The Structure of the Atom

The Structure of the Atom History, Structure, Properties and Forces

I. Early Theories of Matter before the early 1800’s many Greek philosophers thought that matter was formed of air, earth, fire and water

II. Democratus 1) first to propose atomos - matters as small indivisible particles 2) said they move through empty space 3) different properties of matter due to changes in arrangement of atoms

III. Dalton’s Postulates 1) Small- elements are made of atoms 2) Identical -atoms of an element are identical in their masses - different elements have different masses

3) Created - matter is neither created or destroyed - chemical reactions - rearrange atoms 4) Compounds - atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3... In truth, Dalton never ruled out the possibility of subatomic structure. He just knew that the state of the art in the early 1800's did not allow the physical structure of an atom to be probed. Dalton a English School Teacher

Postulate 4 is actually the Law of Definite Proportions,by Joseph Louis Proust in 1797. a) a given chemical compound always contains the same proportion by mass of its constituent elements or... b) the relative amount of each element in a particular compound is always the same, regardless of preparation or source.

IV. Subatomic Particles and the Nuclear Atom 1) J.J. Thomson - a) discovered electron using Cathode ray tube b) was able to determine charge to mass ratio but was unable to calculate exact charge 2) Robert Millikan - determined the charge of the electron using an oil drop experiment - was very close to the accepted value we have today

3) models of atoms - J.J. Thomson purposed a plum pudding model in which a evenly distributed positive circle was imbedded with electrons throughout.

4) Rutherford, Geiger, & Marsden’s propose a different model -Gold Foil Experiment

a) Ernest Rutherford a ex student of Thompson had been studying alpha particles since 1898. In fact, he discovered them. In 1909 he was confronted with some rather bizarre alpha-particle behavior that he had to explain. What was the behavior, exactly? b) Hans Geiger and Ernest Marsden aimed a stream of alpha particles at a thin gold foil for several months in 1909.

Rutherford’s associates discovered c) almost all of the alpha particles went through the gold foil as if it were not even there.. d) some of the alpha particles were deflected only slightly, usually 2° or less. e) a very, very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90° or more. (Rutherford cites 1 in 20,000 for gold in his 1911 paper.)

f) from these results it was concluded * that a extremely small dense area of positive charge existed (area that reflected alpha particles straight back) * that most of the atom was empty space (most particles passed straight through) * electrons have very minimal mass (particles that were only slightly reflected by the alpha particles)

5) The Modern Atom A) about 1920 - eight years after the Gold foil experiment - Rutherford - called positive charged area a nucleus with a positive proton. B) 1932 - James Chadwick discovered the Neutron

Properties of Subatomic Particles

V. Comparing Atoms 1) atomic number - the number of protons in an atom also equals the electrons in a neutral atom A=P=E all atoms of an elements have the same number of protons 2) elements - all have the same type of atoms (same number of protons) but can have different numbers of neutrons and electrons

3) isotopes - atoms of an element that have a different number of neutrons are isotopes of that element a) mass number - is the number of protons + the number of neutrons b) nuclide symbol - is a symbol that notes the number of protons and neutrons of a specific atom - Carbon -14 mass # 14 notes C 6 protons atomic # 6 8 neutrons

Atomic Mass • Weighted average mass of all the naturally occurring isotopes of an element • Calculated by the sum of all the isotopes times their mass in amu’s

4) ions - are atoms of an element that have different numbers of electrons a) cations - have fewer numbers of electrons than the neutral atom and carry a positive charge. (metals form cations) Mg 12+ Mg+2 12+ 12- 10- 0 +2 magnesium loses 2 electrons to form a positive 2 cation as do all members of the alkaline earth metals

b) anions - atoms of an element that have more electrons than their neutral atom (non-metals form anions) Br 35+ Br-35+ 35- 36- 0 -1 bromine gains one electron to become a negative one charged anion as do all halogens.

VI. Nuclear Chemistry 1) protons and neutrons are in the nucleus and compose almost all the mass of the atom 2) they are tightly packed in the nucleus -neutrons have no charge and protons have a positive charge 3) why don’t the positively charged protons repel each other - why does the nucleus stay together?

4) the strong force holds the protons to the neutrons and the protons to the protons in the nucleus a) one of the 4 basic forces and is about 100 times stronger than the electric force b) is a short range force that quickly becomes weak as protons and neutrons get further apart c) the electric force is a long range force, so protons that are far apart repel each other

d) the larger the atom the weaker the strong force is --> larger atoms tend to decay (nuclear decay) e) also, atoms with certain numbers of protons tend to decay--if they have the same number of protons and neutrons they tend to be more stable--different numbers--more unstable

5) 4 forces in nature a) strong force b) weak force c) electrical force d) gravitational force 6) Usages of Nuclear Chemistry a) production of power b) killing bacteria in food products c) medicine -cancer treatments along with other usages

7) Radiation-Radioactivity a) radiation is electrically charged particles or waves emitted by an energy source or decaying atoms. b) radioactivity - is radiation from an unstable atom that is splitting or undergoing decay. There are three types of radioactive materials * Alpha radiation - harmful if inhaled or otherwise enter the body - can be stopped by clothes, skin or a sheet of paper.

Some producers of alpha particles are among the longest lasting waves * beta particles - more penetrating power than alpha but most serious when inhaled or ingested -- tend to concentrate in certain body parts, such as bone -can cause serious health problems with minimal exposure * gamma radiation-- highest energy levels -similar to x-rays, can penetrate the body and cause direct damage to internal organs

Types of Radiation • 1. Alpha Radiation : an alpha particle emitted from radioactive nuclei, consists of 2 protons and 2 neutrons, but no electrons • Not very harmful • Large atoms are not very stable and need to decrease mass • What is the charge and mass of an alpha particle? • +2 and 4amu • Example • 22688Ra → 22286Rn + 42He • Radium Radon Alpha Particle

Types of Radiation • Beta Radiation: fast moving electron emitted from a radioactive element called a Beta Particle. • Can cause serious health problems especially in bones • Atoms want to have a 1 : 1 neutron to proton ratio • Beta emission is used to decrease the neutron to proton ratio. • What is the charge and mass on a beta particle? • -1 and 0 • 146C → 147N + 0-1β

Types of Radiation • Gamma Radiation: Gamma rays are released from radioactive nuclei. • Gamma rays have no mass or charge. • Gamma rays are very harmful and have a very high energy

* Effects of radioactive particles on biological systems --can alter cellular function particularly DNA--carries the cells genetic code-causing birth defects-can create mutated forms of cells that can cause cancerous growths 8) Fusion-Fission a) fusion uniting of nuclei of two light elements to form heavier nucleus- example sun--

b) fission - a heavy nuclide splits into two or more intermediate- sized fragments when hit in a particular way by a neutron --utilized by nuclear power plants --to make nuclear bombs

The Structure of the Atom