Models of the Atom

Objectives Recognize that science is a progressive endeavor that reevaluates and extends what is already known. (SPI Inq.1) Identify the contributions of major atomic theorists: Dalton, Thomson, Rutherford, and Bohr. (SPI ) Compare and contrast historical models of the atom. (SPI )

Class Opener: Create a drawing that illustrates Dalton’s view of an atom. Based on what you learned about atoms in other science classes, create a diagram that represents the structure of an atom.

JJ Thomson – 1897 – discovered that within atoms there are negatively charged particles – called these particles electrons.

Thomson performed experiments that involved passing electric current through gases at low pressure. The result was a glowing beam called a cathode ray.

A cathode ray is deflected by electrically charged plates. Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements.

These observations changed the way atoms were portrayed since it was now determined that atoms could be further broken down.

– from his findings Thomson developed the “Plum pudding” model. – the atom was filled with positively charged material and the electrons were evenly distributed throughout.

Images of Thomson’s Model

Thomson had correctly concluded that the atom had electrons, was spherically shaped, and electrically neutral.

Ernest Rutherford (1911)- – Gold foil experiment – directed a narrow beam of alpha particles at a very thin sheet of gold foil. – Alpha particles are positively charged particles that are released by certain radioactive elements as they decay.

Rutherford’s Gold Foil Experiment

Rutherford’s gold-foil experiment yielded evidence of the atomic nucleus. a)Rutherford and his coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a fluorescent screen. Most of the particles passed through the foil with no deflection at all. A few particles were greatly deflected. b)Rutherford concluded that most of the alpha particles pass through the gold foil because the atom is mostly empty space. The mass and positive charge are concentrated in a small region of the atom. Rutherford called this region the nucleus. Particles that approach the nucleus closely are greatly deflected.

Alpha particles scatter from the gold foil

The Rutherford Atomic Model Rutherford concluded that the atom is mostly empty space. All the positive charge and almost all of the mass are concentrated in a small region in the center of the atom. He called this tiny core the nucleus.

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 the electrons would lose energy and spiral inward toward the nucleus. Niels Bohr came up with a theory that explained why electrons do not spiral inward.

Bohr’s Atomic Model He stated that electrons follow two rules: 1.Electrons orbit at specific distances from the nucleus. ( All electrons can be found at a certain distance form the nucleus in an orbital.) 2.Atoms give off energy when an electron jumps from a higher energy orbit to a lower energy orbit and atoms absorb energy when the electron gets blasted from a low energy orbit to a high energy orbit.

His model was known as the planetary model since the electrons are orbiting around the nucleus much like the planets orbit around the Sun.

Bohr’s Model of the Atom Niels Bohr (1913) - – Planetary Model – electron is found only in specific circular paths, or orbits, around the nucleus.

Summary of the Atomic Models 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 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

What is still unclear about today’s lesson?

After today’s lesson, on your guided notes, draw the Thomson, Rutherford, and Bohr models of the atom.