Modern Physics The Nucleus

Modern Physics Three main particles in an atom

Modern Physics Three main particles in an atom Protons Neutrons Electrons

Modern Physics Three main particles in an atom Mass Protons amu Neutrons amu Electrons amu

Modern Physics Lets look at a Helium atom Helium 4, symbolized as 4 2 He 2 – protons 2 – neutrons 2 – 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

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 ISOTOPES Different 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 MassTotal Mass 2 Protons amu Neutrons amu Electrons amu total combined mass should be =

Modern Physics Three main particles in an atom Actual Mass Total Mass 2 Protons amu Neutrons amu Electrons amu total combined mass should be = HOWEVER THE ACTUAL MEASURED MASS IS amu is MISSING

Modern Physics Three main particles in an atom Actual MassTotal Mass 2 Protons amu amu 2 Neutrons amu amu 2 Electrons amu amu total combined mass should be = amu HOWEVER THE ACTUAL MEASURED MASS IS amu is MISSING THIS DIFFERENCE IS CALLED “THE MASS DEFECT”

Modern Physics Where did the mass go???

Modern Physics Where did the mass go??? E=mc 2 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 Physics Belt 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 Physics Belt 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 Physics Belt of Stability