Chapter 2 Atoms and Molecules

Transcription

1 Chapter 2 Atoms and Molecules 2-1 Elements and their symbols Most of the chemicals you find in everyday life can be broken down into simper substances Key Concepts: A substance that cannot be broken down into simpler substances is called an element A substance that can be broken down into two or more elements is called a compound The above definition of elements is good, but here is a better one: An element is a substance that consists only of atoms with the same nuclear charge (more on this in a minute) There are currently about 120 known elements but only 40 of these make up 99.99% of the materials we know (So 80 elements are very rare) only 10 make up 99% of the total mass of the earth Table 2.2 Key Concept: Elements can be divided into two broad classes: metals and nonmetals Metals Properties of solid metals luster cast into shapes good conductors of heat and electricity malleable - (can be rolled or hammered into sheets) Ductile - (can be drawn into a wire) Table 2.3 lists some of the common metals Key Concept: Chemical Symbols are one or two letter abbreviations used to designate elements Chemical symbols usually consist of the first couple of letters in an elements name. The problem is that sometimes the name dates back to the original latin! See interchaptera (online) if you are interested in where these names came from.

2 2 You need to start memorizing the symbols of the most common elements! They are your basic vocabulary for the rest of this year. Nonmetals vary greatly in appearance over ½ are gases at RT some are solid one (bromine) is a liquid Poor conductors of heat or electricity not ductile or maleable do not have luster Common nonmetals shown in table 2.5 Note that several nonmetal elements have a subscript of 2 This means that the elemental unit consists of two atoms that are joined together Key concept: A molecule is when two are more atoms are joined together to form a single unit A molecule consisting of just two atoms is called a diatomic molecule. When we refer to a naturally occurring diatomic element by name, we are referring to the diatomic molecule. IE oxygen means the O 2 molecule. If we want to refer to a single atom of that element we will say the oxygen atom.

3 3 2-2 States of Matter Not only are we concerned with the elements that compose a chemical, we are also concerned with the physical state of the chemical. The most common states are: KEY TERMS: Solid - When a chemical is in the solid form we write the chemical formula followed by (s) as in NaCl (s). Solids have a fixed shape because they are composed of a rigis, well define matrix locking the atoms into place. Liquids - Denoted by (l) in a chemical formula. Here there are intramolecular forces holding the individual molecules in contact with each other so there is a fixed volume, but the forces do not fix the molecules in a rigid geometry so they are free to move around. Thus we have a fixed volume, but the shape can change. Gases - Denoted by (g). Here there are no forces holding the molecules together, so the molecules move around and expand to fill the space they are in. That is why there is no fixed volume or shape. Aqueous - Denoted by (aq). Here the chemical is dissolved in water to make an aqueous solution. (Next section, but I might as will throw it in now!) 2-3 Separation of mixtures In nature most substances occur in some kind of mixture. A mixture is where component substances are mixed together, but they do not combine chemically so there can be many different molecules all mixed together Mixtures can be classed as KEY TERMS: Homogenous - the same from point to point Or Heterogeneous - different from point to point The usual way you can tell a homogenous mixture from a heterogeneous mixture is with your eyes. If you can see some visibly different components within a mixture it is homogenous. If you can t see different chunks then it is homogeneous KEY TERM: Another name for a homogeneous mixture is solution. Note that you probably think of a solution as something dissolved in water.

4 4 And that is correct, because once it is dissolved in water, you can no longer see the individual components of the mixture. However the Chemical term solution applies to any uniform mixture, so it also can be applied to such diverse mixtures as the air we breath, or fog or a piece of solid brass Many solutions are solids dissolved in liquids. In this case we have more specialized terminology KEY TERMS Solvent - is the liquid that something is dissolved in Solute - is the solid that is dissolved in the liquid Since the components of a mixture are not chemically linked to each other we can separate the components using various physical process. The book mentions several, but I am going to take a rain check on them 2-4 Law of Constant Composition If we start with any mixture, and then apply the various physical process to separate the individual components, we will eventually come up with various materials that have a constant composition. Key Concept: This has lead to the Law of Constant Composition for compounds: The relative amount of each element in a compound is always the same regardless of the source of the compound or how it was prepared. One way to observe this law is by looking at the mass percentage of elements in a compound. Key Equation:

5 5 Example 1: In a 20 g sample of water we find that there is grams of O, what is the % composition of oxygen in water? 17.69/20.00 x 100% = 88.45% And this is true for any sample of water, it will always contain 88.45% by mass oxygen, no mater where it comes from. Note: the % compositions of all the elements in a compound should sum to 100% (Duh!). This can be a nice check on your calculations Example 2: I have a gram sample that contains only sodium and chlorine and is 39.33% sodium. How many grams are chlorine are in this sample? Approach 1 : grams sample grams sodium grams chlorine % = X/20.15 x 100% 39.33%/100% =.3933 = X/ x = g sodium = 7.92 g Na Mass Sample = Mass Sodium + Mass Chlorine = Mass Cl = mass Cl = g Cl Approach 2: % Na % Cl mass Cl 100% = %Na + % Cl 100% = 39.33% + %Cl = 60.67% Cl 60.67% Cl = Mass Cl/Total mass x 100% 60.67%/100% =.6067 = Mass of Cl / x = mass of Cl = g Cl The Law of constant composition lead to another law called the law of multiple proportions. This law is a little harder to explain because you compare ratios of composition in different compounds. It is one of the pieces that lead to Dalton s Atomic theory, but it is not something we use much today, so I will skip any further details or calculations. Clicker question: set up for a % composition problem

6 6 2-5 Dalton s Atomic Theory In the early 1800's Dalton proposed the atomic theory, on which much of our current understanding of chemistry relies. His theory (updated with modern terms and concepts) is: Key Theory: 1. Matter is composed of small, indivisible particles called atoms 2. The atoms of a given element have the same mass and are identical in all ways 3. The atoms of different elements differ in mass and in chemical behavior. 4. Chemical compounds are composed of two or more atoms of different elements joined together. This particle of joined atoms is called a molecule. 5. In chemical reactions atoms are not created or destroyed, rather they are rearranged to form different molecules This theory works well and is consistent with all of the laws we have talked about of to this point, the conservation of mass, the law of constant composition, and the law of multiple proportions. Let s talk a bit about the mass of an atom As we look at different compounds and use our law of constant composition and law of multiple proportions, we can define a quantity called Atomic Mass Ratio Atomic mass ratio = mass of atom A / mass of atom B The beauty of this is that since one mass id divided by another mass, the units cancel out, and the Atomic Mass ratio is a unitless number Using these ratios we can make a mass system that relates the mass of all the atoms on the periodic table to any other atom. All we have to do is to chose one to base everything on. At different times we have used different atoms as our key atom that we base the weights on, H one time, Oxygen another. Currently our mass system is based on carbon (more on this later) The term atomic mass ratio has since been shortened to simply atomic mass. While the Atomic mass ratio is a unitless number, sometimes we assign in a unit called the atomic mass unit or amu which has now been shortened to the abbreviation u in the IUPAC (International Union of Pure and Applied Chemistry) system. Unfortunately for you, biochemists don t use IUPAC nomeclature, so they use the term Dalton, (Da) for the atomic mass ratio. Thus you will see the atomic mass of C as being:

7 , amu, u or Da, depending on who is writing the test you are reading!! 2-6 Molecules Molecules are groups of atoms that are joined together in some way. As we saw earlier some elements are actually diatomic molecules like O 2 or H 2. So some molecules are elements. On the other hand, most molecules have one or more different atoms in them. We call this kind of molecule a compound There are lots of different ways to represent compounds. In the simplest case we simply give a formula that expresses the number of each atom in a compound like H2O for water or C6H12O 6 for glucose. At another level we will write structural formulas that indicate the bonds linking the atoms with dashes. H-O-H, etc. If we get really tricky we can make three-dimensional models that show the relative size and spacing between atoms to scale. It is with these kinds of models that we can really see that last part of Dalton s theory. Look at the following three chemical reactions: Figures 3,4,5 (2H 2 + O2 H2O) (C + O2 CO 2) (C + H2O H 2 + CO) Notice that in each chemical reaction no atoms are created or destroyed, we just rearrange the molecular groupings. Summary clicker question: identify various materials as Mixture, solution, pure subastance, compound, element 2-7 Chemical Nomenclature Chemical nomenclature refers to a system used to assign names to compounds. In this book we will introduce the correct nomenclature with every new class of compounds we study. In ths chapter we will study the simplest nomeclature, one that applies to Binary compounds (only 2 elements) Binary compounds composed of a metal and a nonmetal And that only occur in 1 fixed ratio Key: In this case the naming system goes like this: 1. Name the metal first 2. Name the nonmetal second 3. Change the ending on the nonmetal to -ide

8 8 Try the following examples K2O AlBr 3 Cadmium selenide Magnesium hydride Binary compounds composed of 2 nonmetals These are more difficult because there can be many binary compounds with the same element but different fixed ratios like N O, NO, N O,N O or NO! Since there are all sorts of different ratios, be have to say what the ratio is explicitly in the name For this we will use the Greek prefixes as shown in Table mono- up to 10 deca- So you put the prefix in front of the element to tell how many of each element there is. The one exception is when the first element is a one. In that case you don t bother with the mono- Examples: SO2 SO 3 sulfur dioxide sulfur trioxide Problems to try N2O Dinitrogen monoxide NO Nitrogen monoxide N2O 3, Dinitrogen trioxide Dinitrogen pentaoxide N2O5 Nitrogen dioxide NO 2 Notice there is some funny business when the Greek prefix ends with an a or and o and the name it is combined with starts with a vowel Dinitrogen monoxide instead of monooxide Tricky Hydrogen Hydrogen can act as either a metal or a nonmetal, so sometimes you use one of the above systems, sometimes the other. How can you tell? When Hydrogen is given first in binary formula, it is generally treated as a metal so you use the first system

9 9 H2S hydrogen sulfide When hydrogen is given second in a binary formula it is treated as a nonmetal NaH sodium hydride The Two compounds that we never name properly H2O is always called water NH is always called ammonia 3 Clicker question on names of some simple binary compounds 2-8 Atomic and Molecular Mass Now that we can identify molecules that are made up of combinations of atoms, we will now introduce the idea of a molecular mass Key Concept: The molecular mass is the sum of all the atomic masses of all the atoms in a molecule. Example 1 : What is the molecular mass of water? Water = H2O =2(1.008) = Example 2: What is the molecular mass of cesium chloride? Cesium chloride = CsCl = = Clicker question on molecular mass of some compounds 2-9 The Nucleus Nice historical background on the experiments that helped us to understand the parts of a atom, but lets move directly to the results 2-10 Protons, Neutrons, and Electrons Atom -8 Roughly 1x10 cm diameter -10 1x10 m, 0.1 nm The bulk of this space is occupied but a cloud of electrons electron Charge -1 Mass.00054u

10 10-13 The nucleus is 10 cm, the size of the atom has all the mass of the atom concentrated in it Electron cloud or atom about the size of this room nucleus has about.1 mm radius! -19 Proton Charge +1 (1.602x10 coulombs)mass 1.007u Neutron Charge 0 Mass 1.008u Key Concepts: 1.Number of protons in an atom is called the atomic number (Z) of the element 2.In a neutral atom the number of electrons = number of protons 3.Difference between different elements is due to difference in atomic number 4.Total number of protons and neutrons is called the mass number (A) 2-11 Isotopes If protons and neutrons have a mass of ~1 u, and an atom has in integral number of protons and neutrons, Why do we have non-integral atomic masses? (Cl or Cu ?) Key concept: Isotopes are atoms of one element that contain the same number of protons but different numbers of neutrons. For instance Chlorine has two major isotopes, both with atomic number (Z) of 17 so the nucleus has 17 protons, but one isotope has 18 neutrons so A = 35 and the other isotope has A=20 so A = 37! We differentiate between isotopes with the following convention X = chemical symbol, where A = mass number and Z = atomic number So our two isotopes of Cl are: Many times it is useful to know the number of neutrons in an isotope The number of neutrons N = A-Z Before we go on to problems I want you to notice something. This convention that we use for writing isotopes IS NOT USED on the Periodic table! Notice on the periodic table how the Atomic number is above and to the left, and a mass number is below and to the left, Just the opposite of the above convention!

11 11 Problem: Answer: Symbol Z N A Symbol Z N A Ca Zn H 3 H So now if you go back to Dalton s atomic theory you can see that he had one points wrong! Since an element may be composed of more than one isotope, all atoms of the same element do NOT have the same mass! They gave the same number of protons, but not the same mass! 1 So of a element is composed of isotopes of different masses, where does the mass number that is given on the periodic table come from? Key Concept: Atomic Mass is the average of the masses of the individual isotopes weighted by the natural abundance of each isotope For instance our Chlorine isotopes % of chlorine is the Cl isotope, and 24.22% is the Cl isotope So the mass of the periodic table is x (75.78/100) = x (24.22/100) = = So now you know why the atomic masses on the periodic table are nive interger numbers Let s go back and talk about our atomic mass units again. Remember how I said that atomic masses are more appropriately called mass ratios because we measure one mass against the other? And that the mass we used to set the standard has changed from one time to another? What is our current standard? The current standard that all the numbers are 12 currently based on is that the isotopic mass of C is exactly 12. The discerning student may notice one additional problem. If C-12 has exactly 6 35 protons and 6 neutrons. Why isn t the atomic mass of 17 Cl exactly 35 and Cl exactly 37? Well we saw earlier that the mass of a proton and a neutron are exactly 1, so that can explain some, but not all of the variation. The other part of the explanation lies in nuclear physics. When we bind the neutrons and 6

12 protons into the nucleus some energy is released to hold the particle together. 2 Where does that energy come from? Good old E=mc some of the energy comes from the mass of the particles themselves! If you want more on this you need to talk to Kara Keeter! 2-12 Ions In our atom we cannot change anything in that tiny dense nucleus of the atom, so we can t transmute lead into gold as the alchemists wanted to do. What we can do is to change the number of electrons around the nucleus. And this is by and large what chemistry is all about Key Concepts: 1. When we remove electrons from an atom we give the atom a positive charge and change it into a cation. 2. When we add extra electrons to an atom we give the atom a negative charge and change it into an anion Problems: How many protons, electrons and neutrons are in: (P=14,N=14, E=18) (P=74, N=112,E=69) More clicker questions on same topic 12