 Atom – the smallest particle of an element that retains its identity in a chemical reaction

 Democritus › 460 B.C. – 370 B.C. › Atoms were indivisible and indestructible!!! Challenge – muahhaha (Halloween’s coming) Do we still believe this?  What was wrong with Democritus’ theory? › Could not explain chemical behavior

 John Dalton › – English Chemist › Dalton’s Atomic Theory  All elements are composed of tiny indivisible particles called atoms  Atoms of the same element are identical. The atoms of any one element are different from those of any other element.  Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.  Chemical rxns occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction

 The atom is a tiny, tiny, tiny thing.  Average radii of atoms falls between 5 x m to 2 x m  Let’s put this in perspective › If you put 1,000,000,000 of Cu in a straight line, it would measure 1 cm  Can we see atoms? › Scanning tunneling microscopes

 What are the subatomic particles? › Protons  Relative mass – 1.00 amu  Electrical charge +1 › Neutrons  Relative mass – 1.00 amu  Electrical charge – 0 (neutral) › Electrons  Relative mass 1/1840 amu  Electrical charge -1  Discovered by J.J. Thompson  Cathode ray tube

 Where are those subatomic particles located? › Nucleus  What is the overall charge of the nucleus?  Positive › The Nuclear Model › What is the overall charge of an atom?

 Identity of an atom is determined by › # of protons  What else can we use to distinguish atoms from each other? › Mass number  The sum of protons and neutrons in an atom.

Element symbol Mass number Atomic number

 A nitrogen atom has 7 protons and 7 neutrons. a) What is the atomic number? b) What is the mass number? c) Write the isotopic symbol?

 Atoms that have the same number of protons but different number of neutrons › Results in a different mass number For example carbon-12 and carbon-13

-Passed electric current through gases at low pressure. -The beam (cathode ray) is the result. Travels from the cathode to the anode. -If a magnet is placed near the tube, the ray can be deflected. -Knowing that opposite charges attract, he hypothesized that the ray was negative. -Through further experimentation he found that the mass to charge ratio was constant and independent of the gas in the tube. -Mass of the electron was calculated by Robert A. Millikan