Physics Unit 3 Cont: Nuclear reactions Flashcards Preview

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Flashcards in Physics Unit 3 Cont: Nuclear reactions Deck (25):
1

Alpha Particle decay

radiation = alpha particle -->Emission of a helium NUCLEUS (two protons and two neutrons) →Alpha Particle

-Creating more inward force (strong nuclear force) to prevent exploding caused by outward (electrostatic force)
-Parent atoms have high mass, PROTON rich unstable nucleus --> strong electrostatic forces of repulsion
-result of alpha decay: nuclei becomes less massive and more stable

Ratio of N to P increased

Transmutation: a nuclear decay process in which daughter atoms are different elements from parent atoms

-Alpha particles are positively charged (because helium nucleus does not have any electrons) +2
-Alpha particles are large and slow moving
-can penetrate skin or paper

2

Beta-negative decay

radiation = beta particle --> Emission of an electron from the nucleus of the parent atom

Neutron --> Proton + electron (increase in atomic number, mass the same) + anti neutrino

Nucleus was going to collapse due to strong nuclear force (too many neutrons). A neutron spontaneously decays into a proton and an electron, and the electron is ejected (along with an anti neutrino→conserves momentum and energy)
Therefore produced more outward/electrostatic force

Transmutation

3

Beta-positive decay

Proton --> Neutron + positron (decrease in atomic number, mass the same) + neutrino

Increasing number of neutrons = increasing inward force because too much outward force (nucleus was going to explode) -->too few neutrons

Transmutation

4

Electron capture

Proton + electron (absorbed) --> neutron
• Electron appears on the reactant side (also produces a neutrino on the product side)

Increasing number of neutrons = increasing inward force because too much outward force (nucleus was going to explode)


Transmutation

5

Gamma decay

• In order to return the atom to a lower, more stable energy state, energy in the form of a gamma ray is emitted (a photon – PARTICLE with ZERO mass, ZERO charge, and a high level of energy)
- Photon = gamma particle
-can penetrate most objects, and only a few centimetres of lead
-poor at ionizing other other atoms -->stripping electrons away

Not transmutation

Excited (*) to ground state releases a gamma particle

Gamma rays are electromagnetic waves of very short wavelength and high frequency.

6

Difference between Decay and Nuclear reactions (Fusion/Fission)

Radioactive decay is spontaneous
Nuclear reactions is non spontaneous (triggered in nuclear reactors by bombarding high energy neutrons for fission)

Radioactive decay produces little energy per atom
Nuclear reactions produce huge amounts of energy (due to chain reaction in fission and high temps and pressures in fusion)

7

Subatomic particles

• Proton: positively charged particle in the nucleus of the atom. →mass 1 charge 1
• Neutron: an uncharged/neutral particle in the nucleus of the atom. →mass 1 charge 0
• Nucleons: particles in the nucleus of an atoms; term for protons and neutrons; both have approximately the same mass
• Electron: negatively charged particle found in the space surrounding the nucleus of an atom →mass 0 charge -1

Atom in its normal state has the SAME number of electrons and pronouns (PEA all the same)

Ground state: state in which all electrons are at their lowest possible energy levels
Excited state: state in which one or more electrons are at higher energy levels than in the ground state →absorbs energy to move to a higher energy level and releases energy as electrons drop back to its lowest available energy level

8

Radioactivity

Radioactivity is the process in which UNSTABLE atomic NUCLEI SPONTANEOUSLY decompose to form nuclei with a HIGHER stability by the RELEASE of energetic sub atomic particles.

-Occurs when the electrostatic forces overcome the strong nuclear force or vice versa.
-The atom emits one of three different types of RADIATION (subatomic particles and their related energy).
-Sum of the top must be the same on both sides. Sum of the bottom must be the same on both sides (nothing is lost)

-occurs in radioactive isotopes

9

Isotopes

a form of an element that has the same atomic number (identity), but a different mass number than all other forms of that element
-different number of neutrons
-have same chemical properties

Ex: isotopes of hydrogen:
Deuterium (mass 2)
Tritium (mass 3)

• Radioisotopes: an unstable isotope that spontaneously changes its nuclear structure and releases energy in the form of radiation →beneficial or harmful
-unstable nuclei (due to unbalanced nuclear forces within their nucleus)
ex: carbon 14 -->undergo beta decay to form nitrogen 14
Half life: 5730 years

10

Atomic Number and Atomic mass

Atomic number: # of protons in an atom (Z)
Mass number: # of nucleons in an atom (protons + neutrons) (A)

Standard notation:
A (mass)
then
Z (number)

11

Stable and unstable nuclei

In the nucleus, protons and neutrons are closely packed together. According to Coulomb's law, the protons would exert strong repulsive forces (electrostatic force of repulsion) on each other. --> Outward force

The strong nuclear force is an attractive force between nucleons similar in size to the electrostatic force of repulsion.→inward force
• Holds together the neutrons and protons in the nucleus of an atom
• Stronger than the electrostatic force

Stability is based on the number of protons and neutrons (forces)
Stable nuclei:
-electrostatic force and strong nuclear force are balanced

Unstable Nuclei:
-electrostatic force and strong nuclear force are unbalanced (because the number of protons and neutrons in the nucleus)
-result to become stable is releasing a proton or a neutron through the process known as radioactive decay
-too many neutrons = strong inward force
-too many protons = strong outward force

12

Binding energy

Binding energy: the energy used to hold the nucleus together (related to the strong nuclear force). It is the energy required to separate a nucleus into its component nucleons. This is the same amount of energy required to bound nucleons together to form a nuclide.

• Per nucleon: divide binding energy by number of nucleons (protons and neutrons = mass number) --> does not change in the reaction

13

Beta decay

Electrons are not electrons from the electron shells around the nucleus, but are generated when a NEUTRON in the nucleus splits to form a PROTON and an accompanying ELECTRON.

- faster moving than alpha particles
- can penetrate a few sheets of aluminium foil

-Both beta-positive decay and electron capture transforms a proton into a neutron (decreases outward force0

14

Half-life

• It is possible to predict the decay rate for a LARGE sample of an isotope. Radioactive materials decay at different rates (cannot predict EXACTLY WHEN; billion of years or within seconds)

Half-life: the average length of time it takes radioactive material to decay to half of its original mass. For a given isotope, it is the time it takes for the parent atom to decay to its daughter atom.

Radioactive decay curve: exponential decay with time on the x axis and mass on the y axis

Formula:
o A = A0(1/2)^(t/h)
h = half life
t/h = number of half lives
units of time must be the same (t and h)
units of mass must be the same

Different ways of expressing mass:
o grams
o milligrams
o Counts per time (number of particles decaying per minute)
o Atoms

Percent remaining:
o Amount/original amount = (1/2)^t/h

FOLLOWS the law of conservation of mass and energy:
-Mass decayed/lost by one isotope = mass of another isotope
ex: potassium-42 decays into calcium-42 (mass of K lost = mass of Ca gained)

15

Nuclear Fission

Nuclear fission: the decomposition of LARGE, unstable nuclei into smaller, more stable nuclei and 3 neutrons along with the release of ENERGY when bombarded by a slow moving neutron.

The mass of PRODUCTS from the fission reaction are slightly less than the mass of the parent nucleus. -->Mass defect

The mass that is lost in the reaction has been converted to ENERGY

Mass-Energy Equivalence Formula

E = mc^2

Form of kinetic energy of the daughter nuclei and neutron and electromagnetic radiation in the form of gamma rays.


Ex: Uranium-235 + neutron --> Uranium 236 (unstable) -->repulsions cause the nucleus to split and release a lot of energy -->produces 2 daughter nuclei and 3 neutrons

Chain reaction:
Occurs when the neutrons released initiate other fission reactions AS LONG AS a minimum mass is present (called the critical mass)

16

Nuclear reactor

Energy released can be used to generate electricity
-Nuclear reactors exist to control the chain reactions

Moderators: to slow down neutrons (do not absorb neutrons)
Control rods: control nuclear reaction rates (absorb neutrons)

Electrical energy is produced when the steam spins a turbine, producing electrical energy from mechanical energy

17

Nuclear fusion

Nuclear fusion: the process whereby two or more small nuclei combine to form a new, larger nucleus
ex: two deuterium form one hydrogen
-initially endothermic, but then releases a lot of energy

Fusion reactions are accompanied by a much greater mass to energy conversion than in fission reactions (greater mass defect = greater energy)

Nuclear fusion requires extremely HIGH temperatures and high pressures. This is because the small nuclei require enough KINETIC energy to overcome their electrostatic REPULSION and collide →this allows the nuclei to get close enough to each other for the strong nuclear force to take effect (attraction).

The energy we receive from the sun is from nuclear fusion. The sun is made up mainly of hydrogen and helium. Within the sun the temperature is millions of degrees Celsius, there is the constant fusion of small nuclei into larger nuclei.

18

Binding energy

Binding energy: the energy used to hold the nucleus together. It is the energy required to separate a nucleus into its component nucleons. This is the same amount of energy required to bound nucleons together to form a nuclide.
• Per nucleon: divide binding energy by number of nucleons (protons and neutrons = mass number)

Usually for fusion reactions

19

Nuclear fission and fusion

In both nuclear fission and fusion, energy is released due to mass defect (sum of the masses in the reactants is greater than the products) →according to E = mc2 →EXOTHERMIC

• Fission and fusion reactions occur so that the stability of the products is greater than that of the reactants
• Fission vs. fusion of a nucleus depends on the position on the binding energy per nucleon curve (all elements are becoming Fe56)
o Left of iron undergo fusion (light nuclei)
o Right of iron undergo fission (heavy nuclei)
• *higher the binding energy per nucleon, the more stable the atom is
• Fusion is less harmful than nuclear fission (does not release as much waste)
*On a per mass basis, fusion typically yields more energy than fission

20

Law of Conservation of Mass-Energy

Law of Conservation of Mass-Energy (or mass-energy equivalence): Mass can transform into energy, and energy into mass, such that the total mass-energy in an isolated system remains constant (explains mass defect and energy released) →proposed by Albert Einstein
• 1 J = kgm2s-2 (MASS IS IN KG)

21

Units

Atomic Mass Unit (u): a unit of mass equal to 1.66 x 10-27 kg. One u is equal to the mass of 1/12 of a carbon-12 atom.

Electron-volt (eV): the amount of energy given to an electron when it is accelerated through a potential difference of 1 V.

Mega-electron volt (MeV): one million times the eV. The energy required to accelerate an electron through a potential difference of 1 million volts.
1 MeV = 1.602 x 10-13 J
• J is too large, therefore a new unit is needed

1 u = 931.5 MeV

Process:
1. 4942 x 10^-10 J = 1 u from Einstein's equation
Use 1.602 x 10^-13 J = 1 Mev to find energy in Mev per u

22

Mass of nucleus

A nucleus weights less than its sum of nucleons due to mass defect

Mass defect: Sum of the masses of the subatomic particles – actual atomic mass

23

Radiation

Radiation is energy released from radioactive substances i.e. beta, alpha, gamma radiation

24

Nuclear Notation Form

Mass
Symbol
Charge/atomic number

25

Mass and binding energy graph

Most stable isotope is Iron 56

Fission reactions do not change in binding energy per nucleon very much

Fusion reactions change in binding energy per nucleon very much = release more energy than fission