Physical Science 1 – chapter 20 Name_________________________

Section 1 – The Nucleus

Atoms are composed of protons, neutrons, electrons

Protons are positively charged, are found in the nucleus. The nucleus is positive because of protons

Neutrons are neutral, are in the nucleus.

Electrons are negative, are outside the nucleus. Number of electrons are equal to number of protons in an atom

If an atom was the size of a soccer stadium, the nucleus would be about the size of a marble or 1 inch (2cm)

The protons and neutrons are 2000 times more massive than electrons. ALL OF THE MASS OF AN ATOM IS IN THE NUCLEUS!

Isotopes

Are atoms of the same element with different number of neutrons (same # of protons).

Isotopes have different masses

Atomic number is whole number on periodic table. It tells # of protons (and electrons)

Mass number is defined as the number of protons PLUS the number of neutrons. It is NOT on the periodic table

NEUTRONS = MASS NUMBER – ATOMIC NUMBER

What is the meaning of each number and symbol here? C612 the 12 is the mass number, the 6

is the atomic numberThis isotope is called carbon-12

It has 6 protons, 6 neutrons, 6 electrons

Write the symbol for carbon – 14 C614 it has 6 protons, 8 neutrons, 6 electrons

Write the symbol for cobalt – 60 Co2760 it has 27 protons, 33 neutrons, 27 electrons

Write the symbol for hydrogen – 3 H13 it has 1 proton, 2 neutrons, 1 electron

Forces in nature:

Four fundamental forces: strong force, weak force, electromagnetic (EM) force, gravityStrong: in nucleus EM: makes like charges repel & opposites attractWeak: when nucleus breaks up Gravity: pull of things to massive objects

The strong force__causes protons and neutrons to be attracted to each other_1

The strong force is about 100 times stronger than the electromagnetic (EM) force. Strong forces are short range, EM forces are long range.

Protons are close protons are far apart

Strong force is strongest strong force is weak; electromagnetic force is more important

Small atoms with small nuclei

If atom has only a few protons & neutrons, very strong forces hold them together; nucleus is held together tightly

Large atoms with big nuclei

Each proton has few neighbors; strong forces can’t hold them together; nucleus breaks apart

Neutron to proton ratios

For small atoms, it is approximately 1:1 ratio; for big atoms, it is approximately 3:2 ratio

Radioactivity process of unstable nuclei breaking apart (decaying) and emitting energy & matter

All nuclei that contain more than 83 protons are radioactive

All atoms with more than 92 protons are synthetic/ man made

The darker the element in picture, the more likely to be radioactive

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Section 2 – Nuclear Decays and Reactions

When a nucleus decays, particles and energy are emitted

Nuclear radiation the process of the nucleus decaying and emitting energy & particles

Three types of nuclear radiation: alpha particles, beta particles, gamma rays

Alpha particles a helium nucleus, symbol is He24 , most massive, +2 charge, stopped by paper,

travels least distance

Beta particles a high speed electron, symbol is e−10 , moves very fast, stopped by thick

aluminum foil

Gamma Rays very dangerous, most damaging, no charge, stopped by very thick lead

Damage from nuclear decay

All types of radiation can be dangerous to humans

Transmutation when one element changes into another

Alpha decay when a nucleus loses an alpha particle and some other isotope

Example 1: radium-226 decays by alpha decay; write the equation:

Ra88226 He2

4 + Rn86222

Example 2: write the equation for the alpha decay of polonium-210

Po84210 He2

4 + Pb82206

Beta decay when a nucleus loses a beta particle and some other isotope

Example 1: carbon -14 decays by beta decay; write the equation:

C614 e−1

0 + N714

Example 2: write the equation for the beta decay of iodine-131

I53131 e−1

0 + Xe54131

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Nuclear fission

Nucleus splits apart

Nuclear fission is done by

Bombarding a nucleus with neutrons

Chain reaction

A series of repeated fission reactions due to neutrons being released

If a chain reaction is uncontrolled, enormous energy is produced

and an explosion or bomb can happen

A chain reaction can be controlled by adding control rods that absorb neutrons, or spacing the atoms farther apart

This is how nuclear power plants operate

Nuclear fusion

Two or more nuclei combine to form a bigger nucleus

This kind of reaction occurs on stars/ the sun

Temperatures have to be millions of degrees

Theory of special relativity says mass can be changed into energy

Mass- energy equation E = mc2 which means energy = (mass) (speed of light)2

The speed of light is 186,000 miles/second; nothing can go faster

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Section 3 - Radiation technologies and applications

Detecting nuclear radiation – A Geiger Counter a tube with charged particles & gas that can detect radiation by clicking noises

A wire chamber similar to Geiger counter, but can also trace the path of a particleBubble chambers work similarly

Background radiation low level radiation always around us

Sources of background radiation: rocks, radon, cosmic rays

Radiation and medicine

A tracer a radioactive isotope used to locate molecules in your body

Common tracers are Tc – 99 and I – 131

Thyroid I-131 is given; it emits gamma rays that can be detected in the thyroid if thyroid is properly functioning

Cancers can be treated using radiation therapy

Radioactive decay and half-life:

Half life time it takes for ½ of the nuclei of a radioactive isotope to decay

Parent nucleus the radioactive starting nucleus

Daughter nucleus the nucleus that forms after the decay

Half lives of specific isotopes: C-14 5730 years Po-214 less than 1 second U-238 4.5 billion yrs

Radiometric Dating

Method of determining how old a sample is; uses half-lives

C-14 used to date fossils from living organisms

U-235 used to date rocks and mineral that are really old Others used are K-40, Rb-87, and Sm-147

Problem 1. How much C-14 would be left of a sample after 17,190 years if the original amount was 2.00 grams? C-14 has a half-life of 5,730 years.

Total time 11,460 17,190Time (h.l) 0 5730 5730 5730Mass (g) 2 1 .5 .25

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Problem 2: Radon-222 has a half-life of 4 days. If you had 80 grams now, how much would be left after 20 days?

Total time 8 12 16 20Time (h.l) 0 4 4 4 4 4Mass (g) 80 40 20 10 5 2.5

Nuclear Decay

The following atoms all undergo alpha decay. Write the complete nuclear equation.

→ ___________ + ____________

→ ___________ + ____________

→ ___________ + ____________

→ ___________ + ____________

The following atoms all undergo beta decay. Write the complete nuclear equation.

→ ___________ + ____________

→ ___________ + ____________

→ ___________ + ____________

→ ___________ + ____________

The following all undergo electron capture. This means to add a beta particle as a reactant. Write the complete nuclear equation.

+ ___________ → ____________

+ ___________ → ____________

+ ___________ → ____________

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+ ___________ → ____________

Radiometric dating1. What is a half-life?

2. If we start with 400 atoms of a radioactive substance, how many would remain after one half-life? After 2 half-lives? After 3 half-lives? After 4 half-lives?

3. If we start with 16 grams of a substance, how much will remain after 3 half-lives?

4. A sample of carbon-14 has been decaying for 22,920 years and is now 35.0 grams. What was the size of the original sample? (The half-life of carbon-14 is 5,730 years)

7. How old is a skeleton sample if the current amount of carbon in the bones is 3.12g? (Assume you started with 100g)

8. If you started with 120 grams of a radioactive substance, and now you have 15 grams, how many half-lives have passed?

9. The half-life of isotope X is 2.0 years. How many years would it take for a 4.0 mg sample of isotope X to decay and have only 0.50 mg of it remain?

10.The half-life of Po-218 is three minutes. How much of a 200 atom sample will remain after 15 minutes?

11.The half-life of Au-198 is 2.69 days. How long does it take a 180g sample to decay to 1/8 its original mass?

12.What is the half-life of a radioactive sample if 100.0 grams of it decays to 12.5 grams in 24.3 hours?

13.How long will it take a sample of 131I to decay to 12.5% assuming you start with 100% and its half-life is 8.07 days?

14.Sodium-25 was to be used in an experiment, but it took 3.0 minutes to get the sodium from the reactor to the laboratory. If 5.0 mg of sodium-25 was removed from the reactor,

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how many mg of sodium-25 were placed in the reaction vessel 3.0 minutes later if the half-life of sodium-25 is 60 seconds?

15.Actinium-226 have a half-life of 29 hours. If 10.0 g of actinium-226 disintegrates over a period of 145 hours, how many mg of actinium-226 will remain?

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