Chapter 3 Atoms: The Building Blocks of Matter

Sect. 3-1: The Atom: From Philosophical Idea to Scientific Theory  Democritus vs. Aristotle  Atom vs. infinitely divisible  Aristotle’s idea won out for ~ 2000 yr.  Problem with both ideas…

Foundations of Atomic Theory  Law of conservation of mass  Mass not created or destroyed  Law of definite proportions  Same compound always same ratio of elements  Water is always 2 H and 1 O

 Law of multiple proportions  Compounds of same elements combine in different ratios  Carbon dioxide (1 C : 2 O) and carbon monoxide (1 C : 1 O)

Dalton’s Atomic Theory 1)Atoms 2)Atoms of an element are the same (mass, size, etc), atoms of different atoms are different 3)Atoms can’t be divided 4)Atoms combine in ratios to form compounds 5)Atoms combine, separate, or rearrange during a chemical reaction

 What’s the difference between Democritus and Dalton???  Experimental Data!!!

Modern Atomic Theory  What’s no longer held as true from Dalton’s theory?  Atoms being divisible  Atoms of same element having same mass

Sect. 3-2: The Structure of the Atom  Atoms made up of:  Protons  Neutrons  Electrons

Discovery of the Electron  Cathode Ray Tube experiments concluded that the “cathode rays” had mass and a negative charge /abomb/page_id_15138.html&h=342&w=512&sz=35&hl=en&start=44&tbnid=niL- YcGGzm9sqM:&tbnh=88&tbnw=131&prev=/images%3Fq%3Dcathode%2Bray%2Btube%26start%3D36%26gbv%3D2%26ndsp%3D18%26svnum%3D10%26hl%3Den%26sa%3D N

 J.J. Thomson measured ratio of charge to mass for these particles  concluded particles were the same for all elements  Cathode-ray particles later named electrons

 Millikan determined exact mass of electron (about 1/1837 mass of hydrogen atom)  Confirms negative charge of electron  Inferences made:  must be something positive to balance negative  Atoms must have more particles due to tiny mass of electron and much larger mass of atom

Discovery of Atomic Nucleus  Rutherford’s gold foil experiment

 Significance of Rutherford’s experiment:  Nucleus of the atom  Very dense  Positive charge  Nucleus = marble, whole atom = football field

Composition of the Atomic Nucleus  Protons – positive charge  Very massive compared to electrons  Determine what type of element the atom is  Neutrons – neutral (no charge)  Slightly more massive than protons  Nuclear forces – short range forces that hold protons and neutrons together in the nucleus

Sect. 3-3: Counting Atoms  Atomic number = # protons  Isotopes  Same element, different masses  same # protons, different # of neutrons

 Mass # = protons + neutrons  Hyphen notation  Hydrogen - 3  Nuclear symbol 31H31H31H31H  # neutrons = mass # - atomic #

 How many of each of the following are in the carbon – 14 isotope?  Protons  Neutrons  Electrons  6, 8, and 6 respectively

Relative Atomic Masses  1 atomic mass unit (amu) = 1/12 the mass of a carbon-12 atom  Isotopes differ in mass, but not significantly different in chemical behavior

Average Atomic Masses of Elements  Weighted average of atomic masses of all naturally occurring isotopes of an element  (%of isotope) (mass isotope) + (%of isotope) (mass isotope) = (%of isotope) (mass isotope) = weighted average weighted average

Relating Mass to Numbers of Atoms  Mole – SI unit for amount of substance  Based on Carbon-12  Abbreviated mol  Counting unit (like dozen)

 Avogadro’s Number - # of particles in one mol of a substance  x particles = 1 mol  Ex: How many moles of silver are in 3.01 x atoms of silver?  mol

 Molar Mass – mass of one mole of a pure substance  Units are typically reported in g/mol  What is the molar mass of Cu?  g/mol

 Gram to mole conversion  Use molar mass as conversion factor  What is the mass in grams of 3.50 mol of Cu?  222g