Chapter 2 Atoms and Elements
Law of Conservation of Mass in a chemical reaction, matter is neither created nor destroyed total mass of the materials you have before the reaction must equal the total mass of the materials you have at the end total mass of reactants = total mass of products 7.7 g Na + 11.9 g Cl 2 19.6 g NaCl
Conservation of Mass and the Law of Definite Proportions Law of Definite Proportions (John Dalton 1766-1844) -Different samples of a pure chemical substance always contain the same proportion of elements by mass. -Compound always forms from a fixed ratio of its element How many Hydrogen atoms are there? How many Oxygen atoms are there? What is the ratio between Hydrogen atoms to Oxygen atom? Now using the mass of Hydrogen and Oxygen to show these results are consistent with the law of definite proportion
Proportions in Sodium Chloride a 100.0 g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chlorine mass of mass of Cl Na 60.7 g 39.3 g 1.54 a 200.0 g sample of sodium chloride contains 78.6 g of sodium and 121.4 g of chlorine mass of mass of Cl Na??? a 58.44 g sample of sodium chloride contains 22.99 g of sodium and 35.44 g of chlorine mass of mass of Cl Na??? 4
The Law of Multiple Proportions and Dalton s Atomic Theory Law of Multiple Proportions: If two elements (A and B) form more than one compound (AB, AB2, AB3 ) then the ratios of the masses of the second element which combine with a fixed mass of the first element will be the ratio of small whole number - Show how chemical formulas are put together nitrogen monoxide (NO): nitrogen dioxide (NO2): 7 grams nitrogen per 8 grams oxygen 7 grams nitrogen per 16 grams oxygen Insert Figure 2.2 p37
Example Lead forms two compounds with oxygen as shown: PbO: 2.98 g Pb, 0.461 g O PbO 2 : 9.89 g Pb, 0.763 g O Show these results consistent with the law of definite proportions 6
Dalton s Atomic Theory Dalton proposed a theory of matter based on it having ultimate, indivisible particles to explain these laws 1) Each element is composed of tiny, indestructible particles called atoms 2) All atoms of a given element has the same mass and other properties that distinguish them from atoms of other elements 3) Atoms combine in simple, whole-number ratios to form molecules of compounds 4) In a chemical reaction, atoms of one element cannot change into atoms of another element they simply rearrange the way they are attached 7
J.J. Thomson (1897) believed that the cathode ray was composed of tiny particles with an electrical charge designed an experiment to demonstrate that there were particles by measuring the amount of force it takes to deflect their path a given amount like measuring the amount of force it takes to make a car turn cathode anode 8
Thomson s Experiment investigate the effect of placing an electric field around tube (1) charged matter is attracted to an electric field (2) light s path is not deflected by an electric field cathode +++++++++++ anode (-) (+) Power Supply ------------- - + 9
Thomson s Results the cathode rays are made of tiny particles these particles have a negative charge because the beam always deflected toward the + plate Particles that make cup cathode rays are 2,000 times smaller than a Hydrogen the amount of deflection was related to two factors, the charge and mass of the particles every material tested contained these same particles the only way for this to be true is if these particles were pieces of atoms apparently, the atom is not unbreakable 10
Millikan s Oil Drop Experiment Millikan's experiment involved measuring the force on oil droplets in a glass chamber sandwiched between two electrodes, one above and one below. With the electrical field calculated, he could measure the droplet's charge, the charge/mass of these particles was -1.76 x 10 8 C/g the charge/mass of the hydrogen ion is +9.58 x 10 4 C/g 11
Thomson s Plum Pudding Atom The Plum Pudding Atom the mass of the atom is due to the mass of the electrons within it electrons are the only particles in Plum Pudding atoms the atom is mostly empty space cannot have a bunch of negatively charged particles near each other as they would repel the structure of the atom contains many negatively charged electrons these electrons are held in the atom by their attraction for a positively charged electric field within the atom there had to be a source of positive charge because the atom is neutral Thomson assumed there were no positively charged pieces because none showed up in the cathode ray experiment 12
Rutherford s Results Over 98% of the a particles went straight through About 2% of the a particles went through but were deflected by large angles About 0.01% of the a particles bounced off the gold foil...as if you fired a 15 cannon shell at a piece of tissue paper and it came back and hit you. 13
a few of the a particles do not go through Plum Pudding Atom Nuclear Atom... if atom was like a plum pudding, all the a particles should go straight through most a particles go straight through some a particles go through, but are deflected 14
Rutherford s Interpretation the Nuclear Model 1) The atom contains a tiny dense center called the nucleus the amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom 2) The nucleus has essentially the entire mass of the atom the electrons weigh so little they give practically no mass to the atom 3) The nucleus is positively charged the amount of positive charge balances the negative charge of the electrons 4) The electrons are dispersed in the empty space of the atom surrounding the nucleus 15
Structure of the Atom Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign based on measurements of the nuclear charge of the elements these particles are called protons charge = +1.60 x 10 19 C mass = 1.67262 x 10-24 g since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons 16
Relative Mass and Charge we generally talk about the size of charge on atoms by comparing it to the amount of charge on an electron, which we call -1 charge units proton has a charge of +1cu protons and electrons have equal amounts of charge, but opposite signs we generally talk about the mass of atoms by comparing it to 1/12 th the mass of a carbon atom with 6 protons and 6 neutrons, which we call 1 atomic mass unit protons have a mass of 1amu electrons have a mass of 0.00055 amu, which is generally too small to be relevant 17
Some Problems How could beryllium have 4 protons stuck together in the nucleus? shouldn t they repel each other? If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9.01 amu! Where is the extra mass coming from? each proton weighs 1 amu 18
There Must Be Something Else There! to answer these questions, Rutherford proposed that there was another particle in the nucleus it is called a neutron neutrons have no charge and a mass of 1 amu mass = 1.67493 x 10-24 g slightly heavier than a proton no charge 19
Summary: The Structure of the Atom Atom: The basic unit of an element that can enter into chemical combination They are made of even smaller particles called subatomic particles. Electrons Protons Neutrons Subatomic Particle Mass g Mass amu Location in atom Charge Symbol Proton 1.67262 x 10-24 1.00727 nucleus +1 p, p +, H + Electron 0.00091 0.00055 empty x 10-24 space -1 e, e - Neutron 1.67493 x 10-24 1.00866 nucleus 0 n, n 0 20
Elements each element has a unique number of protons in its nucleus the number of protons in the nucleus of an atom is called the atomic number the elements are arranged on the Periodic Table in order of their atomic numbers 21
Sources of Some Element Names Some elements are named for planets, mythological figures, minerals, colors, scientists, and places. A symbol represents the name of an element. consists of 1 or 2 letters. starts with a capital letter With few elements use their own special symbols 1-Letter Symbols 2-Letter Symbols C carbon Co cobalt N nitrogen Ca calcium S sulfur Au gold Mg magnesium Diatomic Molecule: A molecule that consists of two atoms (N 2, O 2, H 2, F 2, Cl 2, Br 2, I 2 ). The diatomic molecules (H 2,N 2,O 2,F 2,Cl 2 ) are gaseous in their elemental forms at normal temperatures. Br 2 and Hg are liquids in their elemental forms at normal temperatu 2 2
Mendeleev and the Periodic Table order elements by atomic mass saw a repeating pattern of properties Periodic Law When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically put elements with similar properties in the same column used pattern to predict properties of undiscovered elements where atomic mass order did not fit other properties, he reordered by other properties Te & I 23
Groups: The elements in a vertical column of the periodic table. Period: The elements in each horizontal row of the periodic table. Alkali Metals: The Group 1A elements (Li,Na,K,Rb,Cs,Fr). Alkaline Earth Metals: The Group 2A elements (Be,Mg,Ca,Sr,Ba,Ra). Halogens: The nonmetallic elements in Group 7A (F,Cl,Br,I,At). Noble Gases: The nonmetallic elements in Group 8A (He,Ne,Ar,Kr,Xe,Rn). Transition Metals: Group of elements in the middle of the periodic table.
Metals solids at room temperature, except Hg reflective surface shiny conduct heat conduct electricity malleable can be shaped ductile drawn or pulled into wires lose electrons and form cations in reactions about 75% of the elements are metals lower left on the table 25
Nonmetals found in all 3 states poor conductors of heat poor conductors of electricity solids are brittle gain electrons in reactions to become anions upper right on the table except H Sulfur, S(s) Bromine, Br 2 (l) Chlorine, Cl 2 (l) 26
Metalloids show some properties of metals and some of nonmetals also known as semiconductors Properties of Silicon shiny conducts electricity does not conduct heat well brittle 27
Structure of the Nucleus Soddy discovered that the same element could have atoms with different masses, which he called isotopes The observed mass is a weighted average of the weights of all the naturally occurring atoms the percentage of an element that is 1 isotope is called the isotope s natural abundance 28
Isotopes all isotopes of an element are chemically identical undergo the exact same chemical reactions all isotopes of an element have the same number of protons isotopes of an element have different masses isotopes of an element have different numbers of neutrons isotopes are identified by their mass numbers protons + neutrons Atomic Number Number of protons Z Mass Number Protons + Neutrons Whole number A Abundance = relative amount found in a sample Isotopic symbols 29
Examples 1. How many protons, electrons and neutrons are present in an atom of 2. Write isotopic symbols in both forms for Selenium isotope with 40 neutrons 3. An atom has 32 electrons and 38 neutrons. What is its mass number and what is the element?
Isotopes and atomic mass 20 10 Ne 90.48% 21 Ne 10 10 11 21 0.27% 22 Ne 10 10 12 22 9.25% 31
Atomic Mass we previously learned that not all atoms of an element have the same mass isotopes we generally use the average mass of all an element s atoms found in a sample in calculations we call the average mass the atomic mass Atomic Mass fractional abundance of isotope mass of isotope n n 32
Mass Spectrometry masses and abundances of isotopes are measured with a mass spectrometer atoms or molecules are ionized, then accelerated down a tube their path is bent by a magnetic field, separating them by mass similar to Thomson s Cathode Ray Experiment a mass spectrum is a graph that gives the relative mass and relative abundance of each particle relative mass of the particle is plotted in the x-axis relative abundance of the particle is plotted in the y-axis 33
Example Magnesium has three naturally occurring isotopes with masses of 23.99 amu, 24.99 amu, and 25.98 amu and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. If the average atomic mass of lithium is 6.941 amu, which isotope is the most abundant? How do you know?
Reacting Atoms when elements undergo chemical reactions, the reacting elements do not turn into other elements Dalton s Atomic Theory since the number of protons determines the kind of element, the number of protons in the atom does not change in a chemical reaction however, many reactions involve transferring electrons from one atom to another 35
Charged Atoms when atoms gain or lose electrons, they acquire a charge charged particles are called ions when atoms gain electrons, they become negatively charged ions, called anions (Cl - ) when atoms lose electrons, they become positively charged ions, called cations (Na + ) ions behave much differently than the neutral atom e.g., The metal sodium, made of neutral Na atoms, is highly reactive and quite unstable. However, the sodium cations, Na +, found in table salt are very nonreactive and stable 36
Ionic Charge from Group Numbers The charge of a positive ion is equal to its Group number. Group 1A(1) = 1+ Group 2A(2) = 2+ Group 3A(3) = 3+ The charge of a negative ion is obtained by subtracting 8 or 18 from its Group number. Group 6A(16) = 6-8 = 2- or 16-18 = 2-37
Atomic Structures of Ions Nonmetals form anions For each negative charge, the ion has 1 more electron than the neutral atom F = 9 p + and 9 e -, F = 9 p + and 10 e - Metals form cations For each positive charge, the ion has 1 less electron than the neutral atom Na atom = 11 p + and 11 e -, Na + ion = 11 p + and 10 e - 38
Examples Predict the charge and identify the number of proton and electron from that ion Mg Al O P
Collection Terms A collection term states a specific number of items. 1 dozen donuts = 12 donuts 1 ream of paper = 500 sheets 1 case = 24 cans In chemistry, how do chemists know the number of atoms in an element or compound? 2
A Mole of Atoms A mole is a unit of measurement used in chemistry to express amounts of a chemical substance, the same number of particles as there are carbon atoms in 12.0 g of carbon. a collection term dozen 1 mole = N A = 6.022 x 10 23 of anything Avogadro s Number = 6.0221421 x 10 23 3
Relationship Between Moles and Mass The mass of one mole of atoms is called the molar mass The molar mass of an element, in grams, is numerically equal to the element s atomic mass, in amu E.g 1 H atom = 1.01 amu 1 mol H = 1.01g 6.022 x 10 23 atoms of H =???? g 42
Examples Give the molar mass for each A. 1 mole of Li atoms = g B. 1 mole of Co atoms = g C. 1 mole of S atoms = g 43
Converting between mass, moles and atoms g C mol C mol C g C g C mol C atoms
Examples Assuming all pennies are pure copper and each has a mass of 2.5 g Without doing calculation, determine the number atoms of copper present in 1 mole How many pennies does it take to make a mole?
Examples Calculate the moles of carbon in 0.0265 g of pencil lead Calculate the mass (in grams) of 0.473 moles of titanium In a 3.0 moles of O 2 molecules How many oxygen molecules are there? How many oxygen atoms are there?