Atomic Theories and Subatomic Particles (#1).doc

7/27/2019 Atomic Theories and Subatomic Particles (#1).doc

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Atomic Theories andSubatomic Particles

The Atom

With the exception of hydrogen, every atom consists of 3 main parts:

Name Location Mass (amu) ChargeProton Nucleus 1 +1

Neutron Nucleus 1 0Electron Outside Nucleus 1/1836 (approx.

0)-1

Daltons Theory of Matter (Particle Theory 1766-1844)

All matter is composed of tiny, indivisible particles called atoms. All atoms of an element have identical properties.

Atoms of different elements have different properties.

Atoms of two or more elements can combine to form new substances.

During a chemical reaction, atoms are not created or destroyed they are only

rearranged.

Since Daltons time there has been intense investigation into the nature of matter and ithas been discovered that:

Atoms are not indivisible All atoms of the same element do not have exactly the same properties

(isotopes exist)

In accordance with Daltons 5th postulate (above) the Law Of Conservation Of Masswas derived:

In a chemical reaction, the total mass of the starting materials (reactants)

will equal the total mass of the products of the reaction.

Reactants Products


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J. J. Thomson (Plum-Pudding1856-1940)

J. J. used a cathode ray tube (glass tube, emptied of air, with an electrode at

either end) to discover electrons. He then proved that these electrons werenegatively charged.

Since the atom itself was known to be uncharged and yet contained negativelycharged electrons, J. J. reasoned that there must be positive charge in the atom

also, to counteract the electron charges. J. J. proposed the plum pudding

model of the atom, with negatively charged electrons distributed in a positive

sphere that was mostly empty space.

Rutherfords Atomic Model (Planetary1871-1937)

Performed the Gold Foil Experiment

His results led to discovery of the nucleus

Used alpha () particles These are positively charged, fast movingparticles emitted from a radioactive source.

The particles were shot through a thin piece of gold foil (afew atoms thick)

Prior to performing this famous experiment, Rutherford thought thathis teachers theory (J.J. Thomsons Plum-Pudding model) of atomicstructure was correct. He therefore expected the fast moving -particles to fly straight through the gold foil with very little (if any)deflection. After all, there was nothing inside the atom positiveenough and dense enough to cause the - particles to changecourse on their way through.

http://micro.magnet.fsu.edu/electromag/java/rutherford/index.html
http://micro.magnet.fsu.edu/electromag/java/rutherford/index.htmlhttp://micro.magnet.fsu.edu/electromag/java/rutherford/index.html


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He was VERY surprised to see that some of the - particles werescattered slightly but a few were deflected almost straightback!

He concluded that there must be a dense positively charged

centre of the atom he called it a nucleus

Rutherfords Planetary Model

Protons and neutrons make up the nucleus and electrons whirlaround the nucleus in defined orbits like planets around a sun.


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The Bohr Model of the Atom (1885-1962)

The previous model, proposed by Ernest Rutherford, was correct in a numberof ways, primarily:

Positive charge is located in a dense nucleus at the centre of theatom.

Negatively charged electrons whirl around outside the nucleus.

The difficulty lay in two areas:

Any charged particle (electron) orbiting another particle (nucleus)will constantly lose energy due to its turning motion. Therefore,electron orbits should decay, and eventually the electrons crashinto their nuclei.

An experiment was performed by Neils Bohr in which energy(electricity) was passed through hydrogen gas (H2) and light wasemitted from the gas.

Rutherfords theory could not explain these observations and therefore

needed modification. Along came Bohr.

In trying to explain his observations (no crashing electrons, and light from agas) Neils Bohr proposed electrons exist only at discreet energy levels (orenergy shells)

When electricity passes through gaseous elements, light of a colourcharacteristic (and unique) to that element is produced.

If this light is passes through a prism:

a series of colored lines is produced This is calledthe line spectrum for that atom.


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Each element has a unique line spectrum this is a very accuratemethod of identifying elements.

Bohr theorized that electrons exist only at certain energy levels andthat a number of these energy levels exist outside the nucleus.

In order for an electron to inhabit a particular energy level, it mustpossess enough energy to be at that energy level.

An electron may jump from one energy level to a higher energy

level only by absorbing enough energy to do so The difference inenergy between energy levels is called a quanta of energy. A

quanta can refer to different amounts of energy depending on theenergy difference between any two energy levels. Example: Theenergy difference between n = 1 and n = 2 is not the same as thedifference in energy between n = 2 and n = 3, yet both are aquanta.

Electrons are most stable when they are in the lowest possibleenergy level that has room for them. Each level will house 2n2electrons, where n is the energy level.

When all electrons are in their lowest possible energy level theatom is said to be at in ground state.

If an electron absorbs enough energy it will jump to a higher

energy level. At this point the electron is said to be in an excitedstate. Electrons dont like to be excited and therefore willimmediately lose some energy and fall back down to ground state.During this transition, the electron loses a quanta of energy mostlyin the form oflight. Depending on how much energy was lost, lightof a characteristic colour is emitted.

In Summary Electrons exist in stable orbits about the nucleus.


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Only certain orbits are allowed An electron passing from a lower orbit to a higher orbit must absorb

exactly the difference in energy between the two orbits. Similarly, foran electron to move from a higher orbit to a lower one, it must emitenergy corresponding to exactly the difference in energy between thetwo orbits.

For an excellent explanation of these theories and others, please referto http://www.colorado.edu/physics/2000/science_trek.html

Atomic Number, Mass Number, and Isotopes

Atomic Number Symbolized byZ

This is the number of protons in the nucleus Each element has a different number of protons and therefore

the atomic number is used to identify an element.

Ex. Helium (He) has 2 protons and an atomicnumber of 2;Z= 2

Uranium (U) has 92 protons and an atomic number of92;Z= 92

Mass Number Symbolized byA

this is the total mass of the atom measured in atomic massunits (amu)

a single atom doesnt have very much mass at all andtherefore it would be inappropriate to measure the mass of anatom using conventional metric units like grams or milligrams for this reason we use extremelysmall units called amu.
http://www.colorado.edu/physics/2000/science_trek.htmlhttp://www.colorado.edu/physics/2000/science_trek.html


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1 proton is approximately 1 amu1 neutron is approximately 1 amu

1 electron is approximately 1/1836 th of an amu

For all practical purposes we consider the electron to have 0

mass, therefore the mass number of any atom will be:

A = number of protons + number of neutrons

Element Symbol

An element is often symbolized by its letter(s) only. Forexample sulphur is S, or sodium is Na. However, a moreinformative symbol indicates both the elements letter(s) andits atomic number and mass number:

Ex. Sulphur

AZS 3216S ; as you can see, the mass number (A) is

superscripted on the left of the letter symbol,and the atomic number (Z) is subscripted.

Ex. Sodium

2311Na

Isotopes

These are two or more atoms that have the same number ofprotons in their nucleus but a different number of neutrons.

In other words, isotopes have the same atomic number butdifferent mass number.

Example: Two isotopes of uranium called U 235, and U-

238.

The only difference between these two atoms of the sameelement is the number of neutrons in their nuclei.

23592U and 23892U


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Notice that each species has the same atomic number (andtherefore are both species of uranium) but different massnumbers, due to their different numbers of neutrons.

Isotopes have the same chemical properties but different

physical properties. Using the above example, U 238 is verydifficult to use in nuclear reactors while U 235 is very easyto use.

Note: The most common isotope of carbon is C 12. Forthis reason C 12 is assigned a mass of exactly 12amu. This was the first element everto be assigned amass in amu and is therefore the standard by whichall other elements are massed.For example, if hydrogen was found to have a massexactly 1/12th that of the C-12 isotope, then

hydrogen was assigned a mass that was 1/12th themass of C-12 namely 1 amu.

Average Atomic Mass (also called Relative Atomic Mass)

Symbolized by Mr

If you look at your periodic table youll notice that the atomicmass of each element is reported usually to 3 or 4 decimalplaces (Example: Cl has an atomic mass reported as 35.453amu). This doesnt seem to make sense since chlorine has aninteger number of protons and neutrons, like 17 protons and


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18 neutrons, not fractions of protons or neutrons! The reasonis that the mass reported on the periodic table is the averagemass of all of the isotopes of the element the averageatomic mass. Also, each proton and each neutron are notexactlyone amu, but they are veryclose, and so for practical

purposes we consider each to be 1 amu.Example: Naturally occurring chlorine is comprised of twodifferent isotopes. One of these isotopes has a mass of34.9689 amu (called Cl 35) and the other a mass of 36.9590amu (called Cl 37), yet your periodic table reports one mass,namely 35.453 amu.

The number reported on your periodic table is the averageatomic mass ofall the isotopes of that element in a naturallyoccurring sample of the element. Using Cl as an example, if

you were to analyze any sample of pure chlorine atoms youwould find that 75.77% of the sample was Cl 35 atoms, and24.23% was Cl 37 atoms. In order to determine the averagemass ofall of the atoms in the sample the following equationmust be used:Note: The percentages listed above are called the naturalabundances of these isotopes. You can find the naturalabundances of all elements at the following link just click onan element on the periodic table displayed (partially down theweb-page) and the abundances of all isotopes will bedisplayed in graphic form:

http://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htm

Average Atomic Mass =(Mass of Cl 35)(fraction of sample that is Cl - 35) + (Mass ofCl 37)(fraction of sample that is Cl-37)

Average Atomic Mass of Cl = (34.9689 amu)(0.7577) +(36.96590 amu)(0.2423)

= 35.453 amu

This is called a weighted average because it takes intoaccount the masses of each isotope and the relative numberof each of those isotopes in the sample.
http://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htmhttp://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htmhttp://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htmhttp://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htmhttp://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htmhttp://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&zu=http%3A%2F%2Fwww.sisweb.com%2Fmstools.htm


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Ions

Ions are charged particles (positively or negatively) In a neutral atom (uncharged) the number of protons is equal

to the number of electrons, however in an ion these are notequal and therefore the atom is charged.Example: Li has 3 protons and 3 electrons; Li+ has 3 protons

and 2 electronsMg has 12 protons and 12 electrons; Mg2+ has 12 protons and

10 electronsCl has 17 protons and 17 electrons; Cl- has 17 protons and 18

electrons Notice that only the number of electrons changes when

forming an ion from a neutral atom. If the number of protonswere to change the identity of the atom would also change.

Atomic Theories and Subatomic Particles (#1).doc

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