Title: Democritus (460-370 B.C.)
1Democritus (460-370 B.C.)
Democritus was one of a few Greek philosophers
who believed that all matter in the world was
made of of indivisible parts which he called
atomos, which means indivisible.
Although this theory was to be discovered to
later be the truth, Democritus ideas faded until
the seventeenth century in Europe.
2John Dalton (1766-1844)
- Dalton is known for his atomic theory
- Theory states that atoms are the smallest
chemical building block of nature - His four postulates
- Each element is composed of extremely small
particles called atoms - All atoms of a given element are identical the
atoms of different elements are different and
have different properties (including different
masses) - Atoms of an element are not changed into
different types of atoms by chemical reactions
atoms are neither created nor destroyed in
chemical reactions - Compounds are formed when atoms of more than one
element combine a given compound always has the
same ratio and kind of atoms - ATOMS ARE SMALL AND CANNOT BE DIVIDED (later
changed!)
http//en.wikipedia.org/wiki/John_DaltonAtomic_th
eory
3Discovery of Atomic Structure
- By 1850 scientists knew that atoms were composed
of charged particles. - Electrostatic attraction
- Like charges repel
- Opposites attract
4J.J. Thomson
- Experimented with cathode rays and found that the
properties didnt change - Distinguished charges within atoms, positive and
negative charges - Plum-pudding model
- Found electron charge ratio
5Cathode Rays and Electrons
- C.R. 1st discovered in mid-1880s from studies of
electrical discharge thru partially evacuated
tubes (CRTs) - Cathode rays radiation produced when high
voltage is applied across the tube. - The voltage causes negative particles to move
from the negative electrode (cathode) to the
positive electrode (anode). - The path of electrons can be altered by the
presence of a magnetic field.
6Consider cathode rays leaving the positive
electrode through a small hole.
- If they interact with a magnetic field
perpendicular to an applied electric field, then
the cathode rays can be deflected by different
amounts. - Amount of deflection depends on applied magnetic
and electric fields. - Deflection also depends on the charge-to-mass
ratio of an electron. - Thomson determined the charge-to-mass ratio of an
electron in 1897. - Charge-to-mass ratio 1.76 x 108 C/g
- C Coulomb, SI unit of electric charge
7Robert Millikan
- Millikan built on J.J. Thomsons work on
electrons by measuring their charge with his
famous oil-drop experiment. - Using Thomsons ratio, Millikan calculated
electrons mass of - 9.10 x 10-28 g which proved how much smaller
electrons are than nucleus particles.
8Millikan Oil-Drop Experiment
- Sprayed oil drops over the hole in a positively
charged plate and measured the electrostatic
force of attraction. - Found the charge on the electron to determine its
mass - Concluded the charge on the electron must be 1.60
x 10-19 C - Mass of electron 1.60 x 10-19 C 9.10 x 10 -28
g - 1.76 x 108 C/g
9Radioactivity(Spontaneous emission of radiation)
Type Symbol Charge Mass (amu)
Alpha particle He 2 4.002 60
Beta particle ß or 0-1e 1- 0.000 548 6
Positron ß or 01e 1 0.000 548 6
Gamma ray ? 0 0
10Ernest Rutherford
- -Conducted an experiment using alpha particles to
bombard gold foil to conclude - 1. Nucleus of an atom contains positive
particles that we now call protons. - The nucleus is a small dense area in the atom.
- -Studied three types of radioactive emissions
- alpha, beta gamma
- -Concluded that
- alpha
- particles were
- He nuclei
11A Positively Charged Nucleus
- Rutherford shot alpha particles though a thin
piece of gold foil. - Some of these particles were deflected instead of
passing straight through - Recall like repels like.
- When a alpha particle encountered a nucleus of
a gold atom, it was deflected by the dense
positively charged nucleus.
12James Chadwick
- James Chadwick suspected there were neutral
particles when he saw in experiments like
Rutherfords that some of the particles were
deflected backwards, meaning that they had no
charge. Chadwick had discovered the neutron!
13Niels Bohr
- Bohr proposed that an electron circles the
nucleus in allowed orbits at specific energy
levels.
orbital
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15Scientist Contributions
- Thomson
- Discovered electron (1897)
- Cathode ray experiments
- Plum pudding atomic model
- Millikan
- Mass of electron
- Oil-drop experiment (1909)
- Rutherford
- Positively charged nucleus (1911)
- Gold foil experiments
- Discovered proton (1919)
- Chadwick Discovered neutron (1932)
16Small Numbers
- Electronic Charge 1.609 x 10-19 C
- Charge on an electron -1.609 x 10-19 C
- Charge on a proton 1.609 x 10-19 C
- Atomic Mass Unit (amu) 1.66054 x 10-24 g
- Proton mass 1.0073 amu
- Neutron mass 1.0087 amu
- Electron mass 5.486 x 10-4 amu
- Unit of length used to note atomic dimensions 1
Angstrom(Å) 1x10-10 m
17- Atomic Number
- Number of protons or electrons in an element
- Identifies the element
- Atomic Mass
- Nucleus contains most of the mass of an atom.
- Protons and neutrons are each 1.67 x 10-24 g.
- Electrons are each 9.11 x 10-28 g.
- Use atomic mass unit (amu) instead of gram.
- The mass of one proton is 1 amu.
- Mass Number
- The sum of the number of protons and number of
neutrons in the nucleus - Is approximately equal to the average atomic mass
shown on periodic table. - Number of neutrons mass number atomic number
18- Isotopes
- Atoms of the same element with different numbers
of neutrons - Have the same number of protons
- Example Carbon-12 and Carbon-14
- Radioactive Isotopes
- Unstable in nature
- Can be used to date fossils and rocks
- The time it takes for half of the radioactive
atoms in a piece of the fossil to change to
another element is its half-life.
19Isotopes AX
Z
- Isotopes have the same Z, but different A.
- Isotopes have different numbers of neutrons.
- An atom of a specific isotope is called a
nuclide. - Nuclides of hydrogen include
- 1H hydrogen (protium)
- 2H deuterium (heavy hydrogen)
- 3H tritium (3H is radioactive.)
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