II. Atomic and Nuclear Structure

10 Questions out of 100 (10% of the test)

– Atomic models and their experimental bases

– Atomic structure and spectra

– Electromagnetic radiation

– Chemical and physical properties related to electron configuration

– Characteristics of radioisotopes, radioactivity, and nuclear reactions

A.  Atomic models and their experimental bases

Subatomic particles, atomic number, and mass number

Atoms are made up of 3 types of particles electrons , protons  and neutrons .  These particles have different properties.  Electrons are tiny, very light particles that have a negative electrical charge (-). Protons are much larger and heavier than electrons and have the opposite charge, protons have a positive charge.  Neutrons are large and heavy like protons, however neutrons have no electrical charge.  Each atom is made up of a combination of these particles. 

The proton and electron stay together because just like two magnets, the opposite electrical charges attract each other.  What keeps the two from crashing into each other?  The electron is constantly spinning around the nucleus.  The centrigugal force of the spinning electron keeps the two particles from coming into contact with each other much as the earth's rotation keeps it from plunging into the sun. 

What keeps electron from flying away from the nucleus because of the attraction force with opposite charges.  This force is called electrostatic or coulombic attraction.

Because protons in the nucleus have the same charge on them, they would tend to repel each other, and the nucleus would fall apart.  To keep the nucleus from pushing apart, there are neutrons in its nucleus.  Neutrons have no electrical charge and act as a sort of nuclear glue, holding the protons, and thus the nucleus, together.

If an atom gains electrons, the atom becomes negatively charged.  If the atom loses electrons, the atom becomes positively charged (because the number of positively charged protons will exceed the number of electrons).  An atom that carries an electrical charge is called an ion. Anions are negative ions that mean more electrons than protons.  Cations are + charge ions (more protons than electrons).

Four basic ideas in Dalton's atomic theory:

1) chemical elements are made of atoms
2) the atoms of an element are identical in their masses
3) atoms of different elements have different masses
4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

5.  Atoms can be neither created nor destroyed.

6. In a given compound, the relative number and kind of atoms are constant.

J.J. Thompson

Thomson conducted a series of experiments with cathode ray tubes which led him to the discovery of electrons and subatomic particles. Thompson studied a form of radiation called cathode rays or electrons that originated from the negative electrode (cathode) when electrical current was force through an evacuated tube.  Thomson constructed a cathode ray tube with a practically perfect vacuum, and coated one end with phosphorescent paint. Thomson found that the rays did indeed bend under the influence of an electric field. Thomson's conclusions were bold: cathode rays were indeed made of particles which he called "corpuscles", and these corpuscles came from within the atoms of the electrodes themselves, meaning they were in fact divisible. Thomson imagined the atom as being made up of these corpuscles (electrons) swarming in a sea of positive charge; this was his plum pudding model.

Max Planck

Planck determined that energy is transferred by radiation in exact multiples of discrete unit of energy called a quantum.  Planck's constant, , was proposed in reference to the problem of black-body radiation. The underlying assumption to Planck's law of black body radiation was that the electromagnetic radiation emitted by a black body could be modeled as a set of harmonic oscillators with quantized energy of the form:

is the quantized energy of the photons of radiation having frequency (Hz) of (nu) or angular frequency (rad/s) of (omega).  Planck helped lead to the formation of quantum mechanics.

Rutherford

Gold foil experiment or the Rutherford experiment was an experiment done by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the University of Manchester which led to the downfall of the plum pudding model of the atom (electrons swarming in a sea of positive charge).

They measured the deflection of alpha particles directed normally onto a sheet of very thin gold foil. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees; some were even scattered back toward the source. From this observation Rutherford concluded that the atom contained a very physically-small (as compared with the size of the atom) positive charge, which could repel the alpha particles if they came close enough, subsequently developed into the Bohr model.

Early in 1911 Rutherford published an analysis of the alpha scattering results which included a somewhat revised model of the atom, known as the Rutherford atom. The observations indicated that a model of the atom with a diffuse charge was incorrect and that a large amount of atomic charge was instead concentrated at some point, giving it a very high electric field. He concluded that the atom might be mostly empty space, with most of the atom's mass and a large fraction of one of its two kinds of charge concentrated in a tiny center, the nucleus.