Atomic Structure

Greek Theory: Scientific knowledge came about through

pure thought process. Thales of Miletus (600 BC) claimed that

water was the origin of all things Heraclitus (500 BC) claimed fire was the

primordial substance. Empedocles (450 BC) claimed the four

elements were earth, air, fire, and water

Democritus (400BC) was the originator of the “atomic theory” He, along with Leucippus, stated that all

things were made up of atoms which could not be further subdivided (atomicus means smallest particle)

These ideas lasted until the Middle Ages

Middle Ages Alchemy was the science of the practice

of trying to turn all substances into gold. Slow progress of science due to the

divorce between theory and practice. Most of the practioners were frequently

persons of inferior mentality and were hoping to find riches

Beginning of Modern Science Began in the Renaissance

– Lavoisier known as the father of modern Lavoisier known as the father of modern chemistrychemistry

recognized the importance of carefully controlled recognized the importance of carefully controlled experiments and accurate measurements.experiments and accurate measurements.

Disproved “theories up to that time”Disproved “theories up to that time” discovered “Law of Conservation of Mass”discovered “Law of Conservation of Mass”

Dalton developed his Atomic Theory based on the observations and measurements produced experimentally by scientists such as Lavoisier and Proust.Lavoisier determined the “Law of Conservation

of Mass” which says matter can not be created or destroyed.

Joseph Proust determined that when elements combine and form compounds, they do so in definite proportions by mass. This is known as the Law of Definite Proportions.

Dalton’s Atomic Theory 1. All substances are composed of

small, dense, indestructible particles called atoms.

2. Atoms of a given substance are identical in mass, size and shape

3. An atom is the smallest part of an element that enters into a chemical change.

4. Molecules of a compound are produced by the combination of the atoms of two or more different elements.

Dalton theory was unable to explain how or why atoms combine in certain ratios, or what the attractive forces are between particles of matter.

During the 1800’s the particle nature of electricity was discovered and in doing so new particles smaller than the atom were found.

Crookes Cathode Ray Tube help determine the movement of negatively charged particles.

JJ Thomson discovered a charge to mass ratio for this negative particle. (In 1906 JJ earned the Nobel Prize for Chemistry for proving electrons are particles.)

Robert Millikan determined the actual charge of the electron(Nobel Prize in 1923)

E. Goldstein discovered a beam of positive charged particle

Around 1896 some new particles were discovered which led to further investigations of the atom Wilhelm Roentgen discoved the X-ray Henri Becquerrel discovered radioactive

particles.

In 1911, Ernest Rutherford used these particles in a scattering experiment which identified two distinct portions of the atom. The nucleus containing most of the

mass of the atomMuch of this was positively charge He could not account for the remainder of

the mass which was not positively charged.

The outer portion of the atoms made of mostly empty space (the electrons were determined to be located in this portion of the atom)

The neutrons would not be discovered for another 20 years until James Chadwick discovered them in 1932.

Atom’s Structure and Terms

What is meant by the atomic number?Atomic number, Z, = number of protonsWhat is meant by the mass numberMass number, A, = number of protons +

number of neutrons.This accounts for the nucleus of the

atom

Elementary particles include the proton, electron, and neutron

the protons and electrons identify the net charge on the atom or ion.

What is the relationship between the protons and electrons for a neutral atom and for an ion?

For a neutral atom the protons equal the electrons

for an ion the electrons are either greater or less than the number of protons. (the electrons can increase or decrease but the protons have to remain the same for a particular atom

Isotope: two elements with the same number of protons and different number of neutronsHydrogen: 1 proton 0 neutronsDeuterium 1 proton 1 neutronTritium 1 proton 2 neutronsNote that all have the same number of

protons but their mass varies due to the neutrons

Atomic Mass the weighted average of all the isotopes

of a particular element. A weighted average is the mass of each

isotope times the percentage abundance that it is found in nature. Then add them together.

Problem: Ne-20 at 90.92 %; Ne-21 at 0.26%, and Ne-22 at 8.82% Determine the atomic mass.

20 x .9092 = 1.818421 x .0026 = .054622 x .0882 = 1.9404total = 3.8134 amu for Ne

NUCLEAR CHEMISTRY

What are the three types of radiation?Alpha, beta, gammaWhat is the composition of each type of

radiation and their nuclear symbols?Nuclear stability seems to be based on the

relative numbers of neutrons to protons found in each nucleus. The most stable isotopes are found when the

N:P ratio is from 1:1 to 1.5:1

What would be a way for an isotope having a larger ratio to achieve greater stability?

Half-lives: The time required for one half of the mass of the substance to decay If the half life of substance X is 5 days and

you begin with 100 grams then after 5 days there are 50 grams remaining after 10 days there are 25 grams remaining after 15 days there are 12.5 grams remaining

Transmutation reactions are nuclear reactions which show the nuclear symbols of all the substances involved. N-14 + n-1 --> C-12 + H-1 or Ra-226 --> Rn-222 + He-4

Fission reactions are when one nuclide splits apart into two or more smaller fragments

Fusion reactions occur when two nuclei combine to form a nuclide of heavier mass.