Modern Physics The Nucleus. Modern Physics Three main particles in an atom.

Modern Physics

The Nucleus

Modern Physics

• Three main particles in an atom

Modern Physics

• Three main particles in an atomProtonsNeutronsElectrons

Modern Physics

• Three main particles in an atom Mass

Protons 1.007276 amuNeutrons 1.008665 amuElectrons .001097 amu

Modern Physics

• Lets look at a Helium atomHelium 4, symbolized as 4

2He

2 – protons2 – neutrons2 – electrons

The “4” is called the “mass number” and is the sum of protons and neutrons

The “2” is called the “atomic number” and is the number of protons

Modern Physics

• Protons have a positive charge +• Neutrons have NO charge • Electrons have a negative charge -

Modern Physics

• Protons are in the nucleus• Neutrons are in the nucleus• Electrons are in clouds at different energy

levels away from the nucleus

Modern Physics

Modern Physics

• Wait, some elements come in different varieties, like Carbon-12 and Carbon 14

Modern Physics

• Wait, some elements come in different varieties, like Carbon-12 and Carbon 14

• These are called ISOTOPESDifferent in the number of neutrons

• We identify elements by the number of protons

• IF an atom has 6 protons, it is called Carbon

Modern Physics

• Carbon comes in two forms• Carbon -12– 6 protons and 6 neutron

• Carbon -14– 6 protons and 8 neutrons

• Carbon -12 is the most common• Carbon -14 is less common and is called an

ISOTOPE of Carbon

Modern Physics

• Lets go back to the main particles

Modern Physics

• Protons have a positive charge +• Neutrons have NO charge • Electrons have a negative charge –• Remember like charges repel• SO If the nucleus is made up of Positively

charged protons and neutral neutrons – why does it stay together?? Shouldn’t it fly apart?

Modern Physics

• Three main particles in an atom Actual Mass Total Mass

2 Protons 1.007276 amu 2.0145522 Neutrons 1.008665 amu 2.0173302 Electrons .0005486 amu .001097 total combined mass should be = 4.032979

Modern Physics

• Three main particles in an atom Actual Mass Total Mass

2 Protons 1.007276 amu 2.0145522 Neutrons 1.008665 amu 2.0173302 Electrons .0005486 amu .001097

total combined mass should be = 4.032979HOWEVER THE ACTUAL MEASURED MASS IS 4.00260

.03038 amu is MISSING

Modern Physics

• Three main particles in an atom Actual Mass Total Mass

2 Protons 1.007276 amu 2.014552amu2 Neutrons 1.008665 amu 2.017330amu2 Electrons .0005486 amu .001097amu

total combined mass should be = 4.032979amuHOWEVER THE ACTUAL MEASURED MASS IS 4.00260

.03038 amu is MISSINGTHIS DIFFERENCE IS CALLED “THE MASS DEFECT”

Modern Physics

• Where did the mass go???

Modern Physics

• Where did the mass go???• E=mc2

• Mass was changed into the BINDING energy to hold together the positively charged protons

• This is called the “Nuclear FORCE”

Modern Physics

• The Nuclear Force must be strong enough to hold together the positively charged protons

• For smaller atoms, an equal number of protons and neutrons generate a sufficient Nuclear Force to hold the atoms together

• For larger atoms, more neutrons are needed to generate a sufficient Nuclear Force

Modern PhysicsBelt of Stability

Modern Physics

• Atoms are stable if they are inside the Belt of Stability

• Atoms are unstable if they are outside the Belt of Stability

• An unstable atom is said to be RADIOACTIVE

Modern Physics

• Are all ISOTOPES radioactive??

Modern Physics

• Are all ISOTOPES radioactive??• NO, only the ones outside of the belt of

stability• Lets take a look at Carbon -12 and Carbon 14

Modern PhysicsBelt of Stability

Modern Physics

• Carbon – 12 is stable with 6 protons and 6 neutrons

• Carbon – 14 is unstable with 6 protons and 8 neutrons

Modern Physics

• Carbon – 12 is stable with 6 protons and 6 neutrons

• Carbon – 14 is unstable with 6 protons and 8 neutrons

• Carbon -14 is RADIOACTIVE

Modern Physics

• Carbon – 12 is stable with 6 protons and 6 neutrons

• Carbon – 14 is unstable with 6 protons and 8 neutrons

• Carbon -14 is RADIOACTIVE• Carbon – 14 occurs naturally in our

environment – a certain percentage of our bodies contain carbon -14

Modern Physics

• Carbon -14 dating

Modern Physics

• Carbon -14 dating• All living things have a mixture of Carbon -12

and Carbon -14 atoms incorporated into our living cells

• Carbon – 14 is always slowly decaying into Carbon -12

• The ratio of Carbon-12 to Carbon -14 in the cells of living things is representative of the environment

Modern Physics

• When an organism is living, the carbon in our bodies is constantly exchanging with carbon in our environment

• Therefore the ratio of Carbon-12 and Carbon-14 in our cells reflects that of the environment

Modern Physics

• However, when an organism dies, the exchange with the environment stops

• The carbon-14 in the dead organism continues to decay and is not replaced with an exchange with the environment

• This changes the ratio of Carbon-12 to Carbon -14 in the dead organism’s cells

Modern Physics

• We know how fast Carbon -14 decays, therefore by measuring how much is missing we can determine how long the exchange of carbon with the environment has been stopped i.e. when the organism died

• This allows us to use Carbon-14 dating to tell how old things are

Modern Physics

• Mesa Verde cliff dwellings have been dated using Carbon -14 dating

Modern Physics

• By taking samples of the cedar wood beams and measuring the ratios of Carbon -12 to Carbon 14 atoms it was determined that the tree died (was cut down) between 900 and 1300 AD (over 1000 years ago)

• Therefore we believe this is the time the cliff dwellings were built

Modern Physics

• Carbon dating can only be used on previously living things

• Cannot be used to measure the age of rocks• Other similar dating systems exist that use

radioactive isotopes and their decay rates

Modern PhysicsBelt of Stability