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Molecules and Compounds Chapter 19. What we will Learn Relate chemical behavior of an element, including bonding, to its placement on the periodic table.

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Presentation on theme: "Molecules and Compounds Chapter 19. What we will Learn Relate chemical behavior of an element, including bonding, to its placement on the periodic table."— Presentation transcript:

1 Molecules and Compounds Chapter 19

2 What we will Learn Relate chemical behavior of an element, including bonding, to its placement on the periodic table Explain how elements form chemical bonds and identify the role of electrons in bonding Predict chemical formulas of compounds made up of two different elements Write chemical formulas for compounds made up of many different elements Calculate the formula mass of a compound and compare different compounds based on their formula masses

3 The Octet Rule When an atom has eight valence electrons, it is said to have an octet of electrons. Atoms form chemical bonds with other atoms by either sharing electrons, or transferring them in order to complete their octet and move to a lower energy state. This is known as the octet rule.

4 Predicting Atomic Behavior Looses 1 electron to satisfy Octet Rule Octet Rule Satisfied Looses 1 electron to satisfy Octet Rule

5 Types of Bonds Chemical Bonds fall into two categories, depending on whether the valence electrons are transferred or shared Ionic Bonds: When electron(s) are transferred from one atom to the other. Covalent Bonds: When electron(s) are shared between atoms.

6 Ionic Bonds Atoms that either gain or lose an electron become ions. Ions may have either positive or negative electric charge. The positive and negative ions are attracted to each other, creating the bond. Ionic bonds tend to form between more than one pair of atoms at a time. The bond between sodium (Na) and chlorine (Cl) in sodium chloride (salt) is a good example of an ionic bond.

7 Ionic Bonds If you put an alkali (Na) with a halogen (Cl), you get an ionic bond because one atom strongly wants to lose an electron and the other strongly wants to gain one. Alkali Metals Halogens

8 Ionic Bonds When sodium, with its +1 charge, comes into contact with chlorine, with its -1 charge, they become electrically neutral as long as they are together. This is because +1 and -1 cancel each other out. This also explains why sodium and chlorine combine in a 1:1 ratio to make sodium chloride

9 IONIC BONDING PROPERTIES OF IONIC COMPOUNDS Form between a metal and a nonmetal Brittle/crystalline High melting/boiling points Dissolve (ions come apart) in water

10 NAMING IONIC COMPOUNDS The metal (positive ion) goes first in the formula The nonmetal (negative ion) goes last in the formula The name of the metal stays the same, and the name of the nonmetal is changed so it ends in –ide. For example: Oxygen becomes Oxide

11 NAMING IONIC COMPOUNDS NaCl HF MgI 2 KBr Sodium Chloride Hydrogen Fluoride Magnesium Iodide Potassium Bromide This is called Binary Nomenclature

12 Oxidation Numbers A sodium atom always ionizes to become Na+ (a charge of +1) when it combines with other atoms to make a compound. Therefore, we say that sodium has an oxidation number of 1+. An oxidation number indicates how many electrons are lost, gained, or shared when bonding occurs.

13 Oxidation Numbers

14 Writing a Chemical Formula Using Oxidation Numbers 1.Find the oxidation numbers of each element in the compound 2. Determine the ratios of each element and then write the chemical formula

15 Oxidation Numbers Some elements have more than one oxidation number. In this case, roman numerals are used to distinguish the oxidation number. ElementOxidation Number Copper (I)1+ Copper (II)2+ Iron (II)2+ Iron (III)3+ Chromium (II)2+ Chromium (III)3+ Chromium (VI)6+ Lead (II)2+ Lead (IV)4+

16 Polyatomic Ions NameFormulaOxidation # AmmoniumNH AcetateC2H3O2-C2H3O2- 1- HydroxideOH - 1- PhosphatePO NitrateNO HydroniumH3O+H3O+ 1+ SulfateSO Polyatomic Ions: contains more than one type of atom (poly means many)

17 Writing Chemical formulas with polyatomic Ions 1. Find the formula and charge of the positive Ion 2. Find the formula and charge of the negative ion (for poly atomic ions you will not have to memorize them they will be given) 3. Determine how many of each ion are needed so the charges are equal to zero and write the chemical formula (Hint: Least common multiple) Example: Al 2 (SO 4 ) 3

18 Writing Chemical formulas with polyatomic Ions Example: Write the formula for Aluminum Sulfate 1. Find Aluminum’s Oxidation Number (it is 3+) 2. Sulfate’s oxidation number will be given (it is 2-) Al 3+ SO How many of each do we need to satisfy the charges? The common denominator between 3 and 2 is 6. 6/3 = 2 Aluminums 6/2 = 3 Sulfates Al 2 (SO 4 ) 3

19 Covalent Bonds In a covalent bond the electrons are shared between atoms. An important difference between covalent and ionic bonds is that covalent bonds act only between the atoms in a single molecule, while ionic bonds act between all adjacent atoms

20 Covalent Bonds Elements that have two to six valence electrons tend to form covalent bonds with each other since the tendency to take or receive electrons is more matched. Covalent bonds can form between two different types of atoms, or between two or more atoms of the same type. The gases nitrogen (N2), oxygen (O2) and hydrogen (H2) are a few examples. We call these covalently bonded atoms of the same type diatomic molecules.

21 This is how you DRAW a SINGLE COVALENT BOND A single line means each atom shared one electron each to get full. Covalent Bonds

22 Some atoms need more than one electron to become stable. What type of element is oxygen? What kind of bond will two oxygen atoms make? Covalent Bonds

23 –D–Draw the Electron Dot Diagram for Oxygen. –H–How many electrons does each oxygen need to become stable? –H–How many TOTAL electrons are shared? –F–FOUR

24 How would you draw this? This is a DOUBLE COVALENT BOND. 2 pairs of electrons are shared OO

25 –W–What kind of bond would 2 nitrogen atoms form? –C–COVALENT –H–How many electrons does each atom need? –T–Three –H–How many total electrons are shared? –S–Six, 3 pairs of electrons

26 –H–How would you draw this? –T–This is a TRIPLE COVALENT BOND 3 pairs shared

27 –S–Sometimes, one atom is not enough to fill all the “empty” spots another atom has. –H–How many electrons does Carbon need to be stable? –H–How many electrons does Hydrogen need to be stable?

28 How do you write the Chemical Formula for a MOLECULE? H2H2 O2O2 F2F2 N2N2 CH 4

29 Naming Covalent Molecules –The element with the lowest ionization energy goes first (farthest from Fluorine). –Each element gets a prefix to identify how many there are. –Exception: If there is only one of the first element, no prefix is used. –The second element changes so that it ends with –ide. Covalent Bonds

30 NumberPrefixNumberPrefix 1Mono-5Penta- 2Di-6Hexa- 3Tri-7Septa- 4Tetra-8Octa- Prefixes for covalent bonding nomenclature Covalent Bonds

31 Name the following molecules H 2 O CO 2 SF 6 B 2 Si SO 3 P 2 O 5 Dihydrogen monoxide Carbon dioxide Sulfur hexafluoride Diboron monosilicide Sulfur trioxide Diphosphorus pentoxide Covalent Bonds

32

33 Molecular Weight Does a molecule of water (H 2 O) have the same mass as a group of atoms (also called a formula unit) that make up the ionic compound calcium carbonate (CaCO 3 )?

34 Carbon atoms are used as a standard for determining the atomic mass units for the other elements on the periodic table. One carbon atom is equivalent to atomic mass units. Molecular Weight

35 A chemical Formula does 3 things for us: 1. Tells us the types of atoms in the compound 2. Lets you know if polyatomic ions are present 3. Allows you to calculate the Molecular Weight (MW) or Formula Mass Molecular Weight

36 Lets go back to our first example: Water vs Calcium Carbonate. Water has a chemical formula of H 2 O Hydrogen has an atomic mass of 1.01 amu Oxygen has an atomic mass of amu So one molecule of water has: 2(1.01 amu) + 1(16.00 amu) = amu Molecular Weight

37 Lets go back to our first example: Water vs Calcium Carbonate. Calcium Carbonate has a chemical Formula Of CaCO 3 Calcium has an atomic mass of amu Oxygen has an atomic mass of amu So one molecule of Calcium Carbonate has: 1(40.08 amu) + 1(12.01 amu) + 3(16.00 amu) = amu Carbon has an atomic mass of amu Molecular Weight

38 The molecular weight comparison: amu for Water vs amu for Calcium Carbonate Molecular Weight

39

40 Rules for finding Molecular Weight: 1. Write the chemical formula for the compound. 2. List the atoms, number of each atom, and the atomic mass of each atom 3. Add up the values for each type of atom Molecular Weight

41 Isomers Isomers: Compounds that have the same molecular formula but different structural formulas.

42 Isomers Example: C 3 H 6 O How many ways can we bond these atoms? (hint there are 7 ways minimum)


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