ion symbols: ________________ Formula: _____________ b. Potassium nitride Ion symbols: ________________ Formula: _____________

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Method 1 – Make a neutral compound You Do: Write the formulas for these compounds: c. Aluminum sulfide Formula: _______ d. Calcium oxide Formula: _______ e. Sodium oxide Formula: _______

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Method 2 – Criss-cross the charges We Do: Write the formulas for these compounds: a. Aluminum iodide Ion symbols: ________________ Formula: _____________ b. Barium sulfide Ion symbols: ________________ Formula: _____________

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Method 2 – Criss-cross the charges You Do: Write the formulas for these compounds: c. Lithium oxide Formula: _______ d. Beryllium nitride Formula: _______ e. Calcium fluoride Formula: _______ f. Nickel (II) chloride Formula: ________

WHAT ARE POLYATOMIC IONS?  Polyatomic ions are ions that contain multiple atoms but bond as if they are one ion. Parenthesis can be used to show multiple polyatomic ions. NOTE: YOU SHOULD ALWAYS USE A CHART OF POLYATOMIC IONS TO WRITE THESE FORMULAS! (OR MEMORIZE THEM)

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Polyatomic ions We Do: Write the formulas for these compounds: a. Magnesium nitrate Ion symbols: ________________ Formula: _____________ b. Potassium sulfite Ion symbols: ________________ Formula: _____________

WRITING CHEMICAL FORMULAS FOR IONIC BONDS Polyatomic ions You Do: Write the formulas for these compounds: c. Calcium carbonate Formula: _______ d. Aluminum dichromate Formula: _______ e. Barium hydroxide Formula: _______ f. Copper (II) phosphate Formula: ________