Week 5: Chemical Bonding. Part 1: The Octet Rule Part 2: Ionic Bonding

Transcription

1 Week 5: Chemical Bonding Part 1: The Octet Rule Part 2: Ionic Bonding

2 Part 1: The Octet Rule

3 Part 1: The Octet Rule / Objectives After this lesson I can determine the number of valence electrons elements in the main group have....use the octet rule to determine if a main group element wants to gain or lose electrons, how many it wants to gain/lose, and predict what ion it will most likely form. recall that positive ions are called cations and are almost always metals. recall that negative ions are called anions and are almost always nonmetals. identify the metals, semimetals, and nonmetals on the periodic table....recognize metals outside the main group form cations of varying charge. define and identify polyatomic ions.

4 The Main Group & Valence Electrons The Main Group of Elements are the roman numeral or A columns on the periodic table. It includes metal, semimetal an nonmetal elements. It arguable contains the most important elements on the periodic table. The column number system for the main group elements is very important because it indicates the number of valance electrons an element has. The electrons in an atom can be broken down into two categories Valence electrons: The outermost electrons that are available for bonding Core electrons: Electrons in lower energy levels that do not participate in chemical bonding Determining the number of valence electrons a main group element has is simple; just look at the column number

5 Periodic Table Showing Main Group Elements

6 Practice Problems: Valence electrons Directions: Determine the number of valence electrons in the following elements 1) Al 2) P 3) Na 4) F 5) Ca 6) Li 7) Mg 8) O 9) N 10) Cl 11) Rb 12) I

7 The Octet Rule When forming chemical bonds, atoms share or exchange their valence electrons. When exchanging, they want to get to either 8 valence electrons or none at all. Exchanging electrons means that one atom will give up electrons and another atom will accept electrons. The atom that will be giving up or losing electrons and the atom that will be accepting or gaining electrons depends on if they are closer to 8 or 0 valence electrons. If they are closer to 0 than to 8 (elements in columns 1,2, & 3) than they will lose all their valence electrons If they are closer to 8 than to 0 (elements in columns 5,6,7) than they will gain enough valance electrons to get to 8. Elements in column 8 already have 8 valence electrons so they don t chemically bond at all. We won t worry about elements in column 4 for now. This principal of chemical bonding is called the octet rule

8 Practice Problems: Gain or Lose Electrons Directions: Determine the number of valence electrons in the following elements. Indicate if the element below want to gain or lose their valance electrons. Next, indicate how many electrons they will gain or lose. 1) Al 2) P 3) Na 4) F 5) Ca 6) Li 7) Mg 8) O 9) N 10) Cl 11) Rb 12) I

9 Ion Formation Remember that a neutral atom has the same number of electrons as it does protons. Often times atoms are not neutral, they have a charge. When an atom has more protons than electrons or vice versa it is an ion. An ion is an atom that has gained or lost electrons and as a result has a positive or negative charge. There are two basic ways to determine the charge on an atom: 1) Count up all the protons and electrons and subtract the difference 10 protons & 8 electrons = +2 change 59 protons & 56 electrons = +3 charge 8 protons & 10 electrons = 2 charge 2) Just remember that if an atom loses electrons it will form a positive ion equal to the number of ions that is lost and vice versa Lose 4 electrons = +4 charge Gain 3 electrons = 3 charge Lose 5 electrons = +5 charge The charge is written as a superscript on the right side of the symbol.

10 Practice Problems: Ion formation Directions: Determine the number of valence electrons in the following elements. Indicate if the elements below want to gain or lose their valance electrons. Next, indicate how many electrons they will gain or lose. Finally, indicate what Ion the element with form and write it as a charge. 1) Al 2) P 3) Na 4) F 5) Ca 6) Li 7) Mg 8) O 9) N 10) Cl 11) Rb 12) I

11 VIDEOS! Show the Video: Science Post: The Octet Rules and Valence Charges Show the video: Virtual Academy: What are Ions?

12 More on Ions Positive Ions are called cations. Virtually all cations on are metals. The one notable exceptions is hydrogen. Negative ions are called anions. Virtually all anions are nonmetals. Most metals that are not apart of the main group don t always form the same ion. For example Iron sometimes forms Fe 2+ sometimes forms Fe 3+, and sometimes forms Fe 4+. This is a chemical property and the elements that have it are known as multivalent. Some ions are not just a single atom but a group of atoms. These ions are almost always anions. They are called polyatomic ions. You will find table of these on the back of your periodic table.

13 Periodic Table Showing Metals, Semimetals, & Nonmetals

14 Table of Polyatomic Ions Polyatomic Ions 1 2 and 3 Acetate C 2 H 3 O 2 Bicarbonate HCO 3 Chlorite ClO 2 Chlorate ClO 3 Cyanide CN Hydroxide OH Hypochlorite ClO Iodate IO 3 Nitrite NO 2 Nitrate NO 3 Perchlorate ClO 4 Permanganate MnO 4 Thiocyanate SCN Carbonate CO 2 3 Chromate CrO 2 4 Dichromate Cr 2 O 2 7 Oxalate C 2 O 2 4 Sulfite SO 2 3 Sulfate SO 2 4 Phosphate PO 3 4 Phosphite PO 3 3

15 Practice Problems: Polyatomic Ions and multivalent metals Directions: Assign the charge to the atom below if it s a main group element. Label a nonmain group metal as multivalent. Label polyatomic ions as polyatomic ion 1) Se 2) Fe 3) OH 4) SO 4 2 5) Ti 6) Br 7) Sc 8) CO 3 2 9) Ba 10) Cu 11) Mn 12) MnO 4 13) Cs 14) PO ) Co 16) Ni 17) Cr 18) ClO 19) O 20) W 21) F 22) Sr 23) Hg 24) Al 25) NO 3

16 Part 1 Additional Resources Science Post Video: Octet Rule and Valence Charges Fuse School Video: What are Ions? Tyler Dewitt s Video: What s a Polyatomic Ion

17 Part 2: Ionic Compounds

18 Part 2: Ionic Compounds / Objectives After this lesson I can recall that ionic bonding is when electrons are transferred write the formula s for Ionic compounds when presented with a cation and an anion. write the formula s for Ionic compounds when presented with an element and/or the name of a polyatomic ion identify the parent cation & parent anion in an ionic compound

19 Ionic Compounds Ionic Compounds form when a cation combines with an anion. This happens because positive charges attract negative charges. One of the simplest and classic examples of an ionic compound is table salt: NaCl (Na + ion and Cl ion) Another simple example includes Sodium Fluoride, which they put your toothpaste: NaF (Na + ion and F ion) A third example is limestone: CaO (Ca 2+ ion and O 2 ion) Ionic compounds do not always form in a 1:1 ratio though, sometimes the ratio is 2:1, 1:2, 3:1, 1:3, or several other variations. What does always happen is that the charges balance each other out. There are never charges left over

20 Ionic Compounds & The Criss Cross Method The easiest way to determine the chemical formula for an ionic compound is to use the criss cross method. Using this method the positive ion gets a subscript of equal value (but without the charge) to the negative ion s charge and vice versa. Use must reduce if possible. Examples: Ba 2+ & Cl forms BaCl 2 Al 3+ & I forms AlI 3 Na+ & O 2 forms Na 2 O Na + & N 3 forms Na 3 N Ca 2+ & S 2 forms CaS Note: that here we reduced the coefficients

21 Criss Cross Method Graphic Image Credit: UIA chemistry Department

22 Practice Problems: Writing Formulas for Ionic Compounds Directions: Use the Criss Cross Method to determine the chemical formula formed by the combination of the following ions. Reduce to lowest denominator if possible. 1) Cs + & I 2) Fe 3+ & P 3 3) Al 3+ & Cl 4) Li + & S 2 5) Al 3+ & F 6) Mg 2+ & O 2 7) Na + & I 8) K + & O 2 9) Cr 3+ & F 10) Cu 3+ & P 3 11) Ni + & S 2 12) Fe 3+ & Cl 13) Fe 2+ & Cl

23 The Criss Cross Method and Polyatomic Ions Writing Formulas for ionic compounds becomes a bit trickier when a polyatomic ion is involved. Remember that polyatmic ions are anions for the most part. If there is more than one polyatomic ion in the formula (after you have reduced if possible), you will have to use parenthesis to indicate that. Examples: Al 3+ & CN forms Al(CN) 3 Al 3+ & CO 2 3 forms Al 2 (CO 3 ) 3 Na+ & SO 2 4 forms Na 2 SO 4 Cu + & PO 3 4 forms Cu 3 PO 4 Ca 2+ & SO 2 3 forms CaSO 3 Note: that here we reduced the coefficients and there is no need for parenthesis after we do so.

24 Criss Cross Method Polyatomic Ions Graphic Image Credit: UIA chemistry Department

25 Practice Problems: Ionic Compounds w/ Polyatomic Ions Directions: Use the Criss Cross Method to determine the chemical formula formed by the combination of the following ions. Reduce to lowest denominator if possible. 1) Cs + & OH 2) Cu 2+ & CrO 4 3 3) Fe 3+ & PO 4 3 4) Fe 2+ & NO 3 5) Al 3+ & SCN 6) Mg 2+ & PO 3 3 7) Ni + & CO 3 2 8) Cr 2+ & HCO 3 9) Pb 2+ & PO ) Cs + & NO 2 11) Cr 3+ & ClO 4 12) Fe 3+ & ClO 13) Fe 2+ & CO 3 2

26 Part 2 Additional Resources Tyler Dewitt s Video: Writing Ionic Formulas: Introduction Tyler Dewitt s Video: Writing Formulas for Polyatomic Ions