do now: 1. determine how many protons, neutrons, and electrons are in: a. selenium-79 b. xenon-131...
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Do Now:
1. Determine how many protons, neutrons, and electrons are in:
a. Selenium-79b. Xenon-131c. Cesium-133
2. Determine the number of protons, neutrons, and electrons in Mg-24, Mg-25, and Mg-26
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Do Now:
Protons Electrons Neutrons
Selenium-79 34 34 45
Xenon-131 54 54 77
Cesium-133 55 55 78
Mg-24 12 12 12
Mg-25 12 12 13
Mg-26 12 12 14
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YWBAT
• Describe what Bohr proposed in his model of the atom.
• Describe what the quantum mechanical model determines about the electrons in an atom.
• Explain how sublevels of principal energy levels differ
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Revising the Atomic Model
• Rutherford’s atomic model could not explain the chemical properties of elements.
• For example, why does iron first glow dull red, then yellow, then white when heated to higher and higher temperatures?
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The Bohr Model
• Niels Bohr (1913) • Incorporated discoveries about how the
energy of atoms changes when the atom absorbs or emits light.
• Stated that the electrons orbit the nucleus like planets orbit the Earth.
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The Bohr Model
• Each possible electron orbit has a fixed energy known as an Energy Level
Quantum of Energy Amount of energy required to move an electron from one energy level to another
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The Bohr Model
Bohr’s model can be compared to rungs of a ladder.
•Electrons cannot exist between energy levels•The energy levels in atoms are unequally spaced (higher energy levels are closer together)
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The Bohr Model
• When an electron occupies the lowest possible energy level it is said to be in its ground state.
• An electron can absorb energy (become excited) from an external source:– Sun– Fire (heat)– Electricity
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The Bohr Model
• When electrons absorb energy, they can jump to higher energy levels. This is called the excited state.
• When the electron returns to the ground state, it releases energy in the form of light.
Emission Line
Spectra
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The Bohr Model
• Scientists began wondering why the electron had to be in a specific orbit.
• This led to the Heisenberg Uncertainty Principle which states:
At a given instant in time, it is impossible to determine both the position and the velocity of an electron
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Quantum Mechanical Model
• Erwin Schrodinger developed and solved a mathematical equation describing the behavior of the electron in a hydrogen atom.
• The modern description of electrons, the quantum mechanical model, came from the solutions to the Schrodinger equation.
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Quantum Mechanical Model
• The probability of finding an electron is represented as a fuzzy cloudlike region.
• The cloud is more dense where the probability of finding the electron is high.
Atomic Orbital – region of spacewhere there is a high probabilityof finding an electron
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Quantum Numbers
Used to specify properties of atomic orbitals and electrons in orbitals:
1. Principle quantum number (n)2. Angular momentum number (l)3. Magnetic quantum number (ml)
4. Spin quantum number (ms)
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Principle Quantum Numbers (n)
• Indicates the main energy level (shell) occupied by the electron
• Assigned values n=1,2,3,4,5,6,…• Energy level corresponds to the period (row)
of the periodic table– 1st energy level = first period (n=1)– 2nd energy level = second period (n=2)
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Angular Quantum Numbers (l)
• Indicates the shape of the orbital• L = s,p,d,f• Often called the sublevel
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Sublevel
The number of sublevels within a principal energy level is always equal to the principal quantum number
# sublevels = principal quantum number
For example, n=1 has 1 subleveln=2 has 2 sublevels
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Sublevel
There can be several orbital configurations within a sublevel (i.e., s,p,d,f).
S - subshell contains 1 orbitalP - subshell contains 3 orbitalsD - subshell contains 5 orbitalsF - subshell contains 7 orbitals
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Sublevel
Parts of the periodic table corresponds to each orbital shape (angular momentum number)
Groups 1 & 2 – s blockGroups 13-18 – p blockGroups 3-12 – d blockBottom two rows – f block
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Spin Quantum Number (ms)
• Indicates the spin of the electron • Clockwise & Counter-clockwise • A vertical arrow indicates an electron and its
direction of spin.• In any orbital, the spin of the 2 electrons must
be opposite.
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Principal Energy Levels
•# of orbitals in an energy level is equal to n2
•Max of 2 electrons can occupy a single orbital.•The maximum number of electrons that can occupy an energy level is 2n2
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Quantum Numbers
The building = atom
Apartment Floors = Energy Levels (shells)
Specific Apartments = Orbitals (subshells)
Residents = Electrons
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Electron Configuration
• Describes arrangement of electrons in an atom
• Three rules1. Aufbau Principle2. Pauli Exclusion Principle3. Hund’s rule
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Electron Configuration
1. Aufbau Principlea) An electron occupies the lowest energy orbital that
can receive it.b) Fill electrons lowest to highest energy
2. Hund’s Rule- Orbitals of the same energy must be occupied by 1
electron before it can be occupied by two electrons
3. Pauli-Exclusion Principle- Each orbital can only hold 2 electrons with opposite
spins
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