By the 1850’s, scientists began to realize that the atom was made up of subatomic particles

Thought to be positive and negative

Thomson’s Plum Pudding Model

1897, English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles

Thompson understood that all matter was inherently neutral, so there must be a counter

• A positively charged particle, but where to put it
• It was suggested that the negative charges were balanced by a positive umbrella-charge called the Plum Pudding Model

Rutherford and the Nucleus

• Thompson’s theory was replaced with another, more modern one
• Ernest Rutherford (1910) studied angles at which alpha particles were scattered as they passed through a thin gold foil

Voice Notes:

Rutherford and the nucleus slide:

• Discovered that protons were in the center of the nucleus, and that the atom was mainly empty space
• Used a gold foil to slow down something so that it would be easier for the detector to detect things

The Modern Atom is Composed of Two Regions:

Nucleus:

• Contain protons and neutrons
• It’s a bulk of the atom and has a positive charge associated with it

Electron Cloud:

• Responsible for the majority of the volume of the atom
• The electrons can be found orbiting the nucleus (extranuclear) here

TEXTBOOK READINGS Pg 160-166

The evolution of atomic theories, from Dalton to quantum mechanics, exemplifies the scientific process: experimentation, hypothesis formation, and rigorous testing. Chemists collaborate, combining empirical evidence with theoretical concepts to understand matter. Experimentalists work at the observable level, while theorists explore unseen particles and forces. Their synergy may lead to Nobel Prize-worthy breakthroughs, fostering pride at personal and national levels.

The Thomson Atomic Model

J. J. Thomson’s quantitative experiments with cathode rays resulted in the discovery of the electron, whose charge was later measured by Robert Millikan. The Thomson model of the atom (1897) was a hypothesis that the atom was composed of electrons (negative particles) embedded in a positively charged sphere. Thomson’s research group at Cambridge University in England used mathematics to predict the uniform three-dimensional distribution of the electrons throughout the atom. The Thomson model of the atom is often communicated by using the analogy of a raisin bun, with the raisins depicting the electrons and the bun being the positive material of his atom

The Rutherford Atomic Theory

Ernest Rutherford, eventually showed that some parts of the Thomson atomic theory were not correct. Rutherford developed an expertise with nuclear radiation during the nine years he spent at McGill University in Montreal.

Rutherford’s nuclear model of the atom was then created to explain the evidence gathered in this scattering experiment. Rutherford’s analysis showed that all of the positive charge in the atom had to be in a very small volume compared to the size of the atom. Only then could he explain the results of the experiment

Even though these theoretical ideas seemed far-fetched, they explained the experimental evidence. Rutherford’s explanation of the evidence gradually gained widespread acceptance in the scientific community

• An atom is made up of an equal number of negatively charged electrons and positively charged protons.
• Most of the mass of the atom and all of its positive charge is contained in a tiny core region called the nucleus.
• The nucleus contains protons and neutrons that have approximately the same mass.
• The number of protons is called the atomic number (Z).
• The total number of protons and neutrons is called the mass number (A).