Democritus
Thompson
• Matter was very small particles
• Used a cathode ray tube to discover that there is a negatively charged particle in the atom (electron)
• Cannot be divided
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Dalton
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Positive magnet
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+
+
+
+
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Negative magnet
• All elements are composed of atoms, which can not be divided
Rutherford
• All atoms of the same element have the same mass
• Shot alpha particles at gold to discover that the atom has a very small, dense, positively charged mass at the center of the atom (nucleus, he called it the proton)
• Compounds contain atoms of more than one element.
– atoms always combine in fixed ratios (CO2 = 1 C & 2 O)
Alpha (+)
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What is still true?
Expected Actual
Bohr
• Planetary based model, electrons have a fixed orbit (path) around the nucleus.
• Electrons move in fixed energy levels
1st level = 2 e­ max
2nd level = 8 e­ max
3rd level = 18 e­ max
4th level = 32 e­ max
5th level = 32 e­ max
6th level = 18 e­ max
7th level = 8 e­ max
8th level = 2 e­ max
• Electrons can move up and down energy levels
– Normal location = ground state
– Higher energy level = excited state
– When electrons move back down to ground state they release energy in the form of light
Electron Cloud Model
• Shows the most likely place for an electron to be found, NO ORBITS
atom
Contain
nucleus
electron
composed of proton
whose number is called the
atomic number
neutron
whose number added together make the
Atomic Number = # or protons OR # of electrons
mass number
which identifies
Mass Number = # of protons + # of neutrons
Isotopes are the same element, but different numbers of neutrons.
isotopes
Drawing Shells/Atomic Diagrams:
Drawing Lewis Dot Structures:
1. Determine the number of protons, neutrons, & electrons.
1. Determine the number of valence electrons for the element.
2. Indicate the number of protons and neutrons in the middle of small circle to represent the nucleus
2. Write the symbol for the element
3. Use your periodic table to determine how many electrons go in each energy level.
3. Place dots around the symbol ­ one for each valence electrons
a. Imagine the symbol has four sides ­ add one dot to each
side before you pair them up
b. Always add dots going around like a circle (doesn't matter which way).
Mg
Organizing the Elements
Organizing the Elements
Johann Doberiner (1829)
John Newlands (1863)
• Classified elements into groups of 3
• Noticed that certain properties repeated every
– Chlorine, Bromine, & Iodine
« 35.453, 79.904, 126.80
8th element.
• Known as the Law of Octaves
« All reactive gases
– Lithium, Sodium, & Potassium
« 6.941, 22.990, 39.098
« All soft, reactive metals
• Known as the Model of Triads
Organizing the Elements
Dimitri Mendeleev & Lothar Meyer (1869)
Periods
• Separately published a table organizing the elements by increasing atomic mass
• Both left spaces where unknown elements should fit
• Mendeleev was the only one to use properties to help arrange the elements. > He was able to predict the properties of the unknown elements because of this. > This is why Mendeleev is known as the Father/Creator of the periodic table.
G
r
o
u
p
s
Periods
• Each Row
• Elements have similar energies
• There are 7 periods (so far)
Groups/ Family
• Each Column
• Elements have similar properties
M
et
all
Metals
Very Reactive
Least Reactive
oid
s
Very Reactive
Not Reactive
non­metals
Metals
• All elements that are left of the zig­zag line
Metalloids
• Good conductors of heat and electricity
• Located on the zig­zag
• Are ductile & malleable
• Have a mixture of metal and nonmetal properties
Non­metals
• All elements that are right of the zig­zag line
• Poor conductors of heat and electricity
Reactivity
• Ability of an element to form compounds
• Are not ductile & malleable
• Also shown in the dot structure!
Transition Metals
Inner Transition Metals
Nobel Gases
• Elements in the same group have same number of valence electrons (That's why they have similar properties!)
Nitrogen Family
Oxygen Family
Halogens
• Are the electrons responsible for chemical reactions
Boron Family
Carbon Family
• An electron in the highest energy level
Alkali Metals
Alkaline Earth Metals
Valence Electrons