ATOMIC THEORY

Honors Chemistry

Mr. McKenzie

Atomic Structure

• Essential Questions:– How was the modern theory of atomic

structure developed?– What are the significant experiments that led

to the advancement of our understanding of atomic structure?

– What are the components of the atom?

The Atom

• Dalton – first to develop an atomic theory

• Used experimental data on chemical reactions to develop his theory

• Three parts to his theory:– All matter consists of tiny particles called

atoms– All atoms of the same element are the same– Atoms are indivisible (cannot be broken down

into smaller parts

Problems with Dalton’s Theory

• Marie Curie

• Studied radioactivity – she isolated radium and discovered polonium

• Only way to explain radioactivity is to hypothesize that radioactive atoms are disintegrating or decomposing into smaller particles

• If atoms can disintegrate, they must be made up of smaller particles

Smaller particles?

• The search was on for smaller particles

• If atoms could be broken down, what were the parts of the atom?

• Atoms are part of everything – that makes it important to understand them!

The Electron

• Cathode Ray Tubes

• Cathode Rays– flowed from the negative electrode (cathode)

to the positive electrode (anode)– caused gases to glow– caused metals to glow red-hot– can be deflected by a magnetic field– are attracted to positive charges

Cathode Ray Tubes

Cathode Ray Tubes

The Electron

• J.J. Thompson

• Used his knowledge of the properties of electricity and magnetism to study the mass to charge ratio of cathode rays (electrons)

• Found that the cathode rays (electrons) were produced from a range of materials, suggesting that the particles were universal

Charge of an Electron - Millikan

Charge of an Electron – Millikan

The Electron

• Millikan – Oil Drop Experiment

• Used knowledge of electricity and magnetism to calculate the charge of ionized oil drops

• The measured charges were whole-number multiples of a constant, 1.60 x 10-19 C

• Once the charge of an electron was discovered, its mass could be calculated using the mass-to-charge ratio determined by Thompson

The Proton

• The discovery of negatively charged electrons lead scientists to look for the accompanying positively charged particles

• Canal Rays

• Experiments showed that the particles must be significantly more massive than electrons

Canal Rays

Canal Rays

The Proton• Rutherford• Bombarded different elements with alpha

radiation• Noticed that some particles were deflected

rather than passing straight through the element• Deflection depended on atomic mass• Results showed that the positive charge had to

be concentrated in the center of the atom• When N was bombarded, protons were emitted

and observed

The Neutron

• The number of protons in an atom didn’t account for the observed mass of an atom

• Hypothesized that there was another, neutral particle with a significant mass present in the atom to account for the discrepancy between the number of protons and the observed atomic mass

The Neutron

• Chadwick• Bombarded a beryllium target with

polonium, creating a “penetrating radiation” which he directed at a wax target

• Wax target emitted protons• “penetrating radiation” was really neutrons,

non-charged particles with about the same mass as a proton

The Nucleus

• Rutherford

• Gold Foil Experiment

• Created a thin sheet of gold, only a few atoms thick

• Hit the foil with alpha particles – products of radioactive decay that have mass and positive charge

Gold Foil Experiment

The Nucleus

• The discovery of the nucleus was a surprise

• Discovery completed much of the work regarding atomic structure

• Electron configurations will be covered later in the course