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The Magic of Matter Chemistry for 8th Grade Science Warm-up: The word chemistry means different things to different people. 1. In one or two sentences, summarize what the word chemistry means to you. 2. Draw a picture of what you think about when you hear the word matter. The Magic of Matter Part 1: the Structure of Matter (What is matter made of?) TEKS 8.8 (A) describe the structure and parts of an atom; and 8.8 (B) identify the properties of an atom including mass and electrical charge what really MATTERs here? Chemistry is the study of matter, especially its composition, structure, properties and transformation. What do you know? With your learning partner, try and come up with at least three properties of matter (general properties that can be applied to all matter.) what really MATTERs here? Some possible answers you may have come up with are: All Matter has mass All matter occupies space All matter is composed of atoms Matter may have charge what really MATTERs here? There are many ways to classify matter. One way that is useful is to classify matter as either Mixtures or Substances. Copy this chart into your notes. Matter Heterogeneous Mixtures Homogenous Matter (Substances) Homogenous Mixtures Pure Substances Compounds Elements what really MATTERs here? All matter is made up of atoms. An element is a substance that is composed of only one kind of atom. There are some 92 naturally occurring kinds of atoms. They are called the Chemical Elements. Note: There are more than 92 elements listed on the periodic table. The rest are created in the laboratory by scientists but usually only last a few seconds. what really MATTERs here? Here’s where chemistry simplifies the universe: Everything you own, eat, see, touch, or have heard about, all the beauty and complexity of the natural world, is made up only of those 90 distinct kinds of atoms, put together in different arrangements! You see, the world isn’t quite as complicated as you thought. If all elements are made of atoms, what are atoms made of? Here is where chemistry and physics make the universe even less complicated, and a LOT easier to understand. All atoms are made of ONLY three different parts!! Atomic Structure What do you know? Starting from the center of the atom and working outwards, what are the three parts of an atom? Neutrons – No charge (neutral) Protons – Positive charge (+) Electrons – Negative charge (-) Atomic Structure Atoms consist of a nucleus, which is made of protons and neutrons, and electrons which orbit the nucleus. This model of the atom is called the Bohr model, named after the physicist Neils Bohr. Atomic Facts The nucleus is very small. If this classroom were an atom, the nucleus would be in the center of the room and it would be the size of a grain of sand!! Protons and neutrons are about the same size and are slightly bigger than electrons. Atoms are made mostly of empty space. Atomic Structure The number of protons an atom has determine what kind of atom it is. The number of protons is called the Atomic Number of the atom. All atoms of the same element have the same number of protons. (For Example, all carbon atoms have 6 protons. That’s why they are carbon atoms.) Atoms are arranged on the periodic table in order of increasing atomic number. The higher the atomic number, the bigger the atom. Atomic Structure Electrons orbit the nucleus in energy levels. An energy level is really just a specific distance from away from the nucleus. Key Point: In a stable atom, there are the same number of electrons circling the nucleus as there are protons in the nucleus. That is why stable atoms do not have charge. Each negative electron charge is canceled out by a positive proton charge. What are atoms made of? Protons - charge of +1, and a mass of 1 amu Neutrons - charge of 0, and a mass of 1 amu Electrons - charge -1, mass of 1/2000 amu (call it 0) Charge - add up the charge of all the particles in an atom Mass - add up the mass of all the particles in an atom Bohr model Practice What is the mass and charge of an atom that has 10 protons, 12 neutrons, and 10 electrons? What is the mass and charge of an atom with 5 protons, 6 neutrons, and 4 electrons What is the mass and charge of an atom with 7 protons, 7 neutrons, and 10 electrons? Mass: 22 amu, Charge: 0 Mass: 11 amu, Charge: 1 Mass: 14 amu, Charge: -3 The Magic of Matter Part 2: What Holds Atoms Together? Warm-up: Have you ever fallen down or stumped your toe? A force acted on you. There are only a few kinds of known forces that act upon matter. Draw a picture that illustrates as many kinds of forces as you can think of. (Hint: Two of the forces are related to nouns in TEKS 8.8 B) TEKS 8.8 (A) describe the structure and parts of an atom; and 8.8 (B) identify the properties of an atom including mass and electrical charge What Holds Atoms Together? Charge is a property of matter. Most matter has no charge because most matter has an even number of protons (+) and electrons (-). However, matter can acquire charge by gaining or loosing electrons. Particles of the same charge repel each other, and particles of opposite charge attract each other. This is called the electromagnetic force. What Holds Atoms Together? The electromagnetic force is a strong force – a million million million million times stronger than gravity!!! …but, it only acts over short distances. What Holds Atoms Together? The electromagnetic force is what makes electrons orbit the nucleus. The negative electrons are attracted to the positive protons. The electron’s velocity keeps it moving around the nucleus instead of crashing into it. But Wait!!! If like charges repel each other, how can you have so many positive protons in the nucleus without the electromagnetic force blowing the nucleus apart? What Holds Atoms Together? Answer: There is a force stronger than the electromagnetic force. It is called the Strong Nuclear Force. This force only works over very short distances. You must force protons incredibly close together for the strong nuclear force to engage and overpower the repelling force. Neutrons also exert strong nuclear forces and this acts like glue to hold the nucleus together. What Holds Atoms Together? Because the electromagnetic force is so strong, it requires intense pressure and temperatures to overpower it and force protons close enough together for the strong nuclear force to take hold. In fact, it requires temperatures in the tens of millions of degrees!!!! Where can you find temperatures and pressures such as these? Where Do Atoms Come From? Surprisingly, temperatures in the tens of millions of degrees are common in nature. Can you think of where? Where Do Atoms Come From On the insides of stars! Atoms are made on the insides of stars. Stars are giant nuclear reactors, fusing hydrogen atoms together to make the heavier elements. Where Do Atoms Come From When giant stars die, they explode in what is called a Supernova. When this occurs, the star sends all the heavy elements it has created out into space. This is where all atoms (except Hydrogen and Helium) come from. Where Do Atoms Come From Hydrogen atoms are as old as the universe. They have been around since the Big Bang. Great clouds of hydrogen condense in space, drawn together by gravity. These clouds are called Nebulas. Gravity is a weak force, but it acts over enormous distances and pulls matter together. Where Do Atoms Come From As clouds of hydrogen atoms get bigger and more dense, the atoms start bumping into each other more and getting hotter. Eventually, the nebula gets so dense and hot that when hydrogen atoms bump into each other, they fuse together to form a Helium atom. This sets off a fusion reaction, and a new star turns on. Nebula with new stars Important Point!!! There is no difference between two Hydrogen atoms held together by nuclear forces and a Helium atom. They are the same thing! H (atomic #1) + H (atomic #1) = He (atomic #2) He (atomic #2) + He (atomic #2) + enough neutrons to hold the nucleus together = Be (atomic # 4) You are the star! Now you try! Fuse 40 helium atoms together with enough neutrons to hold the nucleus together and what new element do you make? You are given the choice of either adding one proton to your new element or subtracting one proton. Which would you choose to do and why? The Biggest Idea Except for hydrogen and helium, all the atoms in the universe were made in now dead stars. Everything we see and know, from the gasoline in our cars to the uranium in our warheads to the gold in our banks and the carbon in our DNA, even we ourselves, are made of Star Stuff. It is no surprise that we stare up in wonder at the stars in the night sky. We are their children. Estimate the area of the yellow circles Hints: The box is 10 cm on a side There are 100 red dots Use the dots to estimate the size of the yellow circles Notice, the dots are evenly distributed in the area So you can expect the proportion of dots on yellow circles to be the same as the proportion of area of yellow circles # dots in yellow / total # dots EQUALS total area of yellow / total area 23 dots / 100 dots = area of yellow / 100 cm2 Area of yellow dots ~ 23 cm2 That’s how the radius of an atomic nucleus was first measured! But we’re getting ahead of ourselves… Let’s start with a few basic ideas… Democritus of Abderra Lived in Greece, from about 460 B.C. to 370 B.C. Along with Leucippus, was first to suggest that all matter is made of microscopic atoms Reached that conclusion using ONLY logic… never conducted any experiments to check the idea (also first to realize the Milky Way was made of millions of separate stars) John Dalton Experimented with reacting gases Example: Observed that water could be broken down into hydrogen and oxygen… but their masses were not equal. 18 grams of water would give 2 grams of hydrogen and 16 grams of oxygen. And NEVER any other gas! This, and many other observations with other gases, led to: Dalton’s Atomic theory 1. 2. 3. 4. 5. 6. Elements are made of very tiny particles called atoms All atoms of a given element are identical Atoms of different elements have different properties, including mass and chemical reactivity Atoms aren’t changed by chemical reactions; they are merely rearranged into new combinations Compounds are formed when atoms of different elements combine Compounds are defined by the number and This theory could be falsified by: Some examples: Breaking water down into two elements other than hydrogen and oxygen Finding atoms of an element that have some dissimilar properties Observing a chemical reaction that changes the atoms involved What about the structure of the atoms themselves? Dalton knew atoms existed and how they combined to make compounds, but he had no evidence concerning their So hestructure picked the simplest possible structure: tiny, hard spheres J.J. Thomson (1897) Discovered the electron Conducted experiments in which a gas produced electrons Concluded that atoms are positively charged spheres that contain removable electrons Ernest Rutherford (1911) Bounced alpha particles off of atomic nucleus Realized that positive part of atom must be tiny, less than 1/10,000th of the volume! Did not have information about electron location Discovered the proton in 1918 Niels Bohr (1913) Used observations that each element will emit particular frequencies of light to suggest that electrons only exist in precisely defined orbits Erwin Schrodinger (1926) Louis de Broglie showed that a moving particle could be treated, mathematically, as a wave Schrodinger used that result to work with electrons in atoms as probability waves. This led to much more precise predictions of the light emitted by atoms James Chadwick (1932) Identified where the extra mass in atoms came from. There is another particle in the nucleus - the neutron. It has the same mass as a proton, but is electrically neutral. The Magic of Matter Part 4: Where the Real Magic Lies – Electrons and Change Warm-up- Short Writing: Based on what you know about the electromagnetic force, describe why electrons space themselves out around the nucleus instead of grouping tightly around it. TEKS 8.8 (B) identify the properties of an atom including mass and electrical charge 8.9 (A) demonstrate that substances may react to form new substances Bonding Basics Sodium (Na, atomic number 11) is a soft metal that is highly reactive. It explodes on contact with water. Chlorine (Cl, atomic number 17) is a poisonous, corrosive, green gas used to kill soldiers in WW1. What happens when these dangerous elements are placed in a reaction vessel together? When Na is placed into a cylinder filled with Cl, a violent reaction occurs giving off large amounts of heat. Bonding Basics The result is a crystalline substance which is dissolved in our seas and in our veins. It is essential for life. It makes food taste better. We call it table salt. Bonding Basics How can it be that a metal and gas can combine to produce an eatable solid? The answer is that a chemical reaction has taken place to create a new substance with new properties. That means that chemical bonds have broken and reformed in a different arrangement. Chemistry’s Big Question Where are the Electrons? To understand how atoms bond together, we must understand electrons. Electron Configurations An atom has from 1 to Electrons circle the 7 energy levels nucleus of the atom in clouds that are a specific distance from the nucleus. These are called energy levels. Electrons always fill the lowest energy Electron Configurations The horizontal rows of the periodic table are called Periods. They are numbered from 1 to 7. The period an atom is in tells how many energy levels it has. Each energy level can hold only a certain number of electrons. Level 1 = 2 electrons, Level 2 = 8 electrons, Level 3 = 18 electron, Level 4 = 32 electrons Bonding Basics Earlier, we said that atoms normally have no charge because they have equal numbers of protons and electrons. However, atoms can gain or loose electrons. The only place this electron change can happen is in the outer most energy level. Electrons in the outer energy level are called the valence electrons. Bonding Basics Key Point! The vertical columns of the periodic table are called Families or Groups. The are numbered across the top of the periodic table. Elements in the same family behave similarly because they have the same number of valence electrons. Bonding Basics The elements in group 8A, on the far right side of the periodic table, all have full outer energy levels. They are called the Noble Gasses. They are happy having full outer energy levels. They won’t take any more electrons from any other atoms, and they aren’t giving any of theirs away. Therefore, these atoms do not react readily with other atoms, or with each other. Bonding Basics How Noble Gasses see themselves. Bonding Basics Like some people, atoms have a goal. Atoms want to have FULL outer energy levels! Full energy levels are more stable, and atoms are all about stability. Bonding Basics The key to understanding chemistry: Atoms will loose or gain electrons in order to have the Key Point! same number of valence electrons as the Noble Gas closest to them on the Periodic Table. That’s all you need to know to predict most chemical reactions. Bonding Basics The Noble Gasses have 8 electrons in their outer most energy levels. Since all atoms want to be like the Noble Gas closest to them, we can say that atoms will gain or loose electrons in order to have an octet (8) of valence electrons. This is called the Octet Rule. There are exceptions: H and He have only one energy level that can only hold 2 electrons. Also the Transitions Metals in Groups 3B-12B do not follow the rule. You’ll find out why when you take Chemistry in high school. Bonding Basics Example #1 Na, atomic #11, is in Group 1A. It has one valence electron in its outer shell. The Noble Gas it is closest to is Neon, atomic #10. Na must loose one electron to have the same number of electrons as Neon, and that is what Na does in most reaction. Bonding Basics Example #2: Chlorine , atomic #17, is in group 7A. It has 7 valence electrons in its outer shell. Argon, atomic # 18 is the closest gas to Chlorine, so chlorine must gain 1 electron to be like Argon. That is what chlorine does in most reactions. Decide Now you try! if the following atoms will gain or loose electrons in a chemical reaction, and how many: Potassium (K, #19) Phosphorous (P, #15) Magnesium (Mg, #12) Loose 1 electron Gain 3 electrons Loose 2 electrons The Magic of Matter Part 5: Kinds of Bonds – Stealing and Sharing Electrons Warm-up: Draw a model of an atom with an atomic number of 7, a mass of 14, and a charge of -2. Ions Atoms which have gained or lost electrons are called Ions. Ions are just atoms with charge. An atom which looses electrons will have a positive (+) charge. These are called cations. An atom which gains electrons will have a negative (-) charge. These are called anions. Kinds of Bonds Atoms can gain or loose electrons to complete their octets in two ways: 1. They can trade electrons with other atoms. Bonds formed in this way are called Ionic Bonds. 2. They can share electrons with other atoms. Bonds formed in this way are called Covalent Bonds. Ionic Bonds 1. Metal atom looses an electron(s) and becomes positively charged. 2. Non-metal atom gains those electrons and becomes negatively charged. 3. The two ions have opposite charge and are attracted to each other by the electromagnetic force. Properties of Ionic Substances Form Between metals and non-metals. Usually forms high melting point solids. Ionic substances are usually brittle and cleave easily. Dissolve easily in water Conduct electricity in solution. Covalent Bonds Atoms share one or more electrons to fill their outer energy level. Covalent Bonds form between two nonmetals. (Elements to the right of the zigzag line on the periodic table.) Covalent Bonding No ions are formed in covalent bonding. No atoms gain or loose electrons. Atoms simply share valence electrons so that they have full outer energy levels. Hydrogen Molecule Note that electrons actually exist inside electron clouds. We cannot say exactly where the electrons are, only where they are most likely to be at any given time. Properties of Covalent Substances Many are gasses or liquids There are some covalent solids, but they generally have low boiling points. Do not conduct electricity Do not dissolve easily in water. Water is a special covalent molecule. Polar Covalent Bonds Oxygen has a stronger pull on electrons than Hydrogen, so the electrons spend more time around Oxygen, making that region slightly negative. The Hydrogen region becomes slightly positive. Hydrogen Bonds The slightly negative region of water near the Oxygen can be electrostatically attracted to the slightly positive hydrogen region of another water molecule. This is called a Hydrogen Bond. Hydrogen bonds are weak bonds that are easy to form and break. They are of high importance in biology! Hydrogen Bonds in H2O Hydrogen bonds give water its unique properties. Hydrogen Bonds in Water Hydrogen Bonds in Proteins Hydrogen bonds often give structure to biological compounds. Hydrogen Bonds in DNA Why do you suppose… We stated that Hydrogen bonds are very weak bonds, easy to form and break. Yet, the DNA strands of the double Helix are held together by Hydrogen bonds. Can you think of any functional reason why hydrogen bonds are a good choice for this purpose? Chemical Reactions Warm-Up: On a sheet of paper, draw a picture of a chemical reaction that happens in your daily life, or write down three things you think of when you think about chemical reactions. Chemical Reactions When you hear the phrase “Chemical Reaction,” what do you think of? Explosions? Acids dissolving doors? Glowing liquids? Purple clouds? Changes in Matter We can classify changes in matter as either physical or chemical changes. Physical changes only change the shape or state of an object. Chemical changes rearrange atoms among molecules. Can you think of some examples of each? Physical Crushing ice Melting ice Boiling water Bending a piece of metal Separating iron filings and sulfur powder Melting iron and tin, mixing, and letting the mixture cool Chemical Using a battery to break water into hydrogen and oxygen gas Burning a piece of wood Heating iron filings and sulfur until they bond, making pig iron Bonding and Energy Chemical reactions are the rearrangement of atoms What do you have to do in order to rearrange atoms? You have to break their bonds! It takes energy to break bonds. How can we give a molecule energy it can use to break a bond? Giving Molecules Energy Slam other molecules into it (heating) Like when you bake a cake… Pull them apart with an electric field or hit them with electrons (electricity) Like when you put wires from a battery in water to make hydrogen and oxygen Hit them with light Like when your dentist uses a UV light to set a polymer filling Getting Energy from Molecules Of course, you can get energy from molecules, too. That’s why we burn gasoline in our cars - to get the energy to turn the wheels Heat - the new molecules are moving more quickly Electricity - the molecules get rid of electrons (like in a battery!) Light - like in a glowstick Exothermic Reaction If molecules release more energy than they absorb, the reaction is exothermic. Sodium and Water Endothermic Reaction If molecules take in more energy than they release, the reaction is endothermic. Endo or Exo? You mix two substances together, and… … they heat up. Exothermic … they cool down. Endothermic … they glow. Exothermic … nothing happens until you shine light on them. Endothermic Chemical Equations “Methane gas combines with oxygen gas to produce carbon dioxide and water” That’s quite a mouthful, eh? There’s a shorter way to write that CH4 + O2 methane oxygen CO2 + H2O carbon dioxide and water combines with to produce Types of Reactions synthesis - two different elements combine together to form one compound Fe (II) + O FeO (rust!) Decomposition - one compound is broken into its two individual elements H2O 2H + O Types of Reactions (cont.) single displacement - a lone element replaces some part of a compound. 2Li + 2H2O 2LiOH + H2 double replacement - atoms in compounds switch partners BaBr2 + K2SO4 2KBr + BaSO4 Conservation of Atoms Conservation means, in science, “keep the same” or “the same everywhere” It really means, “there’s no such thing as a free lunch” When you have a chemical reaction, the atoms IN must be the same as the atoms OUT. The atoms must come from somewhere! Are atoms conserved in this reaction? CH4 + O2 CO2 + H2O CH4 + O2 CO2 + H2O C - Carbon 1 goes in… 1 comes out H - Hydrogen 4 go in… 2 come out (??) O - Oxygen 2 go in… 3 come out (??) Nope, the atoms aren’t conserved. Balancing Equations Balancing equations is adjusting the numbers of each molecule until there’s the same number of each atom on each side. + O2 CO2 + H2O Hrm… what if I had TWO oxygen molecules on the left? CH4 + 2 O2 CO2 + H2O C - 1, H - 4, O - 4 on the left C - 1, H - 2, O - 3 on the right CH4 Balancing Equations (cont.) Closer… I need 2 more H and 1 more O on the right. How about another water molecule? + 2 O2 CO2 + 2 H2O C - 1, H - 4, O - 4 on the left C - 1, H - 4, O - 4 on the right It is balanced! CH4 Balancing Equations (practice) If I mix aluminum (Al) and sulfur (S) together, what compound will they make? Hrm… Al has an oxidation of 3+, S has an oxidation of 2-… 2 Al + 3 S Al2S3