Atomic Structure and
Timeline
Objectives:
1.
Describe the process why which the atomic structure was discovered
2.
Describe what an atom is an what makes it up.
3.
Identify differences in protons, neutrons and electrons
Democritus
• 400 BC
• Proposed that matter was
composed of tiny indivisible
particles
• Not based on experimental data
John Dalton
• 1807
• British schoolteacher that based his theory on other’s data
• Billiard Ball Model
• Atom is uniform, solid sphere
Dalton’s 4 Postulates
• Elements are composed of small indivisible particles called
atoms
• Atoms of the same element are identical. Atoms of different
elements are different.
• Atoms of different elements combine together in simple
proportions to create a compound.
• In a chemical reaction, atoms are rearranged, but not changed
Henri Becquerel
• 1896
• Discovered radioactivity
• Spontaneous emission of radiation from the
nucleus
• Alpha, beta and gamma (p, neg, neu)
J.J. Thomson
• Cathode Ray Tube Experiments
• Beam of negative particles
• Discovered electrons
• Negative particles within an atom
• Plum-pudding Model
Plum-Pudding Model
Ernest Rutherford
• 1911
• Gold Foil Experiment
• Discovered the nucleus
• Dense, positive charge in center of atom
• Nuclear Model
Niels Bohr
• 1913
• Energy Levels
• Electrons can only exist in specific energy states
Erwin Schrodinger
• 1926
• Quantum mechanics
• E- can only exist in specified energy states
• Electron Cloud Model
• Orbital-region around nucleus were electrons are likely to be found
James Chadwick
• 1932
• Discovered neutrons
• Neutral particles in the nucleus of an atom
Atomic Structure
• An element is composed of one type of atom
• Atom—smallest piece of matter that sill retains the properties
of the element
• Composed of: protons, neutrons and electrons
Atomic Components
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Protons (+1)and neutrons (neutral) found in nucleus
Nucleus--Positively charged
Surrounded by electrons(-1)
Nucleus—center core of atom
ATOM
NUCLEUS
ELECTRONS
PROTONS
NEUTRONS
POSITIVE
CHARGE
NEUTRAL
CHARGE
Most of the atom’s mass.
NEGATIVE CHARGE
Atomic Number
equals the # of...
Atomic Components
Protons and neutrons are made up of quarks
So far, only 6 uniquely different quarks
Scientist theorize that 3 quarks held together with strong nuclear
force produces a proton
A different arrangement of 3 other quarks produces a neutron
Electron Cloud Model
• Electron cloud—area around the nucleus of an atom where
electrons are most likely found
• 100,000 times larger than the diameter of the nucleus
• Electrons in the cloud are a lot smaller than a proton
• Electrons are so small and moves so quickly, it is impossible to
describe its exact location
• Bohr Model:
• Electrons can only exist at certain energy levels
• Low levels—close to nucleus
• Each level can hold 2n2
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Level 1—2
Level 2—8
Level 3—18
Level 4--32
Atomic Mass
• Nucleus contains most of the mass b/c protons and neutrons
are so large
• Mass of proton and neutron are about equal
Atomic Mass
• Unit of measurement—atomic mass unit (amu)
• Mass of a proton and neutron is about 1 amu
Protons
• The number of protons defines the atom
• Ex. Every carbon atom has six protons; all atoms with six
protons are carbon atoms
• # protons=atomic number
Mass Number
• Sum of protons and
neutrons in the nucleus of
an atom
• Always a whole number
Neutrons
• Number of neutrons=mass number-atomic number
• Not all atoms of an element have the same number of
neutrons
• Isotopes—atoms of same element with different number o f
neutrons
• Mass Number
• Atomic Number
Isotopes
Identifying Isotopes
• Models of two isotopes of boron are shown below. Neutrons
are different, so the mass numbers are different
• You use the name of the element followed by the mass
number of the isotope to identify each
• Boron-10 and boron-11
Identifying Isotopes
• Average atomic mass—weighted-average mass of the mixture
of isotopes
• 4 out of 5 atoms of boron are boron-11 and one out of five are
boron-10
Practice
• Using the periodic table of elements in the back of your text
book:
• Identify the number of protons, neutrons and electrons in each of
the following elements
• Ca, B, C, O, N, F, Na, K, Mg, Mn, Si, As, He, H
• Draw an Electron Cloud Model (Bohr Model) for each of the
following elements
• Cl, Br, N, O, C, Mg
Project
• Pick an element on the Periodic Table
• Construct a 3D Bohr Model of that element
• Make a brochure using the same element
• Include:
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Atomic mass
Atomic number
Who discovered it
When was it discovered
How was it named
What is its natural state
Melting/boiling point
Group
Characteristics
Interesting facts
Pictures
Bohr model
• Due Friday