Elements, Compounds & Pure Substances

1. Elements, Compounds & Pure Substances

2. Pure Substance: Made of only one kind of matter.

3. Compound: A new substance made when two or more substances chemically combine. Water is an example of a compound containing 2 hydrogen molecules and 1 oxygen molecule.

4. Why do atoms bond together? - 'electron glue'! Some atoms are very reluctant to combine with other atoms and exist in the air around us as single atoms. These are the Noble Gases and have very stable electron arrangements e.g. 2, 2,8 and 2,8,8because their outer shells are full. The first three are shown in the diagrams below and explains why Noble Gases are so reluctant to form compounds with other elements.

5. Ways to Achieve a Stable Outer Shell If atoms collide with enough energy, their outer electrons may rearrange to achieve a stable octet of valence electrons and the atoms will form a compound. Remember, electrons are matter and cannot change during the reaction (law of conservation). Think about how valence electrons may rearrange among colliding atoms so that each atom has a stable octet. There are only 2 possibilities: The transfer of valence electrons between atoms. The sharing of valence electrons between atoms.

6. All other atoms therefore, bond together to become electronically more stable, that is to become like Noble Gases in electron arrangement. Bonding produces new substances and usually involves only the 'outer shell' or 'valency' electrons and atoms can bond in two ways … The phrase CHEMICAL BONDrefers to the strong electrical force of attraction between the atoms or ions in the structure. The combining power of an atom is sometimes referred to as its valency and its value is linked to the number of outer electrons of the original uncombined atom (see examples later).

7. Ionic Bonds Atoms like to have a filled outer shell of electrons. Sometimes, by transferring electrons from one atom to another, electron shells are filled. The donor atom will take a positive charge, and the acceptor will have a negative charge. The charged atoms or ions will be attracted to each other, and form bonds. The compound NaCl, or table salt, is the most common example.

9. The electron structure of atoms is relatively stable when the outer shells contain eight electrons (or two in the case of the first shell). An element like sodium with one excess electron will give it up so that it has a completely filled outer shell. It will then have more protons than electrons and become a positive ion (charged atom) with a +1 charge. An atom of chlorine, on the other hand, with seven electrons in its outer shell would like to accept one electron. When it does, it will have one more electron than protons and become a negative ion with a -1 charge. When sodium and chlorine atoms are placed together, there is a transfer of electrons from the sodium to the chlorine atoms, resulting in a strong electrostatic attraction between the positive sodium ions and the negative chlorine ions. This explains the strong attraction between paired ions typical of the gas or liquid state.

10. Monatomic Ion Polyatomic Ion

12. Covalent Bonds Some atoms like to share electrons to complete their outer shells. Each pair of shared atoms is called a covalent bond. Covalent bonds are called directional because the atoms tend to remain in fixed positions with respect to each other. Covalent bonds are also very strong. Examples include diamond, and the O-O and N-N bonds in oxygen and nitrogen gases.

16. The gold and silicon in these pictures are elements. The are also pure substances; there are no other materials mixed into the rocks. A glass of distilled water is also a pure substance. It contains only water, no other materials. The water you drink from the water fountain is not a pure substance. It also has chlorine, minerals, and bacteria in it. Tap water is a mixture.

17. Families of the Periodic Table The elements of the periodic table can be classified into three main groups: metals, semimetals or metalloids, and nonmetals. In addition, certain columns of the periodic table have recognizable features. Metalloid Metal Nonmetal

18. A dividing line exists in the periodic table beginning between boron and aluminum and proceeding stepwise down to the right (i.e., passing next between aluminum and silicon and then between silicon and germanium).

19. Metals are those elements found to the left of this line, nonmetals are on the right of this line, with the semimetals or metalloids lying along the line.

20. Families of the Periodic Table Alkaline Earth Metals Nitrogen Family Oxygen Family Alkali Metals Carbon Family Noble Gases Transition metals Boron Family Halogens Lanthanides Actinides

22. MIXTURE BASICS Mixtures are absolutely everywhere you look. Mixtures are the form for most things in nature. Rocks, air, or the ocean, they are just about anything you find. They are substances held together by physical forces, not chemical. That statement means the individual molecules enjoy being near each other, but their fundamental chemical structure does not change when they enter the mixture.

23. Mixture: A combination of two or more substances that keep their properties despite being mixed together. Mixtures can be homogeneous or heterogeneous. • There are several types of mixtures: • Solution • Emulsion • Colloid • Suspension • Composite • Alloy • Amalgum • Foam

24. MIXTURES ARE EVERYWHERE There are an infinite number of mixtures. Anything you can combine is a mixture. Think of everything you eat. Just think about how many cakes there are. Each of those cakes is made up of a different mixture of ingredients. Even the wood in your pencil is considered a chemical mixture. There is the basic cellulose of the wood, but there are also thousands of other compounds in that pencil. Solutions are also mixtures. If you put sand into a glass of water, it is considered to be a mixture. You can always tell a mixture because each of the substances can be separated from the group in different physical ways. You can always get the sand out of the water by filtering the water away. A solution can also be made of two liquids. Even something as simple as bleach and water is a solution.

25. SOLUTIONS AND MIXTURES Before we dive into solutions, let's separate solutions from other types of mixtures. Solutions are groups of molecules that are mixed up in a completely even distribution. Hmmm. Not the easiest way to say it. Scientists say that solutions are homogenous systems. Other types of mixtures can have a little higher concentration on one side of the liquid when compared to the other side. Solutions have an even concentration throughout the system. An example: Sugar in water vs. Sand in water. Sugar dissolves and is spread throughout the glass of water. The sand sinks to the bottom. The sugar-water could be considered a solution. The sand-water is a mixture. Homogeneous: visually the same, each drop of solution has the same concentration. Heterogeneous: Visually different; separate phases; drops from two parts of the mixture will be different.

26. When you see distilled water, it's a pure substance. That fact means that there are just water molecules in the liquid. A mixture would be a glass of water with other things dissolved inside, maybe salt. Each of the substances in that glass of water keeps the original chemical properties. So, if you have some dissolved substances, you can boil off the water and still have those dissolved substances left over. Because it takes very high temperatures to boil salt, the salt is left in the container. A glass of distilled water is a pure substance. It contains only water, no other materials. The water you drink from the water fountain is not a pure substance. It also has chlorine, minerals, and bacteria in it. Tap water is a mixture.

27. Solution: a mixture of 2 or more substances that has extremely small particles and that does not diffuse light. A solution is made of solute and solvent. Solvent: the material that does the dissolving; typically a liquid that something is mixed into; more than 50% of the mixture. Solute: the material that is dissolved; typically a material that is mixed into something; less than 50% of the mixture. Solutions are always homogeneous.

28. CAN ANYTHING BE IN SOLUTION? Pretty much. Solutions can be solids dissolved in liquids. They could also be gases dissolved in liquids (such as carbonated water). There can also be gases in other gases and liquids in liquids. If you mix things up and they stay at an even distribution, it is a solution. You probably won't find people making solid-solid solutions in front of you. They start off as solid/gas/liquid-liquid solutions and then harden at room temperature. Alloys with all types of metals are good examples of a solid solution at room temperature. A simple solution is basically two substances that are going to be combined. One of them is called the solute. A solute is the substance to be dissolved (sugar). The other is a solvent. The solvent is the one doing the dissolving (water). As a rule of thumb, there is usually more solvent than solute. Aqueous Solution: a solution in which the solvent is water.

29. Solutions are very difficult to separate. Solutions cannot be separated by filtration. Solutions are transparent. Nothing settles out of a true solution. Perfume, soda, and white wine are true solutions. Red wine is not a true solution…it will settle out after a few years.

30. MAKING SOLUTIONS So what happens? How do you make that solution? Mix the two liquids and stir. It's that simple. Science breaks it into three steps. When you read the steps, remember... Solute=Sugar, Solvent=Water, System=Glass. 1. The solute is placed in the solvent and the concentrated solute slowly breaks into pieces. 2. The molecules of the solvent begin to move out of the way and they make room for the molecules of the solute. Example: The water has to make room for the sugar molecules. 3. The solute and solvent interact with each other until the concentration of the two substances is equal throughout the system. The concentration of sugar in the water would be the same from a sample at the top, bottom, or middle of the glass.

31. Salt Crystal Water molecules break apart and surround the salt molecules. Water Molecule Water molecules attract the salt molecules and break them away from the salt crystal. As the salt molecules leave the crystal they are surrounded by the water molecules. Once the salt molecules are surrounded by water molecules, they are dissolved in the water and form a salt solution.

32. Solutions can be saturated or unsaturated. Saturated Solution: a solution in which the solvent cannot hold anymore solute. Unsaturated Solution: a solution in which the solvent can hold more solute. For example, sugar can be added to water until no more sugar can be added. The sugar will fall to the bottom, no matter how much the solution is stirred. As long as the sugar continues to dissolve in the solvent, the solution is unsaturated.

33. A saturated solution has the same rate precipitation and dissolution.

34. Super saturated solution: A solution in which the solvent holds more solute than it should be able to, due to increased temperatures. As the temperature falls, crystals of solute will form.

35. CAN ANYTHING CHANGE SOLUTIONS? Sure. All sorts of things can change the concentrations of substances in solution. Scientists use the word solubility. Solubility is the ability of the solvent (water) to dissolve the solute (sugar). You may have already seen the effect of temperature in your classes. Usually when you heat up a solvent, it can dissolve more solid materials (sugar) and less gas (carbon dioxide). Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. They are able to keep the fizz inside because the contents of the can are under higher pressure. Last is the structure of the substances. Some things dissolve easier in one kind of substance than another. Sugar dissolves easily in water; oil does not. Water has a low solubility when it comes to oil. Solubility: the ability of a solute to dissolve in a solvent. Soluble: solute dissolves completely in the solvent (mixes easily). Insoluble: solute dissolves incompletely or not at all in the solvent (does not mix). Oil is insoluble in water. Oil will not mix with water. Salt is soluble in water. Salt will mix easily with water.

36. EMULSIONS These special colloids (another type of mixture) have a mixture of oils and waters. Think about a bottle of salad dressing. Before you mix it, there are two separate layers of liquids. When you shake the bottle, you create an emulsion. As time passes, the oil and water will separate to their original states.

37. A colloid is a solution in which the particles are different sizes. Milk and glass are examples of colloids. Colloids can be easily separated by filtration. Nothing settles out. Typically you cannot see through a colloid (the exception is glass).

38. Suspension: a mixture that has particles of different sizes that do settle out. A suspension can be easily separated by filtration. An example of a suspension is flour in water. Another example is red wine. Suspensions do not last long.

39. Composite: a mixture of 2 solids, one imbedded inside another. Composite materials usually have the best properties of the starting materials. For example, in the bridge, the concrete is good to drive on and the steel rods make the bridge strong. The concrete alone is not strong enough for a bridge. The steel rods alone don’t make a good driving surface. Composites help make things stronger and lighter and make them last longer.

40. ALLOYS Alloys are a type of composite. Elements like calcium (Ca) and potassium (K) are considered metals. Of course, there are also metals like silver (Ag) and gold (Au). You can also have alloys that include small amounts of non-metallic elements like carbon (C). Metals are the key thing to remember for alloys. The main idea with alloys is that they are better at something than any of the metals would be alone. Metallurgists (people who work with metals) sometimes add chromium (Cr) and/or nickel (Ni) to steel. While steel is already an alloy that is a very strong metal, the addition of small amounts of the other metals help steel resist rusting. Depending on what element is added, you could create Stainless Steel or Galvanized Steel. It's always about improving specific qualities of the original. Another good example of an alloy happens when metallurgists add carbon (C) to steel. A tiny amount of carbon (a non-metallic element) make steel stronger. These special carbon-steel alloys are used in armor plating and weapons.

41. AMALGAMS Amalgams are a special type of alloy. We like them because we think mercury (Hg) is a cool element. You might know mercury as "quicksilver" or the metal that is liquid at room temperature. Anyway, amalgams are alloys that combine mercury and other metals in the periodic table. The most obvious place you may have seen amalgams is in old dental work. The fillings in the mouths of your grandparents may have been amalgams. We already talked about mercury's being a liquid at room temperature. That physical trait was used when they made fillings. Let's say you have an amalgam of mercury (Hg) and silver (Ag). When it is created, it is very soft. As time passes, the mercury leaves the amalgam and the silver remains. The silver that is left is very hard. Voila! You have a filling!

42. Foam: is a mixture of liquid and gas, that when mixed expands in size. The particles are different sizes and can easily be separated through filtration. Shaving cream and whipped cream are both foams. Microscopic picture of whipped cream. You can see the air (gas) mixed in with the particles.

43. Mixture Pure Substances

44. Pure Substance Mixture Heterogeneous Element Compound Homogeneous Solution Colloid, composite, emulsion, alloy, amalgam, foam, suspension Nucleus Electron Proton Neutron