Chapter 3Notes

(rev. 10/14/09)

Democritus

Democritus was a Greek philosopher who “…was one of two founders of ancient atomist theory”.

“The atomists held that there are smallest indivisible bodies from which everything else is composed, and that these move about in an infinite void space”.

“The atomists held that there are two fundamentally different kinds of realities composing the natural world, atoms and void. Atoms, from the Greek adjective atomos or atomon, ‘indivisible,’ are infinite in number and various in size and shape, and perfectly solid, with no internal gaps”

. Text http://plato.stanford.edu/entries/democritus/

image http://reich-chemistry.wikispaces.com/file/view/demo._atom_model.gif/97847685/demo._atom_model.gif

• Atoms:Smallest particle of an element that retains the chemical identity of that element.

• Principles of Chemical Behavior:* Lavoisier: Law of Conservation of Matter* Proust: Law of Constant Composition

• a compound always contains the same elements in the same proportions by mass.

John Dalton’s: Atomic Theory of Matter

4 Postulates:• elements are composed of atoms• all atoms of an element are identical, but

different from atoms in other elements• atoms are neither created nor destroyed• a given compound always has the same

relative number and kind of atoms.

Michael Faraday

• atoms contain particles that have an electrical charge

Ben Franklin• studied electricity• he determined the following:

– there are 2 kinds of charge positive and negative

– 2 like charges repel each other

– opposites charges attract each other

– excess negative charge can be discharged as static electricity

Ben Franklin

• Do you remember Ben Franklin’s famous kite experiment?

J.J. Thomson• called the negative particles electrons• determined the charge to mass ratio of an electron• The Plum Pudding Model is Thomson’s name for his

model of the atom

JJ Thomson’s Plum Pudding Model of the Atom

• www.tamucc.edu

Cathode Ray Tube (CRT)• negative end is the cathode• positive end is the anode

A cathode ray is radiation streaming from a cathode to an anode in a CRT

• it is a stream of particles• a magnet can deflect the ray• cathode ray particles have a

negative charge

• http://www.chem.uiuc.edu/clcwebsite/cathode.html

Robert Millikan

• measured the charge of an electron using the Oil Drop Experiment.

• x-rays gave the oil a negative electron • 1.60x10 -19 coulomb is the charge of an

electron• using Thomson’s charge to mass ratio, he

determined the mass of the electron is 9.11x10 -28 g

Robert Millikan’s Oil Drop Experiment

• Robert Millikan received the NobelPrize for his work

• www.68pair.com

Henri Becquerel

• discovered that uraniumexhibits radioactivity

• the chemical propertiesof an element change asit gives off radiation

Ernest Rutherford

• alpha particles have a +2 charge

• beta particles are highspeed electrons

• gamma rays are notcomposed of particles

Rutherford• Gold Foil Experiment (alpha scattering)• he determined that an atom’s positive charge,

and most of its mass, was concentrated in the core (most of the atom is empty space)

• In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. 

• he named the core of the atom “the nucleus”

Rutherford’s Gold Foil Experiment

• Over 98% of the particles went straight through

• Approx. 2% of the particles were deflected• Approx. 0.01% of

the particles bouncedoff the gold foil

www.sci.tamucc.edu

Rutherford’s Experiment

www.sci.tamucc.edu

James Chadwick

• Chadwick proved the existence of neutrons.

http://dev.physicslab.org/img/c2df54c3-7f43-4f21-8c54-36f23218c5f5.gif

Rutherford’s Nuclear Model

• The atom contains a tiny dense center called the nucleus

• The nucleus is essentially the entire mass of the atom

• http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Stylised_Lithium_Atom.svg/180px-Stylised_Lithium_Atom.svg.png

This is an image of a

lithium atom.

The Nucleus

• The nucleus is positively charged• The amount of positive charge in the

nucleus balances the negative charge of the electrons

• The electrons move around in the empty space of the atom surrounding the nucleus

Neils Bohr

• Planetary Model of the Atom

•http://jila-amo.colorado.edu/research/images/bohr.gif

Quantum Mechanical Model

This image is a representation of Schrodinger’s Quantum Mechanical Model of the atom.

This model shows the nucleus in the center surrounded by electrons in different energy levels, but there is no distinct energy level shown as in the Bohr Model.

http://www.physicalworld.org/restless_universe/figs/fig_1_30.gif

Modern Atomic Theory* atoms are composed of protons, neutrons and electrons* Recently, scientists discovered these particles contain

particles (i.e. quarks, gluons)* A nucleus is a positively charged central core containing

protons and neutrons* An individual proton has a charge that is equal to the

charge of an electron, but the charge is positive* In a neutral atom: the number of protons equals the

number of electrons* a proton is much more massive than an electron

Charge and Mass

• a proton’s charge is +1• an electron’s charge is -1• mass is expressed in atomic mass

units (amu)• the mass of a proton is approx. 1 amu• the mass of a neutron is approx. 1 amu• the mass of an electron is

approx. 0 amu

Henry Mosely• an atom’s identity comes from the number of protons in

its nucleus• the number of protons is called the atomic number• every element has a unique atomic number• the atomic number is written above the chemical

symbol on the periodic table• since atoms are electrically neutral, the number of

positive charges equals the number of negative charges• the number of neutrons equals the mass number minus

the atomic number• # of no = mass # - atomic #

IONS

• when an atom loses or gains one or more electrons it acquires a net charge and is called an ion

• if there are more electrons than protons, the ion has a negative charge

• charge of ion = (# of p+) - (# of e-)• charge can be written as +2 or 2+• place the charge to the upper right

corner of the chemical symbol

ISOTOPES

• atoms that have the same number of protons, but different numbers of neutrons

• the chemical properties of an element depend primarily on its electrons and protons

• isotopes of the same element, with different numbers of neutrons have essentially the same chemical properties

ISOTOPES

the major difference between 2 isotopes is their mass

• isotopes with more neutrons have higher mass than those with fewer neutrons

• mass # = (isotope’s # of p+) + (isotope’s # of n0)• to identify an isotope you add the mass number

after the element’s name (i.e. chlorine -37)

• number of neutrons = mass # - atomic #

Carbon-12

• The entire periodic table is based on the carbon-12 isotope.

Example

mass number

p+ e- nºCarbon-12 6 6 6

Carbon-14 6 6 8

2 extra neutrons

Iodine-125

• What is the difference between this isotope of iodine and the iodine shown on the periodic table?

• How many neutrons does iodine-125 have?

Atomic Mass in Brackets

• Atomic mass (the weighted average of all naturally occurring isotopes) is not given for those atoms which have no stable isotopes.

• In these cases the mass number of the most stable isotope is reported, often in brackets, for example Technetium (98). 

• http://www.digipac.ca/chemical/molemass/isotopes.htm

Practice

• Try 3-3 Rev & Rein WS• # 12,13,14

MASS OF AN ATOM* amu can be defined as the mass of a proton

or neutron* Generally: mass of an atom =

number of protons + number of neutrons* an amu is defined in terms of an arbitrary

standard:a carbon-12 atom

* scientists set the mass of a carbon-12 atom to be exactly equal to 12 amu

* therefore, one amu is 1/12 of the mass of a carbon-12 atom

* 1 amu = 1/12 (mass of 126C atom) = 1.66 x 10-24

g

MASS OF AN ATOM• when determining the mass of a large number of

atoms:

• remember that elements have different isotopes• determine the mass of an “average” element’s

atom• the isotopes of some elements are found in

constant ratios• these ratios are called fractional abundances• atomic mass is the average mass of an

element’s atoms• the atomic mass for each element is written on

the periodic table below the element symbol

RADIOACTIVITY

• nuclear reactions change the composition of an atom’s nucleus.

• alpha, beta and gamma radiation are produced by nuclear reactions

• alpha and beta radiation consist of particles that are emitted from the nucleus

• Radioactivity is the spontaneous emission of radiation from an atom

• atoms with stable nuclei are not radioactive

PURPOSE OF NEUTRONS• not all combinations of protons and neutrons

create a stable nucleus• protons in the nucleus should repel each other,

but there is a strong nuclear force (that is an attractive force) that holds the nucleus together

• the nuclear force is only strong between subatomic particles that are extremely close

• the presence of neutrons in the nucleus adds a net attractive force to the inside of the nucleus

• think of neutrons as the glue that holds the nucleus together

STABLE OR NOT?• elements with:• atomic number 1-20: stable nuclei have almost

equal numbers of protons and neutrons• atomic number 21-83: nuclei need more

neutrons than protons to be stable• atomic number >83: no number of neutrons is

sufficient to hold the nucleus together indefinitely.

• All nuclei with atomic numbers greater than 83 are radioactive.

UNSTABLE NUCLEI

• nuclei are unstable if they contain too few or too many neutrons

• nuclei that have excess neutrons are likely to emit beta radiation

• Generally, isotopes that are much heavier (contain more neutrons) or much lighter (contain fewer neutrons) than the most common isotope are likely to be radioactive.

TYPES OF RADIOACTIVE DECAY

* radioactive elements emit different kinds of radiation (three types are alpha, beta and gamma)

* these types of radiation can be distinguished by their:

- charge- mass- penetrating power

ALPHA PARTICLES* consist of a stream of high-energy alpha

particles* alpha particle consists of 2 protons and 2

neutrons(it is identical to the helium-4 nucleus)

* these particles do not have much penetrating power

* these particles travel only a few centimeters in air and can be stopped by paper or clothing

* not normally a health hazard

BETA RADIATION* consists of a stream of high-speed electrons* the electrons come from changes in the nucleus* in the process that produces beta radiation, a neutron

changes into a proton and an electron* the proton remains in the nucleus and the electron (now

the beta particle) is propelled out of the nucleus at high speed

* the mass number for a beta particle is zero because an electron has a very small mass compared with a proton or neutron

* beta radiation is approx. 100 times more penetrating than alpha

* it can penetrate clothing and harm skin

GAMMA RAYS

• gamma rays are not particles

• they are a very energetic form of light that we cannot see

• it can penetrate deeply into solid material (it can only be stopped by concrete or lead)

RADIOACTIVE DECAY

• Radioactive Decay occurs when an atom emits alpha or beta particles or gamma rays.

• The term decay is used because the original nucleus decomposes (decays) to form a new nucleus, releasing radiation in the process.

• Use a nuclear equation to show the radioactive decay.