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