Chemistry Unit Notes 8th Grade Science

Basic Vocabulary Matter: Anything that has mass and volume Mass: Amount of matter in an object Weight: Measure of the force of attraction between objects due to mass and gravity Volume: Amount of space an object takes up Density: Measurement of how much mass is contained in a given volume

More Vocabulary Atoms: Smallest particle of an element that has all the properties of matter: Protons- particles in the nucleus with positive charge Electrons- particles orbiting around nucleus with negative charge Neutrons- particles in the nucleus with no charge Elements: Simplest form of a pure substance Compounds: Two or more elements chemically combined to form a new substance

Sub-Atomic Particles Part of Atom Charge Location Mass/Size Electron - negative outside nucleus .0006 amu (too little to count) Proton + positive inside nucleus 1 amu Neutron no charge

Periodic Table

Using the Periodic Table Atomic Number Equal to # protons = # electrons Periodic Table is arranged by this number Symbol “Shorthand” for the element – Note 2nd letter is always lowercase Atomic Mass Number Total AVERAGE mass of Protons + Neutrons + Electrons 17 Cl 35.5

Electron Energy Levels Electrons are arranged in “Shells” around nucleus in predictable locations Fill “seats” closest to nucleus first (concert – best seats) “Seats” available Shell #1 2 electrons Shell #2 8 electrons Shell #3 8 electrons Shell #4 18 electrons Shell #5 32 electrons Shell #6 50 electrons Ex. Carbon has 6 total electrons so… Two electrons on first energy level Four electrons on second energy level Question: Could we fit more electrons on the second energy level if there were more electrons in carbon??

Atomic Structure Total # of protons and electrons (in a neutral atom) 17 protons in nucleus 17 electrons orbiting nucleus 17 Cl Element Name Chlorine 35.5 Total Mass of Nucleus 36 - 17 = 18 neutrons (Round Atomic Mass) Notice: electrons follow energy level rules from previous slide.

Atomic Mass – Fractions? Look at Chlorine (atomic number 17) Atomic mass of 35.5? I dont’ get it! Where does the 35.5 come from? 0.5 protons? 0.5 neutrons?  No Atomic Mass = average number of protons and neutrons in nature

More Practice Determine the name, number of protons, neutrons and electrons for each element shown and draw… 26 8 15 Fe O P 56 16 31

Isotopes An isotope is a variation of an element (same protons) but can have diff. # of neutrons Ex: carbon (atomic mass = 12.011) Carbon (14) and carbon (12) exist in nature

Ions Change in electrons which gives an atom a charge (+ or -) You can only add or subtract electrons! (protons don’t change) Ex. Count the number of electrons below… Carbon ion (-1 charge) 7 electrons (-) 6 protons (+) Neutral Carbon 6 electrons (-) 6 protons (+) Carbon ion (+1 charge) 5 electrons (-) 6 protons (+)

Valence Electrons An electron on the outermost energy shell of an atom Important to understand because this is a key factor in how atoms will BOND with each other Octet rule – stable atom will have 8 electrons in that outer shell Practice – Valence # of Chlorine? Neon? Nitrogen? Oxygen?

Electron Dot Diagrams a diagram that represents the # of valence electrons in an atom of an element. The amount of electrons is displayed by dots around the symbol of the element. Ex. http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson38.htm

Types of Chemical Bonds Ionic- Two elements bond by transferring electrons to create ions that attract together (+ is attracted to - after an electron is transferred) Covalent- Two elements bond by sharing electrons (strongest bond type) Metallic- Two metals bond and form a “common electron cloud”. This is a cluster of shared electrons (weakest bond type)

Examples of Bonding http://www.youtube.com/watch?v=xTx_DWboEVs http://www.youtube.com/watch?v=1wpDicW_MQQ http://www.youtube.com/watch?v=QqjcCvzWwww http://lc.brooklyn.cuny.edu/smarttutor/core3_22/Bonds.html

Predicting Bonds Ionic Bond = metal to non-metal Covalent = non-metal to non-metal Metallic = metal to metal Do you understand why? HINT: the numbers at the top of the table indicate the # of valence electrons for each column

Oxidation Numbers Oxidation numbers are assigned to each element They represent a predicted “charge” of an atom/ion when it bonds with another element. (tells us if the atom would prefer give or take electrons, and how many). They help us to predict what compounds will form when two elements get together. Oxidation numbers are labeled like this: Na 1+ O 2-

How to Use Oxidation Numbers Oxidation Number indicates the number of electrons lost, gained or shared when bonding with other atoms. Ex. Na wants to lose an electron. If an electron is lost, it becomes a +1 charge SO: oxidation number for Na = 1+ Ex. Cl wants to gain an electron. If an electron is gained, it becomes a -1 charge SO: oxidation number for Cl = 1-

Oxidation Numbers Each column going down the periodic table has elements with the same oxidation number.

Label the oxidation numbers on your periodic table at the top of each column as shown here: 1+ 2+ 3+ 4(+/-) 3- 2- 1- 0

Rules for using oxidation numbers to create compounds Positive ions can only bond with negative ions and vice versa 2. The sum of the oxidation numbers of the atoms in a compound must be zero (the key is to stay balanced) 3. If the oxidation numbers are not equal to zero, then you must add additional elements until they balance at zero. 4. When writing a formula the symbol of the Positive (+) element is followed by the symbol of the negative (-) element.

Examples of Forming Compounds Ex. Na (+1) + Cl (-1) = NaCl Are these oxidation numbers already equal to zero? If so, you don’t need to add any extra elements to combine them into a compound, so the answer is simply NaCl Ex. H (+1) + O (-2) = H2O How many +1 would you need to balance the -2 to zero? Since you need 2 atoms of the 1+ to balance the 2- to zero the resulting compound would be H2O In other words: to combine H with O, you MUST have 2 H to balance the oxidation numbers to zero 2+ and 2- = ZERO Ex. Al (+3) + S (-2) = Al2S3 This one is tricky…we are not even close to balancing + and - to zero. Because of this we must have more than one Al and more than one S in our final equation. By using 2 Aluminums instead of just1 we would have 6+ By using 3 sulfers instead of just 1 we would have 6- Since these are now equal to zero, we combine 2 Aluminums and 3 Sulfers to make Al2S3

Chemical vs. Physical Change Physical Change: A change that can occur without changing the identity of the substance. Ex. Solid, Liquid, Gas (Phase change) Chemical Change: Process by which a substance becomes a new and different substance Ex. Fire

Chemical Reactions Chemical Reaction: a process in which the physical and chemical properties of the original substance change as new substances with different physical and chemical properties are formed

Chemical Reaction Basics H2 + O2 --> H2O Reactants Products Reactants- substance that enters into a reaction Products- substance that is produced by a chemical reaction

Evidence of Chemical Change EPOCH is an acronym that stands for evidence that a chemical reaction has occurred. – Effervescence (bubbles and/or gives off gas) – Precipitate (solid crystals form) – Odor (change of smell is detected) – Color change – Heat (reaction either heats up or cools down) Does sighting evidence of a chemical reaction mean that a chemical reaction has undoubtedly taken place? E P O C H

Types of Reactions Romance Chemistry :) Synthesis- Marriage/Dating A + B = AB Decomposition- Divorce/Breakup AB= A + B Single-Replacement- Dance Cut In A + BC = AC + B Double-Replacement- Dancing couples switch partners. AB + CD = AC + BD

Cartoon Chemistry This is an example of synthesis

Cartoon Chemistry This is an example of a decomposition

Cartoon Chemistry This is an example of a single replacement

Cartoon Chemistry This is an example of a double replacement

Reaction Types Review… Match each chemical reaction with one of the reaction types on your chemical cartoons. Zn + 2HCl  H2 + ZnCl2 N2 + 3H2  2NH3 2KI + Pb(NO3)2  2KNO3 + PbI2 2MgCl  Mg2 + Cl2

Conservation of Mass Atoms cannot be created or destroyed in a chemical reaction. What goes in must come out. So we must balance equations to conserve mass.

Balancing Equations Rules: Ex. 2H2 + O2 --> 2H2O We can not add or subtract subscripts from either side of the equation We can only add coefficients to the front of each compound Ex. 2H2 + O2 --> 2H2O H = 4 H = 4 O=2 O = 2 Before must match After See “Balancing Act” worksheet for more examples…

Solution Chemistry Mixtures: Matter that consists of two or more substances mixed but not chemically combined Solutions: Homogeneous Mixture in which one substance is dissolved into another Solute = Substance that gets dissolved (ex. Kool-Aid powder) Solvent = Substance that does the dissolving (ex. Water) Acid: Compound with a pH below 7 that tastes sour and is a proton donor. Ex. Citrus foods Base: Compound with a pH above 7 that tastes bitter and is a proton acceptor Ex. Cleaning Products (soap)

Acids and Bases Solutions can be acidic or basic Acids and Bases have unique properties when dissolved in water Acids = sour taste Bases = bitter taste Indicators are substances that change color when mixed with a solution, which helps to determine if a substance is an acid or a base. (pH paper, Litmus paper, cabbage juice)

Acids Proton donors (H+) Acids contain hydrogen and produce positive ions (H+) when dissolved in water Acids = good electrolytes Examples of acids: Lemon Juice Citric Acid Carbonic Acid HCl

Bases Proton acceptors Bases contain hydroxide ions (OH-) when mixed with water. Bases = weak electrolytes Examples of bases: Ammonia Soap Bleach (chlorine)

Combining Acids and Bases -Mixing acids and bases is a balancing act. (like a teeter-totter) Acid + Base = neutral (water and salt)

Combining Acids and Bases EXAMPLE: Acid + Base = neutral (water and salt) H+ + OH-  HOH + Salt Acid Base water Ex. HCl + NaOH  H2O + NaCl

Measuring Acids and Bases pH scale- used to measure the acidity of a solution. Measure pH with indicators pH scale goes from 0 – 14 0 = very acidic 14 = very basic 7 = neutral

Acids and Bases