Chapter 2: Atoms and the Periodic Table Outline: Atoms & electrons, protons, neutrons Isotopes Periodic Table Mole & molar mass Chemistry: the science of matter and its structures, interactions, and transformations (example: making paint that doesn t fade in the sunlight) Atomic Theory of Dalton (1808) Published a new system of chemical philosophy : 1. Matter consists of indivisible atoms 2. Atoms are indestructible and retain their identity in chemical reactions 3. All of the atoms of a given chemical element are identical in mass and in all other properties 4. The atoms of different elements are different in mass and other properties 5. A compound forms from its elements through the combination of atoms of unlike elements in small wholenumber ratios
element: a substance that cannot be decomposed into simpler materials by chemical means identical in mass: not quite correct: the number of neutrons can vary (isotopes) which changes the mass but not the chemical properties compound: example: CO 2, CO, caffeine = C 8 H 10 N 4 O 2
Chapter 2.2: Atoms are composed of electrons, protons, and neutrons (Dalton s indivisible / indestructible atom concept not correct) Electron Cathode ray tube (used for computer screens until the mid-1990 s) two metal plates sealed inside a glass tube. When connected to a high-voltage source, the cathode (-ve charged plate) emits cathode rays that travel to the anode (+ve charged plate), passing through a hole and striking a fluorescent screen. The experiment does not depend on what metal the cathode is made from. Figure: Cathode ray tube The cathode ray is deflected by an electric or a magnetic field
J. J. Thomson (1856-1940) used electric and magnetic fields to deflect the cathode ray and concluded that cathode rays consist of a stream of negatively charged particles (electrons) the charge-to-mass ratio is 1.76 x 10 8 C/g. Millikan (1868-1953) performed an experiment using oil drops that determined the electron charge (see animation) oil drops reach a terminal velocity due to air-friction oil drops become charged by exposing them to X-rays charge per drop multiples of 1.60 x 10 19 C
Proton By ~1900 scientists knew that atoms contained electrons had an overall charge of zero Therefore atoms must also contain positive charge. Rutherford (1871-1937) studied α particles hitting gold foil (α particle = He 2+ = helium nucleus)
Rutherford concluded that the +ve charge was concentrated : he called this location the nucleus. The rest of the atom is mostly empty space (electrons are spread out) He called the +ve charge particles in the nucleus protons and found their charge and mass: charge is -1 x (e charge) mass is 1800 x (e mass) Neutron One last problem: the charge to mass ratio for the nucleus of helium was wrong (too heavy for just protons to be in there) Sir James Chadwick (1932) the rest made up of neutrons
Summary: protons/neutrons in nucleus electrons in cloud surrounding nucleus atoms have same # protons and e (neutral) Chemical reactions only involve changes to the outermost electrons!
2.3: Atomic number, Mass number, and Isotopes The only thing that defines an element is the # of protons. # neutrons can vary isotopes # electrons can vary ions A = mass # (# protons + # neutrons) Z = atomic # (# in periodic table = # of protons) X = chemical symbol 12 example: 6C This is often written as 12 C because specifying both the symbol and the atomic number is redundant.
example problem How many neutrons, protons, and electrons are in 15 N? example problem An atom is composed of 60 neutrons and has mass number 107. What element is it? 2.4: Average atomic mass Use silicon to illustrate the concepts in this section. symbol Si, atomic number = 14 (14 protons) Si has 3 stable isotopes: 28 Si is stable with 14 neutrons, mass = 27.9769 amu 29 Si is stable with 15 neutrons, mass = 28.9765 amu 30 Si is stable with 16 neutrons, mass = 29.9738 amu All other isotopes of Si are unstable (radioactive) Something seems wrong with the masses: 1 proton = 1.0073 amu, 1 neutron = 1.0086 amu So how can 28 Si have a mass less than 28 amu???
Answer: some of the mass is converted to energy (E=mc 2 ) to bind the nucleus together!! In the periodic table, silicon is listed at 28.0855 amu. Why? Answer: Natural abundance refers to the abundance of stable isotopes of a chemical element as found from natural sources on the Earth (example: from mining). For silicon, this is 28 Si 92.22%, 29 Si 4.69%, 30 Si 3.09% (If you get some silicon from a meteor it might not have the same isotopic abundance.) Therefore, the average mass of Si is (weighted average): (0.9222)(27.9769 amu) + (0.0469)(28.9765 amu) + (0.0309)(29.9738 amu) = 28.0855 amu This is the mass that is listed in the periodic table. Definition: atomic mass unit (amu) = 1/12 th of the mass of one 12 C atom Thus one 12 C atom has a mass of exactly 12 amu every other mass is referenced to 12 C 1 amu = 1.66054 x 10 24 g
example problem Lithium (symbol Li) has a mass of 6.941 amu in the periodic table. Lithium has two stable isotopes, 6 Li of mass 6.015 amu and 7 Li of mass 7.016 amu. Work out the natural abundance of the isotopes of lithium. 2.5: Periodic Table we ve already learned something about it (# of protons, average atomic mass) we ll look at it in detail in Chapter 4
2.6: The mole and molar mass Recall: one atom of 12 C weighs 12 amu and 1 amu = 1.66054 x 10 24 g Question: how many 12 C atoms do we need to have 12 g of 12 C? Answer: 12 g 1 amu 1.66054 10 24 g 1 12 C atom 12 amu = 6.022 1023 atoms This is a convenient number of atoms in terms of weighing things out in the lab.
Definition: one mole of something = 6.022 x 10 23 of that something. Example: one mole of 12 C atoms = 6.022 x 10 23 atoms of 12 C Terminology: one mole = 1 mol compare to: one gram = 1 g Definition: Avogadro s number = N A = 6.022 x 10 23 mol 1 Definition: The molar mass (M) of a substance is the mass in grams of one mole of the substance. Example: Take our substance to be atoms of phosphorous. Average atomic mass of P = 30.97 amu (from periodic table). Molar mass: 30.97 amu 1.66054 10 24 g N 1 amu A = 30.97 amu 1.66054 10 24 g 6.022 10 23 mol 1 = 30.97 g/mol 1 amu The mass listed in the periodic table for each element: Phosphorous: mass listed = 30.97 30.97 amu (on average) per P atom 30.97 grams per mole (g/mol) of P atoms
Useful conversion chart from textbook example problem How many moles are in 250 g of Ag (silver)? Answer: from the periodic table, Ag = 107.9 g/mol 250 g 107.9 g/mol = 2.317 mol example problem How many atoms are in 1.00 x 10 9 g of Pb (lead)? Answer: from the periodic table, Pb = 207.2 g/mol = 207.2 amu for one atom of Pb (on average). Two options: use N A or use amu to g conversion. Option 1: 1.00 1 10-9 g 207.2 g/mol 6.022 1023 mol 1 = 2.91 10 12 Option 2: 1.00 10 9 g 1 amu 1.66054 10 24 g 1 atom 207.2 amu = 2.91 1012
Schematic of a mass spectrometer How does one determine atomic masses? Avogadro s # in terms of stoichiometry (preview) H 2 + O 2 H 2 O 2H 2 + O 2 2H 2 O (not balanced) (balanced chemical equation) We say that hydrogen gas and oxygen gas combine in a 2:1 mole ratio to form water. Example problem: how much oxygen gas combines with 7.4 g of hydrogen gas to form water? (won t solve until Chapter 5)