chemical bonds and compounds

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Information about chemical bonds and compounds

Published on June 14, 2008

Author: rangerblue


Unit D Chapter 2Chemical Bonds and CompoundsBy Jerry Mullins : Unit D Chapter 2Chemical Bonds and CompoundsBy Jerry Mullins Principle : Principle Matter changes form and moves from place to place The properties of compounds depend on their atoms and chemical bonds Sections 2.1 Elements Combine to Form Compounds : Sections 2.1 Elements Combine to Form Compounds Compounds have different properties from the elements that make them Atoms combine to predictable numbers Section 2.1 Objective : Section 2.1 Objective The Student will: Define Key vocabulary Describe how compounds are made from combinations of atoms Explain how chemical formulas represent compounds Model a compound in an experiment Warm-Up : Warm-Up Draw a diagram of a neutral carbon atom. A neutral carbon atom has six protons in its nucleus. On you diagram, label the nucleus and electron cloud and indicate the total positive or negative charge on each T12 Overhead Vocabulary : Vocabulary Chemical formula Subscript Compounds Chemical bonds Bonds Explore Lab : Explore Lab “Compounds” Question: How are compounds different from elements? Material: Carbon *Water *Sugar *Test tubes *Test-tube holder *Candle Procedure: Examine the lump of carbon, the beaker of water, and the sugar. Record your observations. Pour some sugar into a test tube and heat it over a candle for several minutes. Record you observation What do you think?: The sugar is made up of atoms of the same elements that are in the carbon and water. How are sugar, carbon, and water different from one another? Does heating the sugar give you and clue that sugar contains more than one element? Compounds : Compounds Compound: a combination of two or more elements What makes a compound different from a mixture is that atoms of the elements in a compound are held together by chemical bonds Chemical bonds: can hold atoms together in large networks or in small group (think of it as “glue”) Bonds: help determine the properties of a compound The proportion of atoms are always fixed Slide 9: Compounds Most elements do not exist by themselves Readily combine with other elements in a predictable fashion Compounds : Compounds The properties of a compound depends not only on which atoms the compound contains, but also on how the atoms are arranged. Example: atoms of Carbon 6 (C) and Hydrogen 1 (H) can combine to form many thousands of different compounds, such as: Natural gas; Components of automobile gasoline; Hard waxes in candles; and Many plastics. Compounds : Compounds Remember: the properties of compounds are often very different from the properties of the elements that make them, another example: Water is made from two atoms of hydrogen bonded to one atom of oxygen. at room temperature, Hydrogen 1 (H) and Oxygen 8 (O) are both colorless, odorless gases, and they remain gases down to extremely low temperatures. Water (H2O) is a liquid at temperatures up to 100° C (212° F) and a solid below 0° C (32° F) What melts the ice on our roads? : What melts the ice on our roads? Calcium Chloride: is nonpoisonous white solid, which melts the ice that form on streets in the wintertime this compound is made up of the elements: Calcium 20 (Ca): soft, silvery metallic solid Chlorine 17 (Cl): greenish-yellow gas that is extremely reactive and poisonous to humans + = Calcium + Chlorine = Calcium Chloride Atoms combine in predictable numbers : Atoms combine in predictable numbers A given compound always contains atoms of elements in a specific RATIO Example: Ammonia always has 3 Hydrogen atoms for every 1 Nitrogen atom—3 to 1 ratio of Hydrogen to Nitrogen (3:1). However: if we change the above ratio we get Hydrazoic acid, which also contains atoms of hydrogen and nitrogen but in a different ratio. Hydrazoic acid has a ratio of 1 hydrogen atom to 3 nitrogen atoms (1:3). Chemical Formulas : Chemical Formulas Chemical formula shows the kind and proportion of atoms of each element that occurs in a particular compound Remember that atoms of elements can be represented by their chemical symbols. Therefore, a Chemical Formula uses these chemical symbols to represent the atoms of the elements and their ratios in a chemical compound What is a “subscript” : What is a “subscript” Simple put, it is a number written to the right of a chemical symbol and slightly below it. Used in writing a chemical formulas The subscript of “1” is never written, only number “2” or more Writing a chemical formula : Writing a chemical formula Carbon Dioxide for example, consist of 1 atom of Carbon attached by chemical bonds to two atoms of oxygen. This is how we would write this: Find the symbols for Carbon (C) and oxygen (O) on the periodic table. Write these symbols side by side To indicate that there are two oxygen atoms for every carbon atoms, place the subscript “2” to the right of the oxygen atom’s symbol Because there is only one carbon in carbon dioxide, you need no subscript for carbon. CO2 means 1:3 ratio Investigative Lab : Investigative Lab “Element Ratios” Question: How can you model a compound? Materials: nuts and bolts Procedure: Collect a number of nuts and bolts. The nuts represent hydrogen atoms. The bolts represent carbon atoms. Connect the nuts to the bolts to model the compound methane. Methane contains four hydrogen atoms attached to one carbon atoms. Make as many of these models as you can Count the nuts and bolts left over What do you think?: What ratio of nuts to bolts did you use to made a model of a methane atom? How many methane models did you make? Why couldn’t you make more? Challenge: The compound ammonia has one nitrogen atom and three hydrogen atoms. How could you use the nuts and bolts to model this compound? What are the three main types of chemical bonds? : What are the three main types of chemical bonds? Metallic Ionic Covalent Section 2.2Chemical bonds hold compounds together : Section 2.2Chemical bonds hold compounds together Chemical bonds between atoms involve electrons Atoms can transfer electrons. Atoms can share electrons. Chemical bonds give all materials their structures. Section 2.2 Objective : Section 2.2 Objective THE STUDENT WILL: Define Key vocabulary; Explain how electrons are involved in chemical bonding; Describe what the different types of chemical bonds are; Determine how chemical bonds affect structure; and Observe how a crystal grows in an experiment. Warm-Up : Warm-Up Match each definition to a term T12 Overhead Teacher Demo : Teacher Demo TE 49 Demonstrate how opposite charges attract Misconceptions : Misconceptions TE 48 Vocabulary : Vocabulary Ionic bond Covalent bond Molecule Polar covalent bond Explore Lab : Explore Lab “Think about it” Question: How do you keep things together? TE p 47 Electron role in forming compounds : The tendency of elements to combine and form compounds depends on the number and arrangement of electrons in their atoms. Atoms are most stable when their outer most energy level is filled Electron role in forming compounds Why do atoms naturally combine into compounds? : Most atoms are not stable in their natural state Tend to react (combine) with other atoms in order to become more stable (undergo chemical reactions) In chemical reactions bonds are broken; atoms rearranged and new chemical bonds are formed Why do atoms naturally combine into compounds? Remember why chemical bonds form? : Remember why chemical bonds form? Atoms form bonds in order to become more stable. According to the Octet Rule, atoms will form bonds by gaining, losing, or sharing valence electrons in order to obtain an octet (8 valence electrons). Atoms transfer electrons : Atoms transfer electrons Ions form when atoms gain or lose electrons Gaining electrons changes an atom into a negative ion Losing electrons changes an atom into a positive ion Individual atoms do not form ions by themselves Ions are typically formed in pairs when one atom transfers one or more electrons to another Periodic table can give us clues as to the type of ions the atoms will form Metal Ions : Metal Ions All metals lose electrons to form positive charges (cation) Group 1: metals commonly lose only one electron to form ions with a single positive charge Na+ Group 2: metals commonly lose two electrons to form ions with two positive charges Ca2+ Transition metals Group 3-12: also form positive charges but the number of electrons given away varies Nonmetal Ions : Nonmetal Ions Form ions by gaining electrons to form negative charges (anion) Group 17 nonmetals gain one electron to form ions with a 1- charge Cl- Group 16 nonmetals gain two electrons to form ions with 2- charge O2- Noble gases : Noble gases Do not normally gain or lose electrons and so do not normally form ions What are the three main types of chemical bonds? : What are the three main types of chemical bonds? Ionic Covalent metallic Ionic Bonds : Ionic Bonds Ionic Bonds: the force of attraction between positive and negative ions. Therefore, An ionic bond is formed when an electron is transferred from a metal atom to a nonmetal atom. When the ions are created, therefore, they are drawn toward one another by electrical attraction Electrical forces act in all directions, each ion attracts all other nearby opposite charge ion Sodium chlorideexample of ionic bond : Sodium chlorideexample of ionic bond Notice that both ions now have 8 valence electrons. Each positive ion is surrounded by six negative ions, and each negative ion is surrounded by six positive ion this regular arrangement gives sodium chloride a crystal characteristic cubic shape Rock salt viewed through a magnifying glass Rules for naming Ionic Compounds : Rules for naming Ionic Compounds Based on the names of the ions it is made of The name for a positive ion is the same as the name of the atom from which it is formed Ammonia (NH3) Water (H2O) The name of a negative ion is formed by dropping the last part of the name of the atom and adding the suffix “-ide” Sodium Chloride Hydrogen Peroxide (H2O2) Rules for naming Ionic Compounds : Rules for naming Ionic Compounds The name of an ionic compound will always have the positive ion name first, followed by the name of the negative ion Example the name of salt: sodium chloride Example: : Example: Naming the chemical formula BaI2: take the name of the positive metal element: barium take the name of the negative, nonmetal element, iodine, and give it the ending –ide: iodide combine the two names: barium iodide As a lab group class write out these: KBr MgF2 Covalent Bonding : Covalent Bonding A covalent bond is formed when electrons are shared between two nonmetals. Neither atom gains nor loses an electron, therefore, no ion is formed The overlapping orbitals create the chemical bond. Covalent bonds often are represented via models: Electron cloud model Ball-and-stick model Notice that both atoms now have 8 valence electrons Determining the number of Covalent Bonds an atom can form : Determining the number of Covalent Bonds an atom can form The number of covalent bonds that an atom can form depends on the number of electrons that it has available for sharing Halogen group and hydrogen can contribute only one electron to a covalent bond, therefore only one covalent bond can be formed Group 16 elements can form two covalent bonds Group 15 can form three covalent bonds Carbon and silicon in Group 14 can form four bonds. For example Carbon forms four covalent bonds with four hydrogen atoms to produce CH4 (methane) How to we represent covalent bonds? : How to we represent covalent bonds? Ball-and-stick model The lines, helps to indicate the type of bond Single bond Double bond Triple bond Space-filled model Helps to show general shape of the bonded atom and takes up less space ball-and-stick diagram of covalent bonds : ball-and-stick diagram of covalent bonds Double bond Triple bond What is a molecule? : What is a molecule? Molecule: are a group of atoms held together by covalent bonds Can contain from two to many thousand atoms Most contain the atoms of two or more elements Water (H2O) Ammonia (NH3) Methane (CH4) However some molecules only contain one kind of atoms. These element exist as two-atom molecules Hydrogen H2 Nitrogen N2 Oxygen O2 Fluorine F2 Chlorine Cl2 Bromine Br2 Iodine I2 Polar covalent bonds : Polar covalent bonds Is a covalent bond in which the electrons are shared unequally Word polar refers to anything that has two extremes like a magnet. Water molecule (H2O) oxygen atoms attracts electrons more strongly than hydrogen atoms do. Oxygen nucleus has 8 protons, hydrogen nucleus has 1 proton. Therefore the oxygen atom pulls the shared electrons more strongly toward it oxygen side has slightly more negative charge then hydrogen which has slightly positive charge Ionic COMPOUNDS : Ionic COMPOUNDS Chemical bonds give all materials their structure. Look around you at the different properties Have regular crystals structures, such as, NaCl (salt) Crystals are responsible for bending rays of light, metals shine, and medications attach certain diseases in the body because their atoms are arranged in specific ways One consequence of such rigid structures is that, when enough force is applied to the crystal, it shatters rather than bends. Ionic COMPOUNDS : Ionic COMPOUNDS Dissolve easily in water, separating into positive ions and negative ions Separated ions can move freely so solutions of ionic compounds are good conductors of electricity. Your body uses ionic solutions to transmit impulses between nerve and muscle cells Exercise rapidly deplete the body of these ionic solutions, so a good sports drink contains ionic compounds like potassium chloride Investigative Lab : Investigative Lab “Crystals” Question: How does a crystal grow? Materials: Crystal-growing substance *2 glass beakers *Hot tap water *Stirring stick *Cotton string *Paper clip Pencil *Hand lens Procedure: Add a small amount of the crystal-growing substance to a beaker of hot tap water. Stir until it mixes completely with the water. Keep adding the substance and stirring until no more will dissolves Pour the mixture into another beaker Tie one end of the string to the paper clip and the other end to a pencil. Lower the paper clip into the solution and lay the pencil across the top of the beaker. The paper clip should hand at about the middle of the beaker Use a hand lens to observe the paper clip several times a week for three weeks What do you think? Describe the crystals you see forming on the paper clip. Do the crystals look different as they get larger? Compare your crystals to those of other groups. What similarities do you see among them? What differences? Covalent COMPOUNDS : Covalent COMPOUNDS Exist as individual molecules. Chemical bonds give each molecule a specific three dimensional shape called its molecular structure Molecular structure can influence everything from how a specific substance feels to the touch to how well it interacts with other substances. Basic structure of covalent compounds : Basic structure of covalent compounds Simple linear shape (I2) Bent shapes (H2O) Pyramid (NH3) ammonia Complex shapes (CH4) methane Covalent COMPOUNDS : Covalent COMPOUNDS Have almost the exact opposite properties of ionic compounds The atoms are organized as individual molecules, melting or boiling a covalent compound does not require breaking chemical bonds. Therefore they often melt and boil at lower temperatures The molecules stay together when dissolved in water Therefore poor conductors of electricity Table sugar Sections 2.3Substances’ properties depend on their bonds : Sections 2.3Substances’ properties depend on their bonds Metals have unique bonds Ionic and covalent bounds give compounds certain properties Bonds can make the same element look different Section 2.3 Objective : Section 2.3 Objective THE STUDENT WILL: Define Key vocabulary; Describe how metal atoms form chemical bonds with one another; Analyze how ionic and covalent bonds influence substances’ properties; and Identify different forms of the same element. Warm-Up : Warm-Up Decide if these statements are true. If they are not true, correct them T13 Overhead Vocabulary : Vocabulary Metallic bonds Explorative Lab : Explorative Lab “Bonds in Metals” Question: What objects conduct electricity? Materials: Masking tape * 3 pieces of copper wire (15cm) D cell battery * light bulb and holder Objects to test (paper clip, penny, pencil, eraser, etc) Procedure: Tape on end of a copper wire to one terminal of the battery. Attach the other end of the copper wire to the light bulb holder. Attach a second wire to the holder. Tape the third wire to the other terminal of the batter Touch the ends of both wires to objects around the classroom. Notice if the bulb light or not What do you think? Which objects make the bulb light? How are these objects similar? Metallic Bond : Metallic Bond A metallic bond is formed when metal atoms share all of their valence electrons to form an “electron sea.” Magnesium Metallic Bond : Metallic Bond The attraction between the loose electrons and the positively charged metal cations creates the chemical bond. Magnesium Properties of Metallic Bond : Properties of Metallic Bond Properties of metals are determined by metallic bonds. One common property is that they are good conductors of electric current Due to the electrons ability to flow through the material and carrying the electric current The free movement of electrons also means that metals are good conductors of heat they typically have high melting points except for mercury Metals are solid at room temperature Easily shaped by pounding and can be drawn into wire Chapter Investigative Lab : Chapter Investigative Lab “Chemical Bonds” TE/PE pp60-61

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