Electrons in AtomsChapter 5…the truth about electrons

Atomic Models 1803 Dalton's model was that

the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

     

     

J.J. Thomson “plum pudding” model Discovered the

“electron” Electron embedded in a

sphere of positive electrical charge

1897

Hantaro Nagaoka 1904 Suggested that an atoms has a central nucleus No evidence to back claim

Rutherford’s experiment Gold foil and alpha

particles 1911

Alpha particle (+) and Gold foil

The above diagram shows what we would expect the result of Rutherford's experiment to be if the "plum pudding" model of the atom is correct.

The above indicates the actual result. Most of the alpha particles are only slightly deflected, as expected, but occasionally one is deflected back towards the source.

Rutherford conclusions Atom mostly empty space (most of the alpha

particles passed right through) Tiny positive nucleus Nucleus has large mass (alpha particle did not

move nucleus, it bounced back so mass must be big)

Electrons move around nucleus

Niels Bohr “the old quantum theory” Electrons found in specific

circular paths, planetary model

Energy levels, energy increases to valence electrons

Quantum is the amount of energy it takes to move and energy level

1913

     

     

Quantum These ladder steps are some what like energy levels.  (a) In an ordinary ladder, the rungs are equally spaced.  (b) The energy levels in atoms are unequally spaced, like the rungs in this ladder. The higher energy levels are closer together so the amount of energy is not always the same as you move.

Ground State vs. Excited State Ground State= lowest energy configuration

“normal” Excited State=higher energy configurations

than normal Q: Why does hamburger have lower energy

than steak?

A: Because it's in the ground state.

Why Bohr was not perfect - The Bohr model gave

results in agreement with experiment for the hydrogen atom. However, it still failed in many ways to explain the energies absorbed and emitted by atoms with more than one electron.

De Broglie’s particle-wave dulaity In 1924 de Broglie's proposed that all moving

particles has a wavelength is inversely proportional to momentum and that the frequency is directly proportional to the particle's kinetic energy ; λ = h/p

This concept leads to one of the most important ideas in 20th century science:

The small, light, fast moving electron also exhibits wave-particle duality. It can be conceived of as a particle or as a wave.

This development leads to atomic and molecular orbitals.

Edwin Schrödinger knew of de Broglie's proposal that a electrons exhibited wave-particle duality. With this idea in mind, he devised/constructed a differential equation for a wavelike electron resonating in three dimensions about a point positive charge.

Erwin Schrodinger In the 1920s, a whole new theory of physics, called

quantum mechanics, presented an even more radical picture of the atom. The electrons cannot be pinpointed but exist as a sort of cloud of probability outside the nucleus.

The airplane propeller is somewhere in the blurry region it produces in this picture, but the picture does not tell you its exact position at any instant. Similarly, the electron cloud of an atom represents the locations where an electron is likely to be found.

Quantum Mechanical Model Determines the allowed energies an electron

can have and how likely it is (probability!) to find the electron in various locations around nucleus. Mathematical theory!

n= the Principle Quantum Number or Principle energy level (energy levels the electrons are found in)

THE DISCOVERY OF THE ELECTRON

Atomic Orbitals Region in space with a high probability to

find an electron

Energy Sublevel Each energy sublevel corresponds to an

orbital to a different shape. Different atomic orbitals are denoted by

letters. S= spherical, P= dumbbell D & F more

complex

Summary of Principal energy levels, sublevels, & orbital's

Principal energy level # of sublevels Type of sublevels

n=1 1 1s (1 orbital)

n=2 2 2s(1orbital), 2p(3orbitals)

n=3 3 3s(1 orbital), 3p(3orbitals), 3d(5orbitals)

n=4 4 4s(1 orbital), 4p(3orbitals),

4d(5orbitals),

4f(7orbitals)

James Chadwick – 1932 Nucleus mystery solved

Rutherford speculated in 1920 that there existed electrically neutral particles with the protons that make up the missing mass but no one accepted his idea at the time.

Complete model! James Chadwick discovered a new type of

radiation that consisted of neutral particles. It was discovered that these neutral atoms came from the nuclei of the atom. This last discovery completed the atomic model.

Quarks….where are we now? 1964 Quarks are proposed by Murray Gell-

Mann and George Zweig (math based theory) quarks are subatomic particles thought to be

elemental and indivisible. They are one of the two kinds of spin-½ fermions (the other being the leptons). Objects made up of quarks are known as hadrons; well known examples are protons and neutrons.