SAAT Physics 4 Section 7 (Nuclear Physics) - تحصيلي

Properties of the nucleus

Both protons and neutrons are found in it (referred to as nucleons)

Atoms are composed of

Electrons, neutrons, protons

A nucleus can be specified by

An atomic number and a mass number

A

The mass number → represents the total number of protons and neutrons in the nucleus

Z

The atomic number → represents the number of protons in the nucleus

N

The neutron number → represnets the number of neutrons in the nucleus

The chemical symbol of an element

Often written with its mass number and atomic numberA

Although the atomic number does not change within an element,

Atoms of the same element can have different mass numbers

Isotopes

Atoms that have the same atomic number but different neutron numbers

Density of a neutron

Very dense; approximately spherical and the volume is proportional to the total number of nucleons and thus, to the mass of the nucleus

Nucleons combine to form a

Nucleus as though they were tightly packed spheres

How a nucleus can be visualized

As a cluster of tightly packed spherical protons and neutrons

The unified mass unit and rest energy are used to

Express the mass of a nucleus

Atomic mass unit

A standard unit used to express the mass of atoms and subatomic particles

Atomic mass unit symbol

Dalton (Da)

Relationship between rest energy and mass formula

E_R = mc²

Relationship between rest energy and mass formula in words

Rest energy = (mass)(speed of light)²

Nuclear stability

The balance between the intense strong nuclear forces holding nucleons (protons and neutrons) together and the electrostatic repulsion pushing them apart

Coulomb repulsion

Mutual repulsion of discrete charged particles

Strong/nuclear force

At the smallest level, the strong force holds quarks together to form protons and neutrons

Strong/nuclear force properties

Almost completely independent of electric charge; the force of attraction between two protons, two neutrons, and a proton have the same magnitude; has a very short range (10-15 m)

Neutrons help to

Stabilize a nucleus

For a nucleus to be stable,

The repulsion between positively-charged protons must be balanced by the strong nuclear force’s attraction between all the particles in the nucleus

The repulsive force exists between all protons in a nucleus because

The electrostatic force is long range

As the number of protons increases,

The number of neutrons has to increase even more to add enough attractive forces to maintain stability

When Z (atomic number) is greater than 83

The repulsive force between protons cannot be compensated by the addition of more neutrons

Elements that contain more than 83 protons

Do not have a stable nuclei

A stable nucleus’ mass is less than

The masses of its nucleons

Binding energy of a nucleus

The energy required to disassemble an atomic nucleus into its individual protons and neutrons

Binding energy of a nucleus formula

E_bind = △mc²

Binding energy of a nucleus formula in words

Binding energy = mass defect x (speed of light)²

Mass defect

The difference between the actual atomic mass and the predicted mass calculated by adding the mass of protons and neutrons present in the nucleus

Nuclear decay

There are 400 stable nuclei; hundreds of others are unstable and tend to break apart into other particles

The nuclear decay process

Can be a natural event or can be induced artificially (in either case; when the nucleus decays, radiation is emitted in the form of particles, protons, or both)

Radiation

The emission of particles and photons

Process of radiation

Radioactivity

Alpha decay composition

2 protons, 2 neutrons

Alpha decay charge

+2

Alpha decay effect on parent nucleus

Mass loss; new element produced

Beta decay composition

Electron, positron

Beta decay charge

-1

+1

Beta decay effect on parent nucleus

No change in mass number; new element produced

Gamma composition

Photon

Gamma charge

0

Gamma effect on parent nucleus

Energy loss

Rules for nuclear decay

The total of the atomic numbers on the left is the same as the total on the right because charge must be conserved; the total of the mass numbers on the left is the same as the total on the right because nucleon number must be conserved

Half time life

Defined as the amount of time it takes a given quantity to decrease half of its initial value