FUNCHEM.1: Matter: The Basis of Life. Isotopes

Learning Outcomes

Upon completion of this module, you should be able to:

• Define core terms: 'matter', 'elements', 'atom', 'mass number', 'atomic number', and 'isotope'.

• Recall the four most abundant elements in the human body and describe their functions.

• Identify the number of protons, neutrons, and electrons for the first $20$ elements of the Periodic Table.

• Discuss the diagnostic applications of isotopes, using examples such as $^{99}\text{Tc}$, $^{131}\text{I}$, and $^{24}\text{Na}$.

• Explain the principles and applications of radiation therapy.

• Calculate the relative atomic mass of elements based on their isotopic abundances.

Matter and Chemistry

Chemistry plays a vital role in understanding the biological world, as it is essential for comprehending how the body functions and how it is affected by diseases.

Why Chemistry Matters

To understand the human body and disease, knowledge of:

• The composition of matter is crucial.

• The interactions within matter is equally important.

Historical and Medical Significance

Chemistry has been central to many medical advancements, including:

• The discovery and development of Penicillin by Alexander Fleming in $1928$, leading to semisynthetic antibiotics.

• Alice Hamilton's pioneering work in the development of Occupational Medicine ($1916$), notably her studies on lead poisoning.

• MRI (Magnetic Resonance Imaging), a staple of modern medical diagnostics, which relies entirely on principles of chemistry and physics.

Useful Definitions

• Matter: Anything that occupies space and has mass. All matter is composed of elements.

• Element: A pure substance that cannot be broken down into simpler substances by ordinary chemical means. Currently, $118$ elements have been identified by IUPAC (International Union of Pure and Applied Chemistry).

• Chemistry: The scientific study of the composition, structure, and properties of matter, the changes it undergoes, and the energy associated with these changes.

Elements in the Human Body

The human body is primarily composed of just a few key elements. Remarkably, $99\%$ of the human body consists of substances made from only four elements:

1. Oxygen (O): Essential for cellular respiration, the process that generates energy in cells.

2. Carbon (C): Forms the backbone of all organic compounds, making up the vast majority of biological molecules, including carbohydrates, lipids, proteins, and nucleic acids.

3. Hydrogen (H): A fundamental component of organic compounds and plays a critical role in maintaining the body's acid-base balance.

4. Nitrogen (N): A key component of proteins, nucleic acids (DNA and RNA), and cell membranes.

Beyond these, other elements like Iron (Fe) for hemoglobin, Zinc (Zn) for enzymes (involved in longevity, growth, fertility), Copper (Cu) for oxidative enzymes, and Sodium/Potassium (Na/K) for the nervous system are also vital in smaller quantities.

The Atom

Definition and Properties

• An atom is the smallest unit of an element that retains the chemical properties of that element.

• All atoms of a given element are identical in their most fundamental properties (though isotopes vary in neutron count).

• An atom is composed of subatomic particles: electrons, protons, and neutrons, along with other, smaller particles.

• It is the smallest part of an element that can participate in a chemical reaction.

Subatomic Particles and Their Properties

Atomic Number ($Z$)

• The atomic number ($Z$) is the number of protons located in the nucleus of an atom of that element.

• The number of protons uniquely identifies the element. For example:

  • A hydrogen atom always has $1$ proton.

  • A carbon atom always has $6$ protons.

  • A platinum atom always has $78$ protons.

  • A uranium atom always has $92$ protons.

Atomic Number and Electrons

• An atom is electrically neutral when it contains an equal number of protons and electrons.

• If the number of protons and electrons is not equal, the atom carries a net positive or negative charge and is called an ion.

• For electrically neutral atoms, the number of electrons equals the number of protons (which is the atomic number $Z$). For example:

  • An electrically neutral hydrogen atom has $1$ electron.

  • An electrically neutral carbon atom has $6$ electrons.

  • An electrically neutral platinum atom has $78$ electrons.

  • An electrically neutral uranium atom has $92$ electrons.

Mass Number ($A$)

• The mass number ($A$) represents the total count of protons and neutrons within the nucleus of an atom.

• The number of neutrons in a nucleus can be calculated using the formula:
  $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number} = A - Z$

Molecules

A molecule is a stable aggregate of at least two atoms, arranged in a specific configuration and held together by chemical forces (also known as chemical bonds). Molecules can consist of:

• Atoms of the same element, such as oxygen gas ($\text{O}_2$).

• Atoms of different elements, such as nitric oxide ($\text{NO}$), water ($\text{H}<em>2\text{O}$), or glucose ($\text{C}</em>6\text{H}<em>{12}\text{O}</em>6$).

Isotopes

Definition

Isotopes are atoms of a specific element that share the same number of protons (and thus the same atomic number, $Z$) but differ in the number of neutrons. Consequently, isotopes of an element have different mass numbers ($A$).

Examples of Isotopes

• Hydrogen (H) has three naturally occurring isotopes:

  • Protium ($\text{}_1^1\text{H}$): $1$ proton, $0$ neutrons.

  • Deuterium ($\text{}_1^2\text{H}$): $1$ proton, $1$ neutron (also known as heavy hydrogen).

  • Tritium ($\text{}_1^3\text{H}$): $1$ proton, $2$ neutrons (radioactive).

• Carbon (C) also has several isotopes:

  • Carbon-12 ($\text{}_6^{12}\text{C}$): $6$ protons, $6$ neutrons.

  • Carbon-13 ($\text{}_6^{13}\text{C}$): $6$ protons, $7$ neutrons.

  • Carbon-14 ($\text{}_6^{14}\text{C}$): $6$ protons, $8$ neutrons. This is a naturally occurring radioisotope used in Carbon Dating to estimate the age of carbon-bearing materials up to approximately $58,000$ to $62,000$ years.

Unstable Isotopes and Radiation

Some isotopes are unstable (radioisotopes). These unstable isotopes undergo radioactive decay, breaking down and emitting various forms of radiation (alpha, beta, gamma particles) as they transform into more stable forms.

Medical Applications of Isotopes

Isotopes, particularly radioisotopes, have crucial applications in medicine, broadly categorized into diagnostic and therapeutic uses.

Diagnostic Applications

Radioisotopes are used to image specific organs or track biological processes within the body without invasive procedures.

• Technetium-99 ($\text{}^{99}\text{Tc}$):

  • The most widely used isotope in medical diagnostics globally.

  • Used to obtain images of various organs, including the liver, heart, and lungs. Its short half-life and suitable gamma emission make it ideal for imaging with minimal patient exposure.

• Iodine-131 ($\text{}^{131}\text{I}$) and Iodine-125 ($\text{}^{125}\text{I}$):

  • The thyroid gland requires iodine for proper function. Malfunctions such as hypothyroidism (underactive) or hyperthyroidism (overactive) can be diagnosed using iodine isotopes.

  • Diagnosis: A solution containing a known amount of $^{131}\text{NaI}$ can be administered. By measuring the radioactivity absorbed by the thyroid, physicians can determine if iodine uptake is occurring at a normal rate.

  • Imaging: $^{125}\text{NaI}$ can be used to image the thyroid gland, providing detailed anatomical information.

• Sodium-24 ($\text{}^{24}\text{Na}$):

  • Used to detect blockages in the circulatory system.

  • Administered as a salt solution ($\text{}^{24}\text{NaCl}$) into the bloodstream.

  • Allows medical professionals to trace blood flow through arteries, veins, and capillaries, identifying potential obstructions that could lead to serious health issues.

• Helium-3 ($\text{}^{3}\text{He}$):

  • Used in specialized MRI applications, particularly for lung imaging.

  • Patients inhale a standard liter of $^{3}\text{He}$ before the MRI scan, allowing for detailed visualization of lung structure and function that is not possible with conventional MRI techniques.

Radiation Therapy

While high-energy radiation can induce cancer, it can also be strategically used to destroy cancer cells in a controlled manner.

• Principle: Radiation therapy aims for a compromise, delivering sufficient radiation to damage and kill rapidly dividing cancer cells while minimizing harm to surrounding healthy tissues.

• Radiation Damage Categories:

  • Somatic Damage: Affects the irradiated organism during its lifetime (e.g., sunburn, radiation sickness, increased cancer risk).

  • Genetic Damage: Affects germ cells and can be inherited by offspring (e.g., chromosomal damage or alterations by radiation leading to deformed offspring).

Average Atomic Mass

Most elements exist as a mixture of two or more isotopes. The average atomic mass of an element is the weighted average mass of all its naturally occurring isotopes, relative to one-twelfth the mass of a carbon-12 atom. This accounts for both the atomic mass and the natural abundance of each isotope.

Calculation Method

The average atomic mass is calculated by multiplying the atomic mass of each isotope by its natural abundance (expressed as a decimal) and summing these products.

$\text{Average atomic mass} = \sum (\text{isotopic mass} \times \text{fractional abundance})$

Example 1: Chlorine (Cl)

Chlorine has two main isotopes:

$\text{Average atomic mass} = (0.7577 \times 34.97) + (0.2423 \times 36.97) = 26.49 + 8.95 = 35.44 \text{ amu}$

Example 2: Magnesium (Mg)

Magnesium has three main isotopes:

$\text{Average atomic mass} = (0.7870 \times 23.99) + (0.1013 \times 24.99) + (0.1117 \times 25.99)$
$\text{Average atomic mass} = 18.88 + 2.53 + 2.90 = 24.31 \text{ amu}$

Fascinating Facts about Atoms

Did you know that an average person contains approximately $8,000,000,000,000,000,000,000,000,000$ ($8 \times 10^{27}$) atoms? This immense number underscores the microscopic scale at which chemistry operates within and around us. For more intriguing details about the human body, its chemical constituents, and cellular composition, resources such as BBC Earth's "The making of me and you" are highly recommended.