Chapter 18 The Nucleus

Cool Nuclear Facts Nucleus has a density of 1.6x10 14 g/cm 3 Nuclear material the size of a ping pong ball would be 2.5 billion tons If the nucleus was the size of a ping pong ball the 1s orbital would be 0.3 miles away Enormous amounts of energy are within the nucleus

Vocab Nucleons – protons and neutrons Atomic Number (Z) – Number of protons Mass Number (A) – Protons + Neutrons Isotope – Same number of protons different number of neutrons Nuclide – Specific isotope

Notation Nuclear Symbol Hyphen Notation – Element name and mass number Carbon – 14

Radioactive Decay Of the approximately 2000 known nuclides only 279 are stable in terms of decay In general light nuclides are stable when the n 0 p + to ratio = 1 Heavier nuclides require a slightly higher ratio No nuclide with more than 84 protons is stable

Cont. Certain combinations of protons and neutrons are more stable too ProtonsNeutrons# of Stable isotopes Even 168 EvenOdd57 OddEven50 Odd 4

Types of Decay Radioactive decay occurs to produce a more favorable neutron to proton ratio Almost all change the number of protons Therefore the element changes Mass is always conserved

Alpha Emission Alpha particle (α) is a helium nucleus Usually in very heavy nuclides Mass Number decreases The n 0 to p + ratio increases

Beta Emission Beta particle (β) is an electron emitted from the nucleus Neutron is converted into a proton and electron Mass Number stays the same n 0 to p + ratio decreases Carbon – 14 is a beta producer

Positron Emission Positrons are particles with the same mass as an electron but with a positive charge Proton is converted into a neutron and a positron Mass Number stays the same n 0 to p + ratio increases

Electron Capture Inner orbital electron is captured by the nucleus Combines with a proton to form an neutron Mass Number stays the same n 0 to p + ratio increases

Gamma Emission Gamma rays ( γ ) are high energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state. Usually follows other types of decay

Decay Series Often times many decays are necessary to produce a stable isotope Original nuclide is called the “Parent” Resulting nuclides are called the “Daughters”

Homework P. 906 #’s 10,11,13,18

Penetrating Ability Different emissions have different penetrating abilities Alpha particles can be stopped by skin Beta can penetrate about 1cm Gamma is the highest energy –Needs a lot of lead

Effect of Radiation Different particles ionize molecules in the body differently Alpha particles ionize molecules easily –Ingesting would be very bad Gamma not so much

Measurement of Radiation Energy of radiation measured in rads –1 rad = 1x10 -2 J However different isotopes have different effects on humans so rems are used too The average human receives 193mrem/yr On short term exposure 500 rem causes death in half the population in 30 days

Detection of Radioactivity Measured with a Geiger counter. Uses the fact the radioactive particles ionize atoms Chamber increases in conductivity when radiation is present Measures radioactivy in counts

Radioactive Decay Kinetics ALL radioactive decay follows first order kinetics Just like chapter 12 There is a rate and a half life Rate changes half life remains the same

1 st Order Kinetics ln N = -kt + ln N o N Number of nuclides or counts at time t N o Number of nuclides or counts at t=0 k is the rate constant This is the equation of a ________? –LINE –Y axis is ln N, X axis is t –The slope is the rate constant

1 st Order Half Life The half life is the time required for a reactant to reach half of its previous concentration Meaning N = N o /2 t 1/2 =ln2/k t 1/2 =.693/k Half life is always the same for first order kinetics

#27 page 907

Radioactive Dating Scientists can use the half lives of nuclides to determine the age of samples Carbon -14 has a half life of 5730 years Every 5730 years half of the original sample is gone.

Homework P. 907 #’s 23,25,28