Atoms and Radioactivity: Structure, Isotopes, and Nuclear Chemistry

Atoms and Their Components

Subatomic Particles

Atoms are the fundamental building blocks of matter, composed of three main subatomic particles: protons, neutrons, and electrons. Understanding their properties and arrangement is essential for studying chemical behavior.

• Protons: Positively charged particles located in the nucleus. Each proton has a charge of +1.

• Neutrons: Neutral particles (no charge) also found in the nucleus.

• Electrons: Negatively charged particles (-1) that move in the space surrounding the nucleus, known as the electron cloud.

• Atoms are electrically neutral overall because the number of protons equals the number of electrons.

Structure of the Atom

The atom consists of a dense nucleus containing protons and neutrons, surrounded by a much larger region where electrons are found.

• The nucleus is the central core, containing nearly all the atom's mass.

• The electron cloud is the region where electrons are likely to be found, occupying most of the atom's volume.

• Most of the atom is empty space.

Relative Mass and Atomic Mass Unit (amu)

Because subatomic particles are extremely small, their masses are measured in atomic mass units (amu).

• 1 amu is defined as one-twelfth the mass of a carbon-12 atom (which has 6 protons and 6 neutrons).

• Proton mass ≈ 1 amu; Neutron mass ≈ 1 amu; Electron mass ≈ 0.0005 amu (about 1/2000 the mass of a proton or neutron).

• For most calculations, the mass of electrons is considered negligible.

Subatomic Particle	Symbol	Electrical Charge	Relative Mass (amu)	Location in Atom
Proton	p or p+	+1	1	Nucleus
Neutron	n or n0	0	1	Nucleus
Electron	e-	-1	0.0005	Outside nucleus (electron cloud)

Atomic Number and Mass Number

Definitions and Calculations

Each element is defined by its atomic number and mass number, which relate to the number of subatomic particles present.

• Atomic Number (Z): The number of protons in the nucleus of an atom. This determines the element's identity.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• For a neutral atom: Number of electrons = Number of protons.

• Symbolic notation: AZElement (e.g., 126C for carbon-12).

Formula:

$\text{Mass number (A)} = \text{Number of protons} + \text{Number of neutrons}$

• Given atomic number and mass number, you can determine the number of protons, neutrons, and electrons.

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, and thus different mass numbers.

• Isotopes are represented by their mass number (e.g., carbon-12, carbon-13, carbon-14).

• All isotopes of an element have the same chemical properties but may have different physical properties (such as stability).

Atomic Mass

The atomic mass of an element is the weighted average of the masses of all its naturally occurring isotopes.

• The atomic mass (as seen on the periodic table) reflects both the mass and the relative abundance of each isotope.

Formula for average atomic mass:

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

Isotope	Isotopic Mass (amu)	Abundance (%)
Silicon-28	27.97693	92.223
Silicon-29	28.97649	4.685
Silicon-30	29.97377	3.092

Example: Chlorine has two stable isotopes: Cl-35 (75.779%) and Cl-37 (24.221%). The average atomic mass is calculated using their masses and abundances.

Radioactivity and Radioisotopes

Radioactivity

Radioactivity is the spontaneous emission of energy from the nucleus of an unstable atom. Atoms that emit radiation are called radioisotopes.

• Most naturally occurring isotopes are stable; unstable isotopes undergo radioactive decay to become more stable.

• Radioactive decay can emit different types of radiation: alpha particles, beta particles, gamma rays, positrons, and neutrons.

Types of Radiation

Type	Symbol	Charge	Description
Alpha particle	$\alpha$ or $^4_2\text{He}$	2+	Helium nucleus (2 protons, 2 neutrons)
Beta particle	$\beta$ or $^0_{-1}\text{e}$	-1	High-energy electron
Gamma ray	$\gamma$	0	High-energy electromagnetic radiation
Positron	$\beta^+$ or $^0_{+1}\text{e}$	+1	Positive electron
Neutron	$n$ or $^1_0\text{n}$	0	Neutral particle

• Alpha particles are the least penetrating; gamma rays are the most penetrating.

Penetration and Shielding

Radiation Type	Penetration	Shielding Material
Alpha ($\alpha$)	Few centimeters; stopped by paper or skin	Paper, clothing
Beta ($\beta$)	Few meters; penetrates skin	Plastic, aluminum foil, gloves
Gamma ($\gamma$), X-rays	Fully penetrates body; several meters	Lead, concrete
Neutron	Fully penetrates body; thousands of meters	Concrete, water

Biological Effects of Radiation

• Ionizing radiation can remove electrons from atoms, making them more reactive and potentially damaging biological tissues.

• High doses can cause radiation sickness, cancer, or death; rapidly dividing cells are most susceptible.

• Exposure is measured in sieverts (Sv) or millisieverts (mSv).

Nuclear Equations and Radioactive Decay

Writing Nuclear Equations

Nuclear equations represent the changes that occur during radioactive decay. The sum of mass numbers and atomic numbers must be equal on both sides of the equation.

• Alpha decay: $^A_Z\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + ^4_2\text{He}$

• Beta decay: $^A_Z\text{X} \rightarrow ^A_{Z+1}\text{Y} + ^0_{-1}\text{e}$

• Gamma emission: $^A_Z\text{X}^* \rightarrow ^A_Z\text{X} + \gamma$

• Positron emission: $^A_Z\text{X} \rightarrow ^A_{Z-1}\text{Y} + ^0_{+1}\text{e}$

• Neutron emission: $^A_Z\text{X} \rightarrow ^{A-1}_Z\text{Y} + ^1_0\text{n}$

Example: $^{241}_{95}\text{Am} \rightarrow ^{237}_{93}\text{Np} + ^4_2\text{He}$ (Alpha decay of americium-241)

Producing Radioisotopes

• Radioisotopes can be produced by bombarding stable isotopes with particles (e.g., neutrons, protons, alpha particles).

• In these reactions, the incoming particle appears on the reactant side of the equation.

Example: $^{98}_{42}\text{Mo} + ^1_0\text{n} \rightarrow ^{99}_{42}\text{Mo}$

Radiation Units and Half-Lives

Units of Radioactivity

• Becquerel (Bq): 1 disintegration per second (SI unit).

• Curie (Ci): $1\ \text{Ci} = 3.7 \times 10^{10}$ disintegrations per second.

• Other units: millicurie (mCi), microcurie (μCi).

Unit	Relationship
1 Bq	1 disintegration/second
1 Ci	$3.7 \times 10^{10}$ disintegrations/second
1 mCi	1/1000 Ci
1 μCi	1/1,000,000 Ci

Half-Life

The half-life ($t_{1/2}$) of a radioactive isotope is the time required for half of the atoms in a sample to decay.

• Each isotope has a characteristic half-life, ranging from fractions of a second to billions of years.

• Medical isotopes are chosen for short half-lives to minimize patient exposure.

Formula for remaining isotope:

$\text{Amount remaining} = (\frac{1}{2})^n \times \text{starting amount}$

where $n$ = number of half-lives elapsed.

Radioisotope	Symbol	Physical Half-Life
Hydrogen-3	$^3\text{H}$	12.3 years
Carbon-14	$^{14}\text{C}$	5730 years
Radium-226	$^{226}\text{Ra}$	1600 years
Uranium-238	$^{238}\text{U}$	4.5 billion years

Medical Applications for Radioisotopes

Diagnostic Uses

Radioisotopes are widely used in medicine for both diagnosis and treatment. Diagnostic procedures often use small amounts of radioisotopes (tracers) with short half-lives to minimize radiation exposure.

• Tracers are radioisotopes that concentrate in specific organs or tissues, allowing imaging of physiological processes.

• Technetium-99m is commonly used to assess blood flow in the lungs.

• Areas with abnormal tracer distribution appear as "cold" (no uptake) or "hot" (increased uptake) spots in scans.

Therapeutic Uses

• Radioisotopes can be used to destroy diseased or cancerous tissues (e.g., iodine-131 for thyroid cancer).

• External beam radiation therapy uses gamma rays (e.g., from cobalt-60) to target tumors.

• Brachytherapy involves implanting radioactive seeds directly into tumors.

Positron Emission Tomography (PET)

• PET scans use positron-emitting isotopes (e.g., fluorine-18) to image metabolic activity in tissues, especially the brain.

• When a positron meets an electron, gamma radiation is produced and detected by the scanner.

Summary Table: Key Concepts

Concept	Definition/Key Point
Atom	Smallest unit of an element, composed of protons, neutrons, and electrons
Isotope	Atoms of the same element with different numbers of neutrons
Radioactivity	Spontaneous emission of energy from an unstable nucleus
Half-life	Time for half the atoms in a sample to decay
Tracer	Radioisotope used in small amounts for diagnostic imaging