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Unit 3 Part 1
Chapter 4
Early Theories of Matter
• Early philosophers formed theories based on everyday
• Thought that there were 4 basic elements:
o Earth
o Water
o Air
o Fire
• 2 forces: love & hate
• Greek philosopher (460 – 370 BC)
• Proposed matter made up of individual particles called atomos
o Atoms could not be _____________, _________________, or
further ________________
• Could not answer question “What holds atoms together?”
• Most influential Greek philosopher (384 – 322 BC)
• Rejected atomic theory of Democritus because it conflicted with his ideas on
• Did not believe that “nothingness” of empty space could exist
John Dalton
19 century English school teacher (1766 – 1844)
Revised Democritus’s ideas based on results of scientific research
Proposed his Atomic Theory in 1803
o All atoms of a given element were ___________________,
and atoms of different elements could join to form
o Explains the Law of Conservation of Mass
Defining the Atom
An atom is the smallest particle of an element that still retains
•How small is an atom?
6 billion atoms side by side would be less than 1 m long
Discovering the Electron
Research with cathode ray tubes convinced scientists of the following:
1. Cathode rays were actually a stream of charged particles
2. The particles carried a negative charge that were found in all forms of matter
__________________: negatively charged particles that are part of all forms of matter
J. J. Thomson (1856 – 1940)
•English physicist
•Series of cathode ray tube experiments to determine ratio of
charge to mass of one cathode ray particle
•Measured effect of both magnetic & electric fields on cathode
•Determined charge-to-mass ratio of charged particle
•Compared ratio to other known ratios
•Mass of charged particle much less than hydrogen atom, lightest
known atom
•Conclusion: Dalton’s theory was WRONG!
•Identified 1st subatomic particle: electron
Robert Milliken (1868 – 1953)
•American physicist
•Determined charge of electron (within 1% of currently accepted
•Charge equated to single unit of negative charge
(charge electron = ________ )
Milliken used charge to calculate mass of eo Mass e- = _______________________________
= 1/1840 mass H-atom
_______________________________ Model
•Proposed by J.J. Thompson
Matter is ____________________ so must be something other than electrons
o Mass of e- so extremely small so where does the rest of the mass come from
o __________________________________________________________
Draw a picture showing the Plum Pudding Model. Label the major
parts of the model.
Ernest Rutherford (1871 – 1937)
o New Zealand native, won Nobel Prize in chemistry in 1908
o 1911 – studied how positively charged alpha particles
interacted with matter
o Shot alpha particles at thin sheet of gold foil
Hypothesis: alpha particles would pass straight thru foil with
little deflection by small e-
o Observations: alpha particles deflected at very large angles, some even straight
back toward source
o Proposed new model to explain observations
o Rutherford proposed:
o That an atom consisted of ______________________________________
o Concluded there was a tiny, dense region called _______________
centrally located within atom that contained all of an atom’s
________________________________ and almost all of its mass
o Electrons move through available space around nucleus & are held within
atom by attraction to positive nucleus
Protons & Neutrons
o 1920 – Rutherford revised concept of nucleus
o Concluded that nucleus contained positive particles
o _____________________: subatomic particle carrying charge equal to but
opposite that of an electron
Charge = ______________
o 1932 – English physicist James Chadwick showed nucleus also contained
o Neutron: mass nearly equal to that of proton but has
The Atom
o Neutral particle composed of electrons, protons & neutrons
o Spherically shaped with tiny, dense, positive center surrounded by 1 or more
negative electrons that move quickly through empty space around nucleus
o Nucleus is 99.97% of atom’s mass
Atoms are neutral so ___________________________________
Atomic Number
o Henry Moseley (1887 – 1915)
o English scientist
o Discovered atoms of each element contain a
unique positive charge in nucleus
o Number of __________________________ in atom
identifies it as an atom of particular element
o _______________________________________:
number of protons in atom (and number of electrons)
o Determines element’s place in periodic table
Atomic Mass
Mass proton = _______________________________ amu
Mass neutron = ______________________________ amu
Mass electron = ______________________________ amu
Mass of an atom is approximately the mass of ______________ + mass of
Mass of an element is a weighted average of the mass of the element’s
Bohr Model of the Atom
Niels Bohr
o Danish physicist
o Worked for Rutherford in 1913
o Proposed quantum model of atom that seemed to explain the
discontinuous spectra of elements
o Model correctly predicted frequencies of lines in hydrogen’s atomic
emission spectrum
o Said that electrons moved around nucleus in ________________________
______________________ (like planets orbit the sun)
o Electrons could change energy levels by absorbing or releasing
Modern Atomic Theory
Various scientists contributed to modern atomic theory
Important parts:
o Electrons behave more like waves than particles
o Exact location, speed, and direction of electrons cannot be determined
o __________________________: a region in an atom where there is a high
probability of finding electrons
Energy Levels
Electrons have various amounts of energy and exist in different energy levels
_________________________ Electrons
o Electrons in atom’s outermost orbitals associated with atom’s
____________________________ energy level
o Determine element’s ______________________________ properties
Electron Transitions
Electrons jump between energy levels when an atom gains or loses energy
__________________________________: lowest state of energy of an electron
________________________________: electron is in higher energy level
because atom absorbed energy
o particle of light that is absorbed or emitted by an atom
o Have different amounts of energy that determines the energy level an
electron will transition to
Atomic Emission Spectra
Set of wavelengths of light emitted by atoms of an element when electrons drop
from ________________ energy levels to ___________________ energy levels
Similar to fingerprint
o Each element has unique emission spectra
o Spectra can be used to identify elements
o Only certain colors appear in spectrum therefore, only photons having
certain specific energies are emitted by excited atoms