Basic Nuclear Physics - Atomic Structure

Basic Atomic Structure

• An atom is composed of three main particles: positively charged protons, uncharged neutrons, and negatively charged electrons.
• The nucleus of an atom consists of protons and neutrons, which determines the identity of the element and its atomic mass.

Protons

• Protons are positively charged particles found inside the nucleus of an atom.
• Each element has a unique atomic number, which is the number of protons in an atom.
• The proton number never changes for a given element.

Neutrons

• Neutrons are particles found in the nucleus of an atom with no electrical charge, making them "neutral".
• Atoms of a given element do not always contain the same number of neutrons.

Electrons

• Electrons are negatively charged particles that surround the nucleus in "orbits" similar to moons orbiting a planet.
• The sharing or exchange of electrons between atoms forms chemical bonds, which is how new molecules and compounds are formed.

Electron Binding Energy

• Electrons exist in discrete "shells" around the nucleus, with each shell representing a unique binding energy holding the electron to the nucleus.
• The shells are designated by letters (K, L, M, N …), where K, the shell closest to the nucleus, has the largest binding energy.
• The maximum number of electrons in each shell is 2 in K shell, 8 in L shell, and so on.

Atomic Mass Unit (amu)

• The atomic mass of the proton and the neutron is approximately 1.0073 amu and 1.0087 amu, respectively.
• The atomic mass of the electron is approximately 0.00055 amu, which is much smaller than that of the proton or neutron.

Elements

• The number of protons in an atom dictates the element, and for an uncharged atom, the number of electrons equals the number of protons.

Isotopes

• Atoms of an element that have a different number of neutrons in the nucleus are called isotopes of each other.
• Isotope notation is typically written as: A Xy, where A is the atomic mass (neutron + protons) and Xy is the element symbol.

Nuclear Stability

• A stable or non-radioactive nuclide is one whose atoms do not decay.
• The graph of neutron number N vs atomic number Z for the stable nuclei shows an interesting pattern.
• For the heaviest stable nuclei, N is about 1.5 times Z.

Unstable Nuclei

• Nuclei that do not fall on the line of stability tend to be unstable or "radioactive".
• They are called "radionuclides".
• Radionuclides undergo a process called radioactive transformation or disintegration to adjust their neutron to proton ratio.

Some Common Radionuclides

• Examples of radionuclides include naturally occurring 235U and 238U, 60Co, 137Cs, 90Sr found in nuclear power plants, 192Ir used in radiography, 99mTc used in nuclear medicine, and 131I used in the treatment of thyroid conditions.