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The Atom
Law of Conservation of Mass
• Was discovered by Antoine Lavoisier
• Helped moved alchemy to be chemistry
• Mass cannot be created nor destroyed in normal chemical or physical processes.
Aristotle – the four basic elements
-Earth-Air-Water-Fire
Democritus-Follower of Leucippus-An Atomist-Coined the word atomos meaning uncuttable or the smallest particle of matter
2000 years laterJohn Dalton
– School Teacher
Five main points of Dalton's atomic theory1. Elements are made of tiny particles called
atoms.2. All atoms of a given element are identical.
3. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.4. Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms.5. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.
Daltons Model of the Atom
Cathode Ray Tube
Plum Pudding Model of the AtomJJ Thompson
Gold Foil Experiment – Ernest Rutherford
Rutherford’s Model of the Atom
Particle SymbolRelative Charge Mass (kg)
Mass Number Location
electrone- -1 9.109x10-31 0Around Nucleus
proton p+ +1 1.673x10-27 1Nucleus
neutron n0 0 1.675x10-27 1Nucleus
Atomic Number: The number of protons in the nucleus of an atom
Mass Number: The total number of protons and neutrons on a nucleus
Atomic NumberSymbolAtomic Mass
Isotope: Atoms of the same element that have different masses
Determining the number of protons neutrons and electrons in an isotope.
Cl3517
# of protons = atomic number17
# of neutrons = mass number – atomic number35 - 17= 18
# of electrons = # of protons in a neutral atom17
Bromine-80
# of protons = atomic number35
# of neutrons = mass number – atomic number80 – 35 = 45
# of electrons = # of protons in a neutral atom35
Practice!!1) Aluminum -27
2) Copper-65
3) B
4) Xe
5) Lead-208
11 5
131 54
protons-13 neutrons-14 electrons-13
protons-29 neutrons-36 electrons-29
protons-5 neutrons-6 electrons-5
protons-54 neutrons-77 electrons-54
protons-82 neutrons-126 electrons-82