Historical Development of the Atomic Theory Democritus must

Historical Development of the Atomic Theory

Democritus (must know) Ancient Greek philosopher (circa 400 BC) Matter is made of tiny particles that we can’t see Atomos means indivisible Thought these spheres of matter were solid and could not be broken down further

John Dalton (1766 – 1844) English scientist and teacher Performed experiments 4 points to his atomic theory:

Dalton’s atomic theory: do not need to know his name, you DO need to know the points of his theory… 1. All elements are made of tiny indivisible atoms 2. Atoms of the same element are the same as each other and different from other elements 3. Atoms of different elements can mix together in whole-number ratios 4. Chemical reactions occur when atoms change how they are arranged.

J. J. Thompson (1856 – 1940) you do not need to know his name, you MUST KNOW the cathode ray experiment. English Physicist Passes electric current through a cathode ray tube The cathode ray is a stream of negatively charged electrons Electrons are a sub-atomic particle Must be a corresponding positive part to the atom “discovers” the electron

Credited with the “Plum Pudding” Model

Milliken – found the quantity and charge on electron. Charge is -1. mass is so small, we pretend it’s zero. Goldstein – proposed “canal rays” were positively charged particles, which we now call protons. Chadwick – Confirmed the existence of the neutron, which has NO CHARGE, just mass.

Ernest Rutherford (1871 – 1937) you must know him, you must know his gold foil experiment, you must know what he concluded from it New Zealander who worked in England Gold Foil experiment

Diagram the gold foil experiment: An alpha particle (α)is a Helium nucleus (2 protons, 2 neutrons)

What was learned from the gold foil experiment: Small, massive center called the nucleus Nucleus has a positive charge (+) Electrons (-) are outside the nucleus Most of atom is empty space

Ø Atomic number – the number of protons Ø Atomic Mass – the protons + neutrons Ø Isotopes – same element, same number of protons, different mass, different number of neutrons.

Neils Bohr (1885 – 1962) you may need to know his name associated with his atomic model. Danish Physicist Used Spectral Line Data from hydrogen Organized electrons into discrete shells or orbitals. The Bohr Model is what is used to explain electron configurations in Regents Chemistry

Modern Atomic Theory

All matter is composed of atoms An element is composed of only one type of atom (notice we don’t say exactly the same) Compounds are different kinds of elements bonded together in definite proportions

• Atoms can be broken down into smaller units called subatomic particles: • Proton • Neutron • Electron • amu stands for “atomic mass units” • NYS just shortens this to “u”

Proton positively charged (charge of +1) Mass of 1 amu Found in the nucleus Atomic # = protons

Finding Protons: Drag out periodic table

Neutron No charge (electrically neutral) Mass of 1 amu Also in the nucleus Atomic mass – atomic #

Calculating neutrons: 1. Find the atomic mass 2. Round it to the nearest whole number 3. subtract the Atomic number from it

Electron Negatively charged (charge of -1) Mass of 0 (zero) amu Found outside the nucleus “Shells” or energy levels For all neutral atoms: electrons = protons

There is another super-secret place you can find the electrons Add those numbers They are the “electron configuration” We’ll talk about them again. Shhh. Don’t tell anyone else.

Wave-Mechanical Model AKA (Also Known As): Quantum Mechanical Model Electron Cloud Deals with the probability of finding an electron in a certain area of the atom outside the nucleus.

Electron Shells (energy levels): (crack out your Periodic Table) Row # = # of electron shells.

Valence Electrons electrons in the outermost energy level Determine HOW atoms react Column # = # of valence electrons sort of…

• Column 1 = 1 valence electron • Column 2 = 2 Valence electrons • Columns 3 – 12 there is no obvious pattern

• Columns 13 -18 = 10 fewer valence electrons than column number • e. g. • Column 14 = 4 Valence electrons • Columns 15 = 5 valence electrons • Exception, He has 2 valence electrons.