Atomic Models POGIL



Atomic Models

Name ______________________

Date ___________

Learning Objectives:

• Understand the history of the atomic models, and use this information to draw the structure of an atom

Information

Throughout history the model of the atom has changed with each new discovery.

• Dalton’s Model – shows that an atom is made up of solid particles and cannot be further broken down.

• Thomson’s Model – the “plum pudding” model; the atom is made up of protons and electrons that are scattered throughout

• Rutherford’s Model – protons are found in the center of the atom in the nucleus and electrons are scattered in the empty space that surrounds it

• Bohr’s Model – the “planetary” model; protons and neutrons are found in a central nucleus and electrons are placed in orbits that circle around the nucleus at a specific distance.

• Quantum Mechanical Model – electrons are found in a cloud around the nucleus; their location around the nucleus is defined by probability

Label the models (Dalton, Thomson, Rutherford, Bohr, or Quantum Mechanical model) according to their description.

[pic] [pic] [pic]

___________ ___________ __________

[pic] [pic]

___________ __________

History Behind the Models:

Dalton came up with the theory that described atoms in five postulates. The first stated that all elements are composed of tiny indivisible particles called atoms that cannot be broken down further. Second, atoms of a given element are all similar in their physical and chemical properties. Third, atoms of different elements differ in their physical and chemical properties. Atoms of different elements combine in simple, whole number ratios to form compounds. Lastly, in chemical reactions, atoms are combined, separated, or rearranged, but never created, destroyed, or changed. Dalton’s model summarizes these principles by showing that an atom is made up of solid, indivisible particles.

Using a cathode ray tube, Sir Joseph Thomson discovered the electron. A cathode ray tube is composed of an cathode or negative electrode and a anode or positive electrode inside of an evacuated glass container. Thomson applied a charge to the cathode ray and observed the flow of electrons from cathode to anode through the open space of the container. These observations changed the way that atoms were portrayed since it was now determined that atoms could be further broken down. Thomson’s model shows electrons scattered throughout an atom that is mostly a positively-charged cloud.

[pic]

In 1911, the model of the atom changed once again after Ernest Rutherford discovered the nucleus. By aiming alpha particles (radiation from the nuclei of helium) at a thin sheet of gold foil, Rutherford determined that protons are located in a concentrated area in the atom, and the electrons are dispersed in the empty space around it. His observations of deflected alpha particles and particles that traveled straight through the gold sheet showed that protons and electrons were not evenly dispersed. This means that Thomson’s model was no longer correct, and that protons were found in a dense area of the atom called the nucleus. Electrons, on the other hand, are found in the empty space around the atom.

[pic] [pic]

As Rutherford’s model of the atom was being analyzed, it predicted that, according to the theory of electricity and magnetism, opposite charges attract each other and will therefore cause the electrons to lose energy and spiral inward toward the nucleus. In 1912, Niels Bohr came up with a theory that explained why electrons do not spiral inward. He stated that electrons follow two rules. The first being that electrons orbit at specific distances from the nucleus. In other words, all electrons can be found at a certain distance from the nucleus in an orbital. Second, atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit, and absorbs energy when the electron gets boosted from a low-energy orbit to a high-energy orbit. This model is known as the planetary model since the electrons are orbiting around the nucleus much like the planets orbit around the sun. The first orbital only holds two electrons. Orbital two can hold up to eight electrons, and orbital three can hold up to eighteen electrons, and orbital four and higher can hold up to thirty-two electrons.

Fifteen years passed before another model of the atom was put into the mix. The next model, the quantum mechanical model, is a mathematical model that predicts the probability of electron locations and paths within an electron cloud. This model resulted from scientists such as deBroglie, Schrödinger, and Heisenberg. These scientists started to look at electrons acting as both particles and waves, and determined that there can be no certainty in finding the position of an electron if you know the momentum at which it travels. This led to the idea of electrons existing in an electron cloud, and hence came the quantum mechanical model.

Answer the following questions using the information about each scientist.

Even though new models were discovered after Dalton created his five principles, which of these principles still holds true?

What is the charge of an electron? In Thomson’s experiment the electrons traveled from the cathode, the negative electrode, toward the anode, the positive electrode. Why did this happen?

Explain why Rutherford’s discovery discredited Thomson’s model. What aspect(s) of Thomson’s model were disproved?

Why is Bohr’s model called the “planetary model”?

Draw models from each scientist for the following elements. (Hint: refer to models on page 1)

Example:

Boron:

Atomic #:__5___ # Protons:__5___ # Electrons:___5___

| | | | | |

| | | | |[pic] |

| | | | |Quantum Mech |

| | | | | |

| | | | | |

|Dalton |Thomson |Rutherford |Bohr | |

Helium:

Atomic #:______ # Protons:______ # Electrons:_______

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

|Dalton |Thomson |Rutherford |Bohr |Quantum Mech |

Sodium:

Atomic #:______ # Protons:______ # Electrons:_______

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

|Dalton |Thomson |Rutherford |Bohr |Quantum Mech |

Oxygen:

Atomic #:______ # Protons:______ # Electrons:_______

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

|Dalton |Thomson |Rutherford |Bohr |Quantum Mech |

Carbon:

Atomic #:______ # Protons:______ # Electrons:_______

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

|Dalton |Thomson |Rutherford |Bohr |Quantum Mech |

Answer the following.

Of the five different models, which model do you think is the most useful for learning about the structures of atoms? Why?

Of the five different models, which model do you think is most accepted today? Why?

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