The Atomic NucleusHolding it all together – or not!

Nuclear Components

• Nucleons• Protons & Neutrons• Bound together by Strong Nuclear Force at short distances

• Quarks Make Up Nucleons• 6 total

• 3 quarks comprise protons• 3 different quarks make up neutrons

• Fanciful names• Colors; Directions (“Up” & “Down”); Flavors

• Sub-subatomic particles/forces• Mesons; Gluons; Muons; Bosons; Gravitons; Pions, etc…

Balancing Forces

• Strong Nuclear Force• Between proton-protons, neutron-protons, & neutron-neutrons• Operates at short ranges• Is 100X stronger than electromagnetic repulsion

• Electromagnetic Force• Like charges repel one another• Opposite charges attract one another• Protons are (+); electrons are (-)

• Role of Neutrons• Buffer proton-proton repulsion

Imbalance of Forces

• Most stable neutron-to-proton ratio is 1:1• Elements with lower atomic numbers tend to be very stable.• Isotopes of these, however, are radioactive.

• Edge of stability occurs at a 1.5:1 ratio• All elements with higher atomic number than Bismuth’s are unstable• Bismuth has 83 protons & 126 neutrons; 1.52:1 ratio

• When the balance shifts, the isotope becomes radioactive• Nucleus natural decays, emitting particles and rays

Radioactive Elements = Nuclides• Alpha particles [4

2He]• a particles = 2 protons + 2 neutrons (Helium nucleus)• Heaviest particle; relatively slow; few cm penetration; shield with paper

• Beta particles [0-1b]

• b particles = 10n 1

1p + 0-1b ; faster & penetrates further

• Shield with Aluminum foil or ½-inch of Plexiglass

• Positron [0+1b]

• 11p 1

0n + 0+1 ; b results when the neutron/ proton ratio is too small

• Electron Capture • 0

-1e + 11p 1

0n ; results when the neutron/ proton ratio is too small

• Gamma rays [g]• g rays = electromagnetic radiation (photon); much higher frequency than light• Needs lead plates &/or tons of concrete (depending on the amount)

Detecting Radiation

• Film badges• Film is worn inside of plastic “badge” & keeps count of total exposure

• Geiger-Muller counters• Detects by counting electric pulses carried by gas ionized by radiation• Has wands and emits beeps proportional to amount of radiation

• Scintillation counters• Converts scintillating light to an electric signal to detect radiation• Uses small paper wipes that are put into vials with scintillation fluid• Can count all types of radioactive decay particles

Isotopes• Isotope = form of an element having a particular # of neutrons • Element’s identity is established by its # of protons (Atomic Number)• Elements are arranged in order of Atomic # on Periodic Table of Elements• H – 1; He – 2; Li – 3; Be – 4; B – 5; C – 6; N – 7; O – 8; F – 9; Ne – 10, etc…

• Element’s Mass number = # protons + # neutrons• Carbon-12 has 6 protons + 6 neutrons• Carbon-14 has 6 protons + 8 neutrons

• Find # neutrons by subtracting Atomic # from Mass #• Nuclear symbol shows Mass # as superscript & Atomic # as subscript• 12

6C; 146C; 14

7N

Hydrogen’s Isotopes• The most common isotope is the most stable & is called “hydrogen”• 1

1H = 9• This isotope has 1 proton & 0 neutrons

• Radioactive hydrogen with 1 neutron is called “deuterium”• 2

1H• “Heavy water” has been labeled with deuterium.

• Radioactive hydrogen with 2 neutrons is called “tritium”• 3

1H

• The average atomic mass is calculated from the weighted average

Average Atomic Mass

• Displayed on Periodic Table of Elements below the Symbol• Units are either “grams/ mole” (molar mass) or “amu”, i.e., atomic mass unit

• Calculated as the weighted average of all natural isotopes’ masses• S (mass)(abundance of isotope)• Oxygen-16 is 99.757% abundant with amu of 15.994915• Oxygen-17 is 0.038% abundant with amu of 16.999132• Oxygen-18 is 0.205% abundant with amu of 17.999160

• Calculations for Oxygen• Avg atomic mass = (15.994915 amu)(0.99757) + (16.999132 amu)(0.00038) +

(17.999160)(0.00205) = 15.9994 amu

Radioactive Half-Life

• Half-Life = time in which ½ of the radioactive material has decayed• At the end of 1 half-life 50% of the original radiation is gone.• At the end of the 2nd half-life, 50% of the residual 50% is gone. 25% is left.• At the end of the 3rd half-life, 50% of the remaining 25% is gone. 12.5% is left.• At the end of the 4th half-life, 50% of the residual 12.5% is gone. 6.25% stays.

• Different elements have different half-lives• U-238, 4.46 billion yrs; C-14, 5715 yrs; H-3, 12.32 yrs; P-32, 14.28 days;

Po-218, 3.0 min; At-218 1.6 s; Po-214, 163.7 ms

• All half-lives appear to be absolutely constant

Transmutations, Natural & Artificial• Elements become different elements during radioactive decay• This initiates naturally or when nuclei are bombarded with particles, charged

or uncharged, such as occurred at Fermilab, IL or now at CERN, Switzerland• Transuranium elements are made by artificial transmutations

• Parent nuclide, original/ heaviest nuclide, decays to daughter nuclides• Transmutations continue until a stable isotope is attained• A decay series traces successive radioactive decay nuclides• There are 3 natural decay series: U-238, U-235, & Th-232