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Unit 3 Part 1
Chapter 4
Early Theories of Matter
• Early philosophers formed theories based on everyday
observations.
• Thought that there were 4 basic elements:
o Earth
o Water
o Air
o Fire
• 2 forces: love & hate
Democritus
• 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?”
Aristotle
• Most influential Greek philosopher (384 – 322 BC)
• Rejected atomic theory of Democritus because it conflicted with his ideas on
nature
• 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
compounds.
o Explains the Law of Conservation of Mass
th
Defining the Atom
An atom is the smallest particle of an element that still retains
_________________________________________________________________.
•How small is an atom?
o
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
ray
•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
value)
•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
•Reasons:
o
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
•Model:
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
o
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
Nucleus
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
o
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
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