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

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

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

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?

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.

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

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)

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

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

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

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.

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

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)

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)

Angular Quantum Numbers (l)

• Indicates the shape of the orbital• L = s,p,d,f• Often called the sublevel

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

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

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

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.

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

Quantum Numbers

The building = atom

Apartment Floors = Energy Levels (shells)

Specific Apartments = Orbitals (subshells)

Residents = Electrons

Electron Configuration

• Describes arrangement of electrons in an atom

• Three rules1. Aufbau Principle2. Pauli Exclusion Principle3. Hund’s rule

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