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Chapter 11 The Atom The Atom • The smallest particle into which an element can be broken down. • size: 3x10-8 cm = .00000003 cm 3 hundred millionths cm • Structure - Subatomic particles located in the nucleus and the electron cloud Nucleus • small, dense, positively charged center of the atom Contains: + Protons – positively charged particles • Mass: 1amu (atomic mass unit) = 1.7x10-24 g 0 Neutrons – no charge • Mass: 1 amu • Most of the mass of an atom is found in the nucleus. • The volume is small which is why it is dense. Electron Cloud • mostly empty space. Contains: – Electrons – negatively charged particles • Mass: 0 amu • Can only predict where the electrons can be found around the nucleus. • Most atoms have an equal number of protons and electrons. Forces in the Atom • Electromagnetic force – holds the electrons around the nucleus. Negative electrons are attracted to the positive protons, opposites charges attract. • Strong force – holds the nucleus together. Stronger than the electromagnetic force that pushes the protons apart, like charges repel. • Weak Force – plays a key role in radioactive atoms where neutrons turn into protons and electrons Atomic Number • Number of protons in the nucleus • Determines the elements • Found in each elements square on the periodic table. Isotopes • Atoms of the same element with a different number of neutrons. • You can tell them apart by their mass numbers • Mass number = number of protons plus neutrons Naming Isotopes • Element name – mass number Hydrogen – 1 (1 proton + no neutrons =1) Hydrogen – 2 (1 proton + 1 neutron = 2) Number of Neutrons in an Isotope # neutrons = mass # – atomic # p+n–p=n Unstable Isotopes • Some isotopes are unstable making them radioactive (break apart over time) Carbon – 14 used in carbon dating Uranium – 238 used in nuclear chain reaction Atomic Mass Weight • average of all the known naturally occurring isotopes of an element. Calculating Atomic Mass Weight Copper’s atomic mass = 63.6 amu Known Isotopes: copper – 63 (69%) copper – 65 (31%) Equation: mass# X percentage + mass# X percentage +.…= atomic mass 63 amu (.69) + 65 amu (.31) = 63.6 amu Try These Chlorine = 35.5 amu Chlorine – 35 (76%) Chlorine – 37 (24%) Thallium = 204.4 amu Thallium – 203 (30%) Thallium – 205 (70%) Gallium = 69.8 amu Gallium – 69 (60%) Gallium – 71 (40%) Uranium = 237.978 amu Uranium – 238 (99.284%) Uranium – 235 (0.711%) Uranium – 234 (0.005%) Titanium = 47.9 amu Titanium – 46 (8%) Titanium – 47 (7.3%) Titanium – 48 (73.8%) Titanium – 49 (5.5%) Titanium – 50 (5.4%) Ions • Atoms with a charge because they gained or lost an electron. • Have an unequal number of protons and electrons charge = protons – electrons Positive Ion Sodium atom has 11 protons Sodium Ion has 10 electrons +11 – 10 = +1 Negative Ion Chlorine atom has17 protons Chlorine Ion has 18 electrons +17 – 18 = – 1