Nuclear Structure

Nucleon

Collection of protons and neutrons that make up the nucleus

1 atomic unity (amu)

Proton Mass: 1.673×10×10^-27 kg

Neutron Mass: 1.675×10×10^-27 kg

since they’re so similar, each proton/neutron has a mass of one atomic unit

1u=1.661×10^-27=931.5MeVc^-2

Nuclide

Combination of protons and neutrons that form a nucleus

A=Z+N

Atomic Number (Z): number of protons

Mass Number/Nucleon Number (A): number of nucleons

Neutron Number (N): number of neutrons

E=mc²

Units: MeV=(MeVc^-2)(c²)

E= Energy

m= mass

c= speed of light

Isotope

Atoms with the same number of protons but a different number of neutrons

Isotope Characteristics

Chemically the same element

No. of protons=No. of neutrons (in a neutral atom)

Electron structure permits chemical bonding

Masses are different

Each isotope has a different number of neutrons

Does not affect chemical bonding but affects radioactivity

Bainbridge Mass Spectrometer

Allows measurements of the mass of a nucleus

Usually run with singly charged (+1) ions since it is difficult to remove ALL electrons from an atom

Consists of:

• velocity selector set to v of incoming particles (crossed fields)

• uniform magnetic field at right angles to path of nucleus

Isotopes & Mass Spectrometers

Isotopes have different masses & thus follow different circular radiuses

Provides evidence for the existence of isotopes which in turn is evidence for the existence of neutrons

Relative concentration of each isotope can be measured by comparing the number of hits at each position on the photographic plate

Forces in the Nucleus

Because of forces present in the nucleus some isotopes are stable and others are radioactive

Forces must be balanced in a stable nucleus

• Large stable nuclei have more neutrons than protons because of short-range nature of strong nuclear force

• Atoms with too many neutrons are also unstable

Two principle interactions between particles

Coulomb Force

• Repulsion between protons

• Long-range; felt between all protons in atom

Strong Nuclear Force

• Attractions between nucleons

• Short-range; felt only between neighboring nucleons

• Stronger than Coulomb force (only over short distances)

Mass Number

Approximate value of the mass of the nucleus in atomic units (amu)

Mass of the Nucleus

If you add the masses of the protons and neutrons, it would be greater than the mass of the nucleus—the “extra” mass is lost as energy (E=mc²)

Neutron Mass

1.008665u=1.675×10^-27kg=940MeVc^-2

Proton Mass

1.007267u=1.673×10^-27kg=938MeVC^-2

Mass Defect

The extra mass between the actual weight of the atom and the mass of the nucleus.

Not looking at overall values but rather the small difference.

Binding Energy (BE)

Energy released in the formation of a nucleus from individual nucleons

Equals to mass times c²

• Conversion of energy to mass: E=mc²

• Units of energy (binding energy scale): MeV=10^6 eV

• Equivalent units of mass: MeVc^-2

Binding Energy Per Nucleon

A measure of the relative stability of nuclei

Natural system tends toward lower energy states

Therefore: atoms with a higher BE (i.e. more energy released in their formation) are more stable.