Atoms and Radioactivity – GOB Chemistry Chapter 2 Study Notes

Atoms and Their Components

Subatomic Particles

Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge, electrons carry a negative charge, and neutrons are uncharged. The number of protons equals the number of electrons in a neutral atom, resulting in no overall charge.

• Proton (p+): Positive charge, located in the nucleus.

• Neutron (n0): No charge, located in the nucleus.

• Electron (e-): Negative charge, found in the electron cloud surrounding the nucleus.

Structure of an Atom

The nucleus, containing protons and neutrons, is at the center of the atom. Electrons move rapidly in the surrounding electron cloud, occupying most of the atom's volume. The mass of protons and neutrons is approximately 1 atomic mass unit (amu) each, while electrons have negligible mass.

Atomic Number and Mass Number

Definitions and Calculations

The atomic number (Z) is the number of protons in an atom and defines the element. The mass number (A) is the sum of protons and neutrons in the nucleus. The number of neutrons can be calculated as:

$\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

• Atomic number (Z): Number of protons (and electrons in a neutral atom).

• Mass number (A): Number of protons + number of neutrons.

Atoms are often represented in symbolic notation:

$^{A}_{Z}\text{X}$

Example: Nitrogen

Nitrogen has atomic number 7, so it has 7 protons and 7 electrons. If its mass number is 15, it has 8 neutrons (15 - 7 = 8).

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers. Isotopes are represented by their mass number, e.g., carbon-12, carbon-13, and carbon-14.

Atomic Mass

The atomic mass listed on the periodic table is the weighted average of all naturally occurring isotopes of an element. The most abundant isotope is usually the one whose mass number is closest to the atomic mass.

Example: Magnesium

Magnesium has three naturally occurring isotopes: magnesium-24, magnesium-25, and magnesium-26. The most abundant isotope is the one closest to the atomic mass (24.305), which is magnesium-24.

Radioactivity and Radioisotopes

Radioactivity

Radioactivity is the spontaneous emission of energy from the nucleus of an unstable atom. Isotopes that emit radiation are called radioisotopes. Most naturally occurring isotopes are stable, but radioisotopes undergo radioactive decay to become more stable.

Forms of Ionizing Radiation

There are several types of ionizing radiation:

• Alpha (α) particles: Positively charged, low penetration.

• Beta (β) particles: Negatively charged, moderate penetration.

• Gamma (γ) rays: Neutral, highly penetrating electromagnetic radiation.

• Positrons: Positively charged, similar to electrons.

• Neutrons: Neutral particles.

Biological Effects of Radiation

Ionizing radiation can remove electrons from atoms, making them more reactive and potentially damaging biological tissues. The biological effect depends on the type and amount of radiation exposure.

Nuclear Equations and Radioactive Decay

Writing Nuclear Equations

Nuclear equations represent the transformation of one nucleus into another during radioactive decay. The sum of atomic numbers and mass numbers must be equal on both sides of the equation.

• Alpha decay: Emission of an alpha particle ($^4_2\text{He}$).

• Beta decay: Emission of a beta particle ($^0_{-1}\text{e}$).

• Gamma decay: Emission of a gamma ray ($^0_0\gamma$), no change in atomic or mass number.

• Positron emission: Emission of a positron ($^0_{+1}\text{e}$).

Example: Beta Decay of Carbon-14

In beta decay, a neutron is converted to a proton, and a beta particle is emitted:

$^{14}_6\text{C} \rightarrow ^{14}_7\text{N} + ^0_{-1}\text{e}$

Production of Radioisotopes

Radioisotopes can be produced by bombarding stable isotopes with particles such as neutrons. For example, cobalt-60 is produced by bombarding cobalt-59 with a neutron:

$^{59}_{27}\text{Co} + ^1_0\text{n} \rightarrow ^{60}_{27}\text{Co}$

Radiation Units and Half-Lives

Units of Radioactivity

The activity of a radioactive sample is measured in units such as the becquerel (Bq), curie (Ci), millicurie (mCi), and microcurie (μCi). The biological effect is measured in sieverts (Sv) or rems (rem).

Half-Life

The half-life of a radioisotope is the time required for half of the radioactive atoms in a sample to decay. The amount remaining after n half-lives is given by:

$\text{Remaining amount} = \text{Initial amount} \times \left(\frac{1}{2}\right)^n$

• After 1 half-life: 50% remains

• After 2 half-lives: 25% remains

• After 3 half-lives: 12.5% remains

• After 4 half-lives: 6.25% remains

• After 5 half-lives: 3.125% remains

Medical Applications for Radioisotopes

Diagnostic Uses

Radioisotopes are used in medicine for both diagnosis and treatment. For diagnosis, small amounts of radioisotopes (tracers) with short half-lives are administered to visualize organs or detect abnormalities. For example, technetium-99m is used for imaging blood flow, and fluorine-18 is used in PET scans to study brain function.

Therapeutic Uses

Radioisotopes can also be used to treat diseases such as cancer. Gamma radiation from cobalt-60 is used in external beam therapy, while radioactive seeds are implanted in tumors for brachytherapy. Iodine-131 is used to treat thyroid cancer by selectively destroying cancerous cells.

Summary Table: Types of Ionizing Radiation

Emission	Symbol	Charge
Alpha	α or $^4_2\text{He}$	2+
Beta	β or $^0_{-1}\text{e}$	1−
Gamma	$^0_0\gamma$	0
Positron	$^0_{+1}\text{e}$	1+
Neutron	$^1_0\text{n}$	0

Summary Table: Radiation Exposure and Clinical Effects

Exposure in mrem	Clinical Effect
5,000–20,000	Cannot be detected.
20,000–50,000	Temporary decrease in white blood cells (WBCs).
50,000–100,000	Mild radiation sickness. Large decrease in WBC, nausea, vomiting.
100,000–200,000	Light radiation poisoning. Delayed appearance of appetite loss; nausea; diarrhea; hair loss; spontaneous abortion in pregnant women; recovery in 3 months.
200,000–300,000	Moderate radiation poisoning. Hair loss; fatigue; general illness; permanent sterility possible; recovery takes several months.
300,000–400,000	LD50: Lethal dose for half the population after 30 days.

Summary Table: Common Units of Radioactivity

Common Unit	Relationship to Other Units
Becquerel (Bq)	1 Bq = 1 disintegration per second
Curie (Ci)	1 Ci = $3.7 \times 10^{10}$ disintegrations per second
Millicurie (mCi)	1 Ci = 1000 mCi
Microcurie (μCi)	1 Ci = 1,000,000 μCi