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Bohr Model of the Atom Electrons reside in specific energy levels most of the time. When electrons are excited, they can jump from one energy level to another. They then fall back to their ground state and give off light based on how far they fell. This produces a line emission spectrum Electrons have wavelike properties, which was confirmed by experiments.

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**Current Model of the Atom**

It turns out that Bohr’s model is too simple. Very difficult math is involved in trying to find the location of an electron and the best we can do is estimate where it is at any given time. About 90% of the time an electron resides in an orbital with given energy. There are 4 descriptions to say where an electron resides: We can say that we live in Oklahoma and then get more descriptive and say Tulsa. We can then get more descriptive and say our zip code and then our street. -- These descriptions for electrons are called quantum numbers.

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**Quantum Numbers PRINCIPAL: (n) energy level, distance from the nucleus**

ANGULAR MOMENTUM: (l) sublevel (s,p,d,f) MAGNETIC: (ml) spatial orientation, orbital SPIN: (ms) spin

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**Energy Levels (n) The principal quantum number has the symbol n.**

n = 1, 2, 3, 4, “shells” The electron’s energy depends principally on n. n = 1 (2 electrons) n = 2 (8 electrons) n = 3 (18 electrons) n = 4 (32 electrons) Distance from the nucleus is directly proportional to energy.

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**Sublevels The angular momentum quantum number has the symbol .**

= s, p, d, f, g, h, (n-1) tells us the shape of the orbitals. These sublevels are the volume around the atom that the electrons occupy 90-95% of the time. Sublevels are regions of space where the probability of finding an electron about an atom is highest.

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Orbitals The symbol for the magnetic quantum number is m, representing the spatial orientation or the orbital. m = - , (- + 1), (- +2), , , ( -2), ( -1), If = 0 (or an s orbital), then m = 0. If = 1 (or a p orbital), then m = -1,0,+1. If = 2 (or a d orbital), then m = -2,-1,0,+1,+2. If = 3 (or an f orbital), then m = -3,-2,-1,0,+1,+2, +3. Theoretically, this series continues on to g,h,i, etc

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Spin quantum number The last quantum number is the spin quantum number which has the symbol ms. The spin quantum number only has two possible values. ms = +1/2 or -1/2 An electron can spin clockwise or counterclockwise Spin quantum number effects: Every orbital can hold up to two electrons. The two electrons are designated as having one spin up and one spin down Spin describes the direction of the electron’s magnetic fields.

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**The s sublevel s sublevels are spherically symmetric**

There is one s sublevel per energy level. = 0 for the s sublevel There is only one orbital (m) in every s sublevel. m = 0. The s sublevel holds 2 electrons in its one orbital—one spinning “up” and one spinning “down.”

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**The p sublevel The first p orbitals appear in the n = 2 energy level**

p orbitals are peanut or dumbbell shaped. There are 3 p orbitals per n level. The three orbitals are named px, py, pz. = 1. m = -1,0,+1 The first p orbitals appear in the n = 2 energy level The p sublevel hold 6 electrons (2 in each orbital)

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**The d sublevel d orbital properties:**

The first d orbitals appear in the n = 3 shell. The d sublevel can hold 10 electrons. The five d orbitals have two different shapes: 4 are clover leaf shaped. 1 is peanut shaped with a doughnut around it. The orbitals lie directly on the Cartesian axes or are rotated 45o from the axes. There are 5 d orbitals per n level. The five orbitals are named They have an = 2. m = -2,-1,0,+1,+2 5 values of m Each orbital hold 2 electrons

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d orbital shapes

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**f orbital properties: The f orbitals have the most complex shapes.**

The first f orbitals appear in the n = 4 shell. The f orbitals have the most complex shapes. There are seven f orbitals per n level. The f orbitals have complicated names. They have an = 3 m = -3,-2,-1,0,+1,+2, values 14 electrons can be placed in the f sublevel (2 in each orbital)

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f orbital shapes

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**Placing Electrons in the electron cloud**

Pauli Exclusion Principle No two electrons in an atom can have the same set of 4 quantum numbers. Only 2 electrons can reside in an orbital (one spinning up and one spinning down) The Aufbau Principle describes the electron filling order in atoms. --Electrons are placed in the lowest energy orbital available. Hund’s rule: Place one electron in each orbital of a sublevel before doubling up.

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**Electron Configurations and Orbital Notation**

1s22s22p s22s22p s22s22p s22s22p4

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**Specific quantum numbers for each electron**

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