Basic Nuclear Physics: Atomic Structure

Basic Atomic Structure

• The atom is composed of positively charged protons, uncharged neutrons, and negatively charged electrons.

Atomic Structure Models

• Thomson's model, Rutherford's model, and Bohr's model are three different models that describe the structure of the atom.

Nucleus

• The nucleus is composed of protons and neutrons, which determines the identity of the element and its atomic mass.
• Protons have a positive charge, while neutrons have no charge.

Protons

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

Neutrons

• Neutrons are uncharged particles found in the nucleus of an atom.
• Atoms of a given element do not always have the same number of neutrons.

Electrons

• Electrons are negatively charged particles that surround the nucleus in "orbits".
• The sharing or exchange of electrons between atoms forms chemical bonds.

Electron Binding Energy

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

Atomic Mass Unit (amu)

• The atomic mass of the proton and neutron is approximately 1 amu.
• The atomic mass of the electron is approximately 0.00055 amu, which is much smaller than the proton or neutron.

Elements

• The number of protons in an atom determines the element.
• For an uncharged atom, the number of electrons equals the number of protons.

Isotopes

• Atoms of an element with a different number of neutrons in the nucleus are called isotopes of each other.
• Isotopes are typically written as A Xy, where A is the atomic mass (neutron + protons) and Z is the atomic number (protons).
• Isotopes can have more neutrons than protons, and some are stable while others are unstable (radioactive).

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 stable nuclei shows a pattern where N is about 1.5 times Z for the heaviest stable nuclei.
• The presence of extra neutrons overcomes the positively charged protons' tendency to repel each other and disrupt the nucleus.

Unstable Nuclei

• Nuclei that do not fall on the line of stability tend to be unstable or "radioactive" and are called radionuclides.
• Radionuclides undergo radioactive transformation or disintegration, emitting particles to adjust their neutron to proton ratio.
• This process tends to move the radionuclide towards the line of stability.

Examples of Radionuclides

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