Models of the AtomThe Nucleus

Early Beliefs2500 ya, early Greek philosophers

believed that if you continued to divide matter eventually you would have only one particle left they called atom (means cannot be divided)

Models of the AtomIn the 18th Century man began to study

why some substances could go together and how some could be taken apart

Scientists called substances which couldn’t be broken down into simpler substances as elements

Elements were pure and made of only one kind of atomsSilver, gold, iron, carbon and oxygen are

types of elements (90 natural occuring elements)

Dalton’s ConceptEarly 19th century John Dalton proposed ideas

about matter1. Matter is made up of atoms2. Atoms cannot be divided into smaller pieces 3. All the atoms of an element are exactly

alike4. Different elements are made of different

kinds of atomsProposed that an atom looked like a tiny hard

marble that was the same throughout

Crookes Cathode ExperimentLate 19th Century William Crooke took a

vacumn tube, attached battery to two electrodes (anode-positive charge, and cathode-negative charge)

Placed a cross in middle a of tubeConnected the electric current and a shadow

was shown on far end of tube (anode)What was the beam? Crooke thought it was a

beam of charged particles… called cathode rays

Crookes Cathode Tube

Discovering Charged ParticlesJJ Thomson placed magnet next to cathode

tube and bent the green glow light (light can’t be bent, so it must have been charged particles)

Thomson concluded they were negative charged particles, now charged electrons

Thomson’s Cathode Tube

Thomson’s Atomic ModelAtoms are neutral, but if there were negative

charged electrons there must also be positive charged particles

There would have to be positive charges to balance the negative charges

Thomson’s model of an Atom A sphere of positive and negative charged

particles evenly spaced kind of like cookie dough and chocolate chip

Rutherford’s ExperimentTested cookie dough theoryFired alpha particles through thin gold foilIf positively charged particles are evenly spaced

in atom, the alpha beam will pass through without deflection

Found some deflection which indicated the particles would have to be concentrated in atom not randomly scattered

Proposed a new shape of atom with a concentrated nucleus of positive charged particles

1920 proposed the positive charge particles as protons

Rutherford’s Experiment

Atomic Model Changed againThe nucleus has two much mass for just

positive protonsProposed that there is particles the same size

as protons but have no charge in the nucleusCalled these new particles neutrons Now the atomic model is a tightly spaced

nucleus composed of protons(+) and neutrons (no charge) surrounded by a cloud of much light weighted electrons (-)

The NucleusAn element is defined by the number of protons

present in nucleus. (another name for number of protons is atomic number)

Neutrons however can have varying numbers of neutrons in nucleus

When atoms of the same element varying in number of neutrons it is called an isotope

Carbon normally has 6 proton and 6 neutrons, but carbon atoms can sometimes have 7 or 8 neutrons in the nucleus, thus those are isotopes of Carbon

Number of NeutronsCarbon normally has 6 proton and 6

neutrons, but carbon atoms can sometimes have 6, 7 or 8 neutrons in the nucleus, thus those are isotopes of Carbon

Isotopes of Carbon’s mass is written as totally the numbers of protons and neutrons:Carbon 12 (6 protons and 6 neutrons)Carbon 13 (6 protons and 7 neutrons)Carbon 14 (6 protons and 8 neutrons)

Mass numberMass number of an isotope is the

number of neutrons plus protonsTo find the number of neutrons in an

isotope subtract the atomic number from the mass numberCarbon 14 means it has a mass of 14Carbon has 6 protons thus an atomic

number of 614(mass#) – 6 (protons) = 8 (neutrons)

Strong Nuclear ForceBecause the nucleus is composed of positive

charged protons, you would think that the like charges would repel each other

Rather strong nuclear forces hold the protons together only when they are closely packed as they are in the nucleus of the atom

Radioactive DecayNucleus are stable when the number of protons

and neutrons are similar (ie: 6 protons, 6 neutrons)

Some nuclei are unstable because they have too many or too few neutrons

These unstable nuclei release particles to become more stable

The release of nuclear particles and energy is called radioactive decay

When particles like protons are emitted from nucleus, the Atomic # changes and a new atom is formed

Transmutation is changing of one element into another through radioactive decay

Loss of Alpha ParticleWhen an alpha particle is emitted from a

radioactive element 2 protons and 2 neutrons are lost from nucleus (atomic mass of 4)

The resulting atom has 2 less protons and atomic mass is 4 less

U-238 releases alpha particle (α) and Th-234 forms

238 234 4

92U 90Th + 2

α

New element forms

Loss of Beta particlesSome unstable elements undergo

transmutation where a neutron becomes unstable and splits into an electron and a proton and a beta particle (β) is released

The electron is emitted as high energy and the proton remains in the nucleus increasing the atomic number by one and changing the element

Atomic number therefore is increased by one but the atomic mass stays the same because a neutron changes to a proton (same mass)

234 234 0

90Th 91 Pa + -1 β ↝

Rate of DecayRadioactive decay is randomRate of decay of a nucleus is measured by its

half-lifeHalf-life of radioactive isotope is the amount

of time it takes for half of a sample of the element to decay

For every half-life, ½ of the original mass is gone and you can calculate how much will be left after each half life

Half life tableNumber of

half-liveselapsed

Fractionremaining

Percentageremaining

0 1/1 100

1 1/2 50

2 1/4 25

3 1/8 12 .5

4 1/16 6 .25

5 1/32 3 .125

6 1/64 1 .563

7 1/128 0 .781

... ... ...

Carbon DatingHalf life of Carbon is 5730 yearsC14 is taken in by plants just as C12 is and

when organism dies radioactive decay continues and can be calculated into age

We can calculate age of fossils by calculating how much C14 (radioactive) remains in a sample (accuracy to 35000 years)

Uranium also is used to date rocks, however its half life is 4.5 billions years and decays to lead (Pb) scientists calculate age of earth and rocks by comparing amount uranium remaining and lead (Pb) formed

Making Synthetic ElementsSynthetic elements are man made elements

caused by smashing elements with alpha and beta particles

These new elements are not found in natureAtomic numbers 93 to 112 and 114 have

been made this way

Uses of Radioactive IsotopesTracer elements: radioactive isotopes that are

introduced into an organism where it is used to diagnose disease and study its surrounding

Isotopes have short half-lifesOther radioactive elements are introduced to

detect cancer, digestion problems and circulation problems

Tumors and fractures can be found using isotopesRadioactive isotopes can be used to trace

phosphates in plants, trace pesticides as it moves through ecosystems