Chemical Reactions History of the Atomic Theory John Dalton - Billiard ball model Solid, dense balls with no charge. History of the Atomic Theory JJ Thompson - Plum Pudding Model Positive nucleus, negative electrons spread spread throughout History of the Atomic Model Rutherford - Solar System Model Negatively charged electrons orbit a positively charged nucleus. Atom is mostly empty space. History of the Atomic Model Multiple contributors - Modern Atomic Model Densely packed nucleus with positively charged protons and neutrally charged neutrons. Negative electrons orbit in specific shells based on their energy level, creating an electron cloud Vocabulary Element- The pure form of a substance (i.e. Neon) Compound- Two or more atoms bonded together to make a new substance (i.e. Calcium carbonate) Mixture- Two or more substances that are mixed together but do not bond chemically (i.e. grains of sand) Vocabulary Valence electron- Electrons located on the outermost orbit or shell of an atom. Covalent bond- A bond made between atoms when they share electrons. (H2O, CO2) Generally very strong bonds that release large amounts of energy when broken Ionic bond- A bond made between atoms because they have opposite charges. (NaCl, MgO) Generally a weak bond that does not release much energy when broken. Vocabulary Polar Covalent Bond- A covalent bond where there is an unequal sharing of electrons, resulting in a slightly positive or slightly negative charge Can result from stronger attractions of different nuclei, or because of the shape of the molecule that results from the bonding. Metallic Bond- Positively charged metal ions embedded in a sea of valence electrons Chemical Reactions When old bonds break, and/or new bonds form between chemicals, we get a chemical reaction Chemical reactions result in one or more new chemicals The goal of an atom is to get a full valence shell of electrons by accepting, donating or sharing electrons with other atoms. When atoms work together like this, a chemical bond is created, and a new substance with unique properties results Valence Electrons When creating the periodic table, Mendeleev noticed that he could group elements by their combining capacity. Mendeleev did not know how elements bonded but he was able to observe these changes in substances. By studying the periodic table, we can see why some elements more readily react than others. Valence Electrons Valence electrons are often symbolized in dot diagrams Each dot around the element shows a valence electron Patterns can be seen when you compare these dot diagrams to the families on the periodic table Ionic Bonds When an atom loses an electron, it loses a negative charge and becomes a positive ion (cation) When an atom gains an electron, it gains a negative charge and becomes a negative ion (anion) Ionic Bonds In general, ionic bonds are hard, brittle compounds that have high melting points and form crystal structures. Covalent Bonds When the nuclei of two atoms share electrons, we have a covalent (co- meaning together; -valent meaning valence) bond. The force that holds atoms together in covalent bonds is the attraction of the nucleus to a shared pair of electrons. Covalent Bonds In this molecule, three hydrogen atoms have bonded with one nitrogen. The nitrogen and hydrogen now have a full valence shell. Covalent Bonds An oxygen molecule contains one double bond, a carbon dioxide molecule contains two double bonds and a nitrogen molecule will contain one triple bond. Polar Covalent Bonds Sometimes, bonds are shared slightly unequally. Can result by stronger attractive forces of one atom, or because of the shape the molecule takes. This will result in a polar molecule that has a slightly more negative or more positive charge. Polar Covalent Bonds A carbon dioxide molecule is a nonpolar molecule because of its straight-line shape. In contrast, a water molecule is a polar molecule because of its bent shape. This is why water has surface tension and the properties of adhesion and cohesion. Metallic Bonds Metallic bonds form when positive metal ions are embedded in a sea of electrons. This gives the metal its properties of conductivity, malleability, ductility and luster. Conductivity- Ability to transfer heat and/or electricity Malleability- The ability of metal to be shaped and pounded into thin sheets (Tin, Aluminum) Ductility- The ability of a metal to be pulled into thin strands (Copper, Gold) Luster- The shine of a metal (Silver, Gold) Graphic Organizer FeatureIonic Bond Covalent Bond Polar Covalent Bond Metallic Bond How Bond Forms Attraction between oppositely charged ions Equal sharing of electrons Unequal sharing of electrons Attraction between embedded ions and surrounding electrons Charge on Bonded Atoms? Yes, positive or negative NoYes, slightly positive or slightly negative Yes, positive Example NaCl (table salt) molecule CO 2 moleculeH 2 O moleculeAluminum Physical vs. Chemical Change Physical changes occur when objects or substances undergo a change that does not change their chemical composition. Only change the form of a substance Chemical changes occur when objects or substances undergo a change that causes chemical bonds to break and/or form. Results in one or more new substances Physical vs. Chemical Changes Physical Shape Size Freezing/ Boiling Hardness Flexibility Density Conductivity Chemical Heat Light Odor Sound Color change Bubbles Chemical Reactions We have a shorthand way of describing chemicals and chemical reactions. We call this an equation. The shorthand is much like math equations, but instead of an equal sign, we use arrows to show which direction the reaction is moving. Chemical Reactions The formula of a chemical compound shows what atoms are present and in what ratios. For example, in a sucrose molecule there are 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms. Baking soda has one sodium, one hydrogen, one carbon, and three oxygen atoms. Balancing Equations The Law of Conservation of Mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. This means that there has to be the same number of the same kinds of atoms on each side of the equation. Balancing Equations The equation H 2 + O 2 H 2 O is not a balanced equation. In this equation, there are 2 hydrogen and 2 oxygen on the reactants side, but on the products side, there is only one hydrogen and 2 oxygen. How would you balance this equation without changing the molecules on each side? Try It! Magnesium (Mg) will react with atmospheric oxygen (O 2 ) to produce magnesium oxide (MgO) 1. Write an equation to symbolize this reaction Mg + O 2 MgO 2. Write a BALANCED equation to symbolize this reaction 2Mg + O 2 2MgO Try It! Hydrogen peroxide (H 2 O 2 ) will decompose when exposed to light and turn into water (H 2 O) and oxygen (O 2 ) 1. Write and equation that shows this reaction. H 2 O 2 H 2 O + O 2 2. Write a BALANCED equation that shows this reaction. 2H 2 O 2 2H 2 O + O 2 Energy in Reactions When a reaction occurs, energy is either released or absorbed. If energy is released, it is an exothermic (exo- meaning outside, -thermic meaning heat) reaction and the substance gets hot. If energy is absorbed, it is an endothermic (endo- meaning inside, -thermic meaning heat) reaction and the substance cools down. Energy in Reactions Whether a reaction is exothermic or endothermic, reactions need energy to get started. This is called activation energy Energy in Reactions Energy, chemicals, substances, or environments that speed up a chemical reaction- but are themselves not part of the reaction- are called catalysts. Even your cells need catalysts to function. Catalysts that are involved in biological processes are often called enzymes. Enzymes in your body are responsible for helping speed up the process of breaking down food in your mouth and your gut, ultimately releasing energy. Energy in Reactions Cause Effect Increase temperature Increase surface area Increase concentrations of reactants Add a catalyst Increased rate of reaction Graphic Summary Endothermic reactions Chemical reactions Exothermic reactions Equations Activation energy Chemical symbols Arrows and plus signs Coefficients Can be Are written as Balanced with Which require Written using