Models of the AtomModels of the Atom

Prentice-Hall Chapter 5.1Prentice-Hall Chapter 5.1Dr. YagerDr. Yager

ObjectivesObjectives

IdentifyIdentify the inadequacies in the Rutherford the inadequacies in the Rutherford atomic model.atomic model.

IdentifyIdentify the Bohr model of the atom and its the Bohr model of the atom and its associated fixed energy levels.associated fixed energy levels.

DescribeDescribe the energies and positions of electrons the energies and positions of electrons according to the quantum mechanical model.according to the quantum mechanical model.

DescribeDescribe the shapes of atomic orbitals and their the shapes of atomic orbitals and their relationship to different energy sublevels.relationship to different energy sublevels.

The Development of Atomic ModelsThe Development of Atomic Models Rutherford’s atomic model could not explain Rutherford’s atomic model could not explain the chemical properties of elements. the chemical properties of elements. Rutherford’s atomic model could not explain Rutherford’s atomic model could not explain why objects change color when heated.why objects change color when heated.

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The Development of Atomic Models: 1803 -1911The Development of Atomic Models: 1803 -1911

The Development of Atomic Models: 1913 -1932The Development of Atomic Models: 1913 -1932

The Bohr ModelThe Bohr Model Niels Bohr (1885-1962) (Rutherford’s student) Niels Bohr (1885-1962) (Rutherford’s student)

proposed that an electron is found only in proposed that an electron is found only in specific circular paths, or orbits, around the specific circular paths, or orbits, around the nucleus. (think planets around the sun!)nucleus. (think planets around the sun!)

Each electron orbit in Bohr’s model has a fixed Each electron orbit in Bohr’s model has a fixed energy that prevents it from falling into the energy that prevents it from falling into the nucleus.nucleus.

The fixed energies an electron can have are The fixed energies an electron can have are called called energy levelsenergy levels..

A A quantumquantum of energyof energy is the amount of energy is the amount of energy required to move an electron from one energy required to move an electron from one energy level to another energy level.level to another energy level.

The Bohr ModelThe Bohr Model

Like the rungs of the strange Like the rungs of the strange ladder, the energy levels in an ladder, the energy levels in an atom are not equally spaced.atom are not equally spaced.

The higher the energy level The higher the energy level occupied by an electron, the occupied by an electron, the less energy it takes to move less energy it takes to move from that energy level to the from that energy level to the next higher energy level.next higher energy level.

The energies of electrons are The energies of electrons are quantized.quantized.

The The Bohr modelBohr model gave results in agreement with gave results in agreement with experiments for the hydrogen atom, but failed to experiments for the hydrogen atom, but failed to explain the energies absorbed or emitted by explain the energies absorbed or emitted by atoms with more than one electron.atoms with more than one electron.

Need a new theory!Need a new theory!

The The quantum mechanical modelquantum mechanical model determines determines the allowed energies an electron can have and the allowed energies an electron can have and how likely it is to find the electron in various how likely it is to find the electron in various locations around the nucleus.locations around the nucleus.

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The Quantum Mechanical ModelThe Quantum Mechanical Model

Austrian physicist Erwin Schrödinger (1887–Austrian physicist Erwin Schrödinger (1887–1961) used new theoretical calculations and 1961) used new theoretical calculations and results to devise and solve a mathematical results to devise and solve a mathematical equation describing the behavior of the equation describing the behavior of the electron in a hydrogen atom.electron in a hydrogen atom.

The modern description of the electrons in The modern description of the electrons in atoms, the atoms, the quantum mechanical modelquantum mechanical model, , comes from the mathematical solutions to comes from the mathematical solutions to the Schrödinger equation.the Schrödinger equation.

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The Quantum Mechanical ModelThe Quantum Mechanical ModelThe propeller blade of an airplane has the same The propeller blade of an airplane has the same probability of being anywhere in the blurry region, probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a electron cloud of an atom can be compared to a spinning airplane propeller.spinning airplane propeller.

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In the quantum mechanical model, the probability In the quantum mechanical model, the probability of finding an electron within a certain volume of of finding an electron within a certain volume of space surrounding the nucleus can be represented space surrounding the nucleus can be represented as a fuzzy cloud. The cloud is more dense where as a fuzzy cloud. The cloud is more dense where the probability of finding the electron is high.the probability of finding the electron is high.

Atomic OrbitalsAtomic Orbitals

An An atomic orbitalatomic orbital is often thought of as a is often thought of as a region of space in which there is a high region of space in which there is a high probability of finding an electron.probability of finding an electron.

Each energy sublevel corresponds to an Each energy sublevel corresponds to an orbital of a different shape.orbital of a different shape.

An atomic orbital represents a mathematical An atomic orbital represents a mathematical solution to the Schrödinger Equation and is the solution to the Schrödinger Equation and is the statistical probability of finding an electron in a statistical probability of finding an electron in a defined volume and with a specific energy.defined volume and with a specific energy.

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Atomic OrbitalsAtomic Orbitals

Different atomic orbitals are denoted by letters. Different atomic orbitals are denoted by letters. The The ss orbitals orbitals are spherical, and are spherical, and pp orbitals orbitals are are dumbbell-shaped.dumbbell-shaped.

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Atomic OrbitalsAtomic Orbitals

Four of the five Four of the five dd orbitals orbitals have the same have the same shape but different orientations in space.shape but different orientations in space.

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Atomic f OrbitalsAtomic f Orbitals

Atomic OrbitalsAtomic OrbitalsThe numbers and kinds of atomic orbitals The numbers and kinds of atomic orbitals depend on the energy sublevel.depend on the energy sublevel.

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Atomic OrbitalsAtomic Orbitals

The number of electrons allowed in each of The number of electrons allowed in each of the first four energy levels are shown here.the first four energy levels are shown here.

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1. Rutherford's planetary model of the 1. Rutherford's planetary model of the atom could not explainatom could not explain

a)a) any properties of elements.any properties of elements.

b)b) the chemical properties of elements. the chemical properties of elements.

c)c) the distribution of mass in an atom. the distribution of mass in an atom.

d)d) the distribution of positive and the distribution of positive and

negative charges in an atom. negative charges in an atom.

1. Rutherford's planetary model of the 1. Rutherford's planetary model of the atom could not explainatom could not explain

a)a) any properties of elements.any properties of elements.

b)b) the chemical properties of elements.the chemical properties of elements.

c)c) the distribution of mass in an atom. the distribution of mass in an atom.

d)d) the distribution of positive and the distribution of positive and

negative charges in an atom. negative charges in an atom.

2. Bohr's model of the atom proposed 2. Bohr's model of the atom proposed that electrons are foundthat electrons are found

a)a) embedded in a sphere of positive embedded in a sphere of positive charge.charge.

b)b) in fixed positions surrounding the in fixed positions surrounding the nucleus.nucleus.

c)c) in circular orbits at fixed distances in circular orbits at fixed distances from the nucleus.from the nucleus.

d)d) orbiting the nucleus in a single fixed orbiting the nucleus in a single fixed circular path.circular path.

2. Bohr's model of the atom proposed 2. Bohr's model of the atom proposed that electrons are foundthat electrons are found

a)a) embedded in a sphere of positive embedded in a sphere of positive charge.charge.

b)b) in fixed positions surrounding the in fixed positions surrounding the nucleus.nucleus.

c)c) in circular orbits at fixed distances in circular orbits at fixed distances from the nucleus.from the nucleus.

d)d) orbiting the nucleus in a single fixed orbiting the nucleus in a single fixed circular path.circular path.

3. What is the lowest-numbered principal 3. What is the lowest-numbered principal energy level in which energy level in which pp orbitals are found? orbitals are found?

a)a) 11

b)b) 22

c)c) 33

d)d) 44

3. What is the lowest-numbered principal 3. What is the lowest-numbered principal energy level in which energy level in which pp orbitals are found? orbitals are found?

a)a) 11

b)b) 22

c)c) 33

d)d) 44