Chapter 3

Atoms: Basic Building Block of Matter

Atoms• Democritus coined term in 400 B.C.• Dalton’s Atomic Theory– Atoms compose all matter. – An element’s atoms are identical is size, mass, &

properties. {Didn’t know about isotopes}– Atoms cannot be subdivided, created, or

destroyed. {Didn’t know about fusion or fission}– Atoms of different elements combine in simple

whole-number ratios. – Atoms combine/ separate/ rearrange in reactions

Structure of Atom

• Proton: 1 amu; (+) charge; Goldstein found it using cathode ray tube

• Neutron: 1 amu; 0 charge; Chadwich • Electron: 0 amu; (-) charge; Thomson found it

with cathode ray tube; Millikan measured charge & mass; Bohr proposed concentric electron orbitals

• Nucleus: composed of protons & neutrons; Rutherford found it using gold-foil experiment

Counting Atoms• Atomic Number = # of protons– # found in corner of element’s box on periodic

table• Mass Number = # of protons + # of neutrons– Rounded value of mass below symbol on periodic

table– Mass # minus Atomic # = # neutrons• Na’s atomic mass 23 [protons + neutrons] • minus atomic number 11 [ protons] • equals 12 [neutrons]

Counting Atoms II• Average Atomic Mass = Weighted average of all

naturally found isotopes of that element• S (% abundance of isotope)(mass of isotope)– S means “the sum of “

For oxygen: O-16 99.757% @15.994915 amu; O-17 0.038% @ 16.999132 amu; O-18 0.205 % @ 17.999160 amu

(0.99757)(15.994915 amu)+ (0.00038)(16.999132) + (0.00205)(17.999160amu) = 15.9994053 amu

Counting Atoms III

• Molar Mass = grams/ mole– Noted on periodic table under the symbol of the element– Mole = same # of atoms of C-12 in 12 grams of C-12

• Avogadro’s number = 6.022 x 1023 particles/mole

• Relative atomic masses– Noted on periodic table (Same # as the molar mass)– amu = atomic mass unit– 1 amu = 1/12 of C-12 mass (i.e., avg mass of no & p+)

Problems

• How many grams are in 2.30 x 1018 H2 atoms?(2.30 x 1018 atoms)/(6.022 x 1023 atoms/mole) =

(2.3/6.022)(1018/1023) = 0.382 x 1018-23 mole = 3.82 x 10-6 mol;(3.82 x 10-6 mol)(2.01 g/mol)= 7.68 mg

• How many atoms are in 3.50 mol Cu? (3.50 mol Cu)(6.022 x 1023 atom/mol) = 21.1 x 1023 = 2.11 x 101 x 1023 = 2.11 x 1024 atom

• What is the mass in grams of 3.50 mol Cu?(3.50 mol Cu)(63.55 gram/mol) = 222 g Cu