Atoms and Radioactivity

What is an Atom?

An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons.

• Protons: Positively charged particles located in the nucleus. Relative mass ≈ 1 amu.

• Neutrons: Neutral particles also found in the nucleus. Relative mass ≈ 1 amu.

• Electrons: Negatively charged particles found in orbitals surrounding the nucleus. Relative mass ≈ 1/1836 amu.

Example: The nucleus of an atom is extremely small compared to the overall size of the atom (like a BB in a 500-yard stadium).

Atomic Number and Mass Number

The atomic number (Z) is the number of protons in the nucleus of an atom and defines the element. All atoms of a given element have the same atomic number. In a neutral atom, the number of electrons equals the atomic number.

• Mass number (A) is the sum of protons and neutrons in the nucleus.

• To find the number of neutrons:

Example: Carbon has atomic number 6. If its mass number is 12, it has 6 neutrons.

Isotopes

Isotopes are atoms of the same element (same atomic number) with different numbers of neutrons (different mass numbers).

• Isotopes have identical chemical properties but different physical properties.

• Example: Carbon has three naturally occurring isotopes: C-12, C-13, and C-14.

• C-12 and C-13 are stable; C-14 is radioactive and used for carbon dating.

Ions are created from losing or gaining eletrons the aim is to become like a noble gas in do so.

A metal tends to lost eletrons to become a (+) charged ion called a cation. a positive sign =losing eletrons

A non metal tends to gain eletrons to become a negativity(Ca^-3) would gain 3 eletrons charged ion called an anion

Isotope notation: (mass number on top, atomic number below).

Atomic Mass (Atomic Weight)

The atomic mass of an element is the weighted average of the masses of all naturally occurring isotopes, not the mass of a single atom.

• Atomic mass is listed on the periodic table.

Radioactivity and Radioisotopes

An atom is radioactive if its nucleus is unstable and emits radiation. Such atoms are called radioisotopes.

• Nuclear radiation is the energy and particles emitted from the nucleus during radioactive decay.

• The process of emission is called radioactive decay.

Forms of Nuclear Radiation

There are several types of nuclear radiation, each with distinct properties:

• Alpha (α) particles: Consist of 2 protons and 2 neutrons (same as a helium nucleus). Charge: +2. Mass number: 4. Least penetrating; blocked by skin.

• 

• Beta (β) particles: High-energy electrons. Charge: -1. Mass number: 0. Can penetrate skin but not deeply.

• 

• Gamma (γ) rays: High-energy electromagnetic radiation. Charge: 0. Mass number: 0. Most penetrating; can pass through the body.

• Positrons: Positive electrons. Charge: +1. Mass number: 0.

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Additional info: X-rays are not considered nuclear radiation because they originate from electron transitions, not the nucleus. Nuclear radiation is called ionizing radiation because it can remove electrons from atoms, creating ions.

Measuring Radiation and Biological Effects

Radiation exposure is measured in units of biological damage:

• Sievert (Sv) and millirem (mrem) are units of biological effect.

• Average annual exposure: about 300 mrem.

• Clinical effects occur above 20,000 mrem.

Natural background radiation comes from cosmic rays, rocks, and other sources.

Radioactive Decay Equations

Radioactive decay changes the atomic number and/or mass number of an atom:

• Alpha emission: Atomic number decreases by 2, mass number decreases by 4.

• Beta emission: Atomic number increases by 1, mass number unchanged.

• Positron emission: Atomic number decreases by 1, mass number unchanged.

• Gamma emission: No change in atomic number or mass number.

Example nuclear equation:

Half-Life of Radioisotopes

The half-life is the time required for half of a radioactive sample to decay.

• Physical half-life: Decay in a laboratory setting.

• Biological half-life: Time to eliminate half from the body.

• Effective half-life: Combines both; most useful in medicine.

Radioisotopes used in medicine typically have short half-lives (minutes to days).

Measuring Radioactivity

Activity is measured in:

• Becquerel (Bq): 1 disintegration per second.

• Curie (Ci): disintegrations per second.

• Medical uses often require millicuries (mCi) or microcuries (μCi).

Devices such as Geiger counters are used to measure radioactivity.

Medical Applications of Radioisotopes

Radioisotopes are used as tracers for imaging and in cancer treatment.

• Imaging: Small doses, gamma emitters preferred for detection outside the body.

• Tumor treatment: External beam radiation, brachytherapy (radioactive seeds), proton therapy, neutron capture therapy.

PET Scans (Positron Emission Tomography)

PET scans use radioisotopes that emit positrons. When a positron meets an electron, they annihilate, producing gamma rays detected by imaging devices.

• F-18 is commonly used, can be incorporated into molecules like glucose for brain and cancer scans.

Additional info: PET stands for Positron Emission Tomography.