HISTORY of the Atom Atomic Model Development 1807 1896 1903 400 BC 1911 1913 1932 HISTORY of the Atom Atomic Model Development Rev 6/7/06

Scientist to Know Ernest Rutherford 1871-1937 John Dalton 1766-1844 J.J. Thomson 1856-1940 Neils Bohr 1885-1962

Dalton’s Postulates - 1807 All elements are composed of tiny invisible solid particles called atoms Atoms of same element share the same mass and properties and different elements have different masses and properties Atoms cannot be created nor destroyed, only rearrange to make new compounds Atoms join together in whole number ratios to form compounds. Example: H2O water is always found in a 2:1 ratio

Thomson’s Atom - 1897 Discovered electrons w/ cathode ray Plum Pudding model (for us Americans it looks like a chocolate chip cookie model) Electrons - charge + charge “pudding”

Rutherford’s Gold Foil Experiment The experiment involved firing positive (alpha) particles through a sheet of gold foil The alpha particles were detected by using zinc sulfide screens. A black burn mark was left where an alpha particle hit. Rutherford found that the majority of particles passed straight through. Very rarely the alpha particles were deflected at different angles.

Rutherford’s Atom - 1909 Discovered 2 Facts: 1) Atom contains a dense positively charged nucleus 2) Atom is made of mostly empty space electrons protons Gold Foil Animation

Bohr’s atom - 1913 Rutherford’s proposal of the atom made it unstable so Bohr applied quantum energy to fix the stability Electrons orbit in energy levels around the protons in the nucleus (like our solar system…) p+

Scientist Recap Dalton J.J. Thomson Bohr Model Rutherford Experiment Solid Sphere Plum Pudding Model Positive nucleus with lots of empty space Solar System Model Experiment No experiment Discovered electron with cathode ray Gold Foil Experiment No Experiment Conclusion Dalton’s Postulates of Atomic Theory Atom is filled with positive charges with sprinkles of negative charges Atom has a central positive nucleus and made up of mostly empty space Central nucleus with electrons moving around in circular energy levels

Structure of an Atom

Nucleus responsible for most of the mass of the atom protons (+ charge) neutrons (no charge) Mass = 1 amu

(-) charge responsible for most of the volume of the atom Electrons (-) charge responsible for most of the volume of the atom negligible mass (0)

How are the protons, neutrons, and electrons arranged in the atom? In the picture can you determine what element is being represented? What does each color represent? + - + +

Let’s review on the periodic table… 3 Li 6.941 Let’s review on the periodic table… Atomic number – this is the number of PROTONS Average atomic mass- the average from all its naturally occurring ISOTOPES (decimal) Mass number(atomic mass)- the number of protons + neutrons (whole number) In a neutral atom, there are the same number of electrons as protons - - + + +

Isotopes Kind of like twins…alike, but different Isotopes -Same element, but with different mass due to different # of neutrons Same number of protons

How to Find Protons, Neutrons, & Electrons Protons = Atomic Number Electrons (in neutral atom) = Atomic Number Mass number = Protons + Neutrons Neutrons = Mass Number - # of Protons

K 39 19 mass # --> <--element Symbol atomic #--> neutrons + Nuclear Notation neutrons + protons--> mass # --> <--element Symbol atomic #--> 39 K 19 protons-->

Back to the atom’s nucleus… Hyphen notation looks like this: C-12 & C-14 The number after the hyphen = mass number (atomic mass) Carbon

Practice (Neutral Atoms) Protons Electrons Neutrons Carbon - 14 18O He - 5

Practice Bohr Models Electrons Practice 1st energy level: 2 2nd energy level: 8 3rd energy level: 8 4th energy level: 2 Helium – 5 Magnesium – 24