Magnetism and magnetic fields Revision Notes

Atomic Structure and Electron Transitions

An atom consists of a nucleus with protons and neutrons, surrounded by electrons orbiting in specific energy levels. Electrons can move between these energy levels. When an electron moves from a higher energy level to a lower one, it emits electromagnetic radiation in the form of a photon. The energy of this photon directly corresponds to the difference in energy between the two levels. The frequency of the emitted radiation is proportional to its energy. Conversely, an electron can absorb a photon of specific energy to move to a higher energy level. These transitions are responsible for much of the visible and ultraviolet radiation emitted or absorbed by matter.

Nuclear Changes and Radioactive Decay

The nucleus of an atom can also undergo changes, particularly in unstable isotopes, a process known as radioactive decay. This decay involves the spontaneous transformation of an unstable nucleus into a more stable one, accompanied by the emission of radiation. Common types of radioactive decay include alpha decay (emission of a helium nucleus), beta decay (emission of an electron or positron), and gamma decay (emission of high-energy electromagnetic radiation, gamma rays). These nuclear changes release significant amounts of energy and typically generate radiation of very high frequencies, such as gamma rays and X-rays.

Frequency Range of Radiation

The radiation generated by atomic and nuclear changes covers a vast range of frequencies within the electromagnetic spectrum. Electron transitions within an atom typically produce radiation in the lower-energy parts of the spectrum, such as radio waves, microwaves, infrared, visible light, and ultraviolet radiation. In contrast, changes within the nucleus, such as radioactive decay, release much higher energies, resulting in higher-frequency radiation like X-rays and gamma rays. This broad spectrum of radiation is a key indicator of the fundamental processes occurring within atoms and their nuclei.

Defining Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the number of protons defines the element, isotopes of an element will always have the same chemical symbol and atomic number (Z). The difference lies in their mass number (A), which is the total count of protons and neutrons in the nucleus. For example, hydrogen has three main isotopes: protium (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons).

Nuclear (Nuclide) Notation

The conventional representation for nuclei is written as ${}_{Z}^{A}\text{X}$. In this notation:

• X represents the chemical symbol of the element (e.g., C for carbon, O for oxygen).
• A is the mass number, which is the total number of protons and neutrons in the nucleus. It is always a whole number.
• Z is the atomic number, which represents the number of protons in the nucleus. This number uniquely identifies the element.

From this notation, the number of neutrons can be calculated by subtracting the atomic number from the mass number: Number of neutrons = A - Z.

Relating Notation to Isotope Differences

When comparing two isotopes of the same element using nuclear notation, you will observe that the chemical symbol (X) and the atomic number (Z) are identical, as they are both atoms of the same element. However, their mass numbers (A) will be different. This difference in mass number directly reflects the difference in the number of neutrons between the isotopes. For instance, consider oxygen-16 (${}_{8}^{16}\text{O}$) and oxygen-18 (${}_{8}^{18}\text{O}$). Both have Z=8 (8 protons), but oxygen-16 has A=16 (8 neutrons) while oxygen-18 has A=18 (10 neutrons).

Understanding Nuclear Symbols

In nuclear equations, each particle or nucleus is represented by a symbol that includes its mass number and atomic number. The mass number (A) is the total number of protons and neutrons, written as a superscript before the element symbol (e.g., xA). The atomic number (Z) is the number of protons, written as a subscript before the element symbol (e.g., xZ). For example, a carbon-14 isotope is written as ¹⁴₆C, meaning it has a mass number of 14 and an atomic number of 6 (6 protons, 8 neutrons).

Balancing Nuclear Equations

For any nuclear reaction, the total mass number and total atomic number must be conserved. This means the sum of the mass numbers on the left side of the equation (reactants) must equal the sum of the mass numbers on the right side (products). Similarly, the sum of the atomic numbers on the left must equal the sum of the atomic numbers on the right. This principle is crucial for determining unknown decay products or identifying missing particles in a decay equation.

Types of Radioactive Decay and their Equations

There are three primary types of radioactive decay: alpha, beta-minus, and gamma.

• α-decay: An alpha particle (⁴₂He or ⁴₂α) is emitted. The mass number of the parent nucleus decreases by 4, and the atomic number decreases by 2.

Example: aAZ → a-4Z-2Y + ⁴₂He

• β-{-}decay: A beta-minus particle (⁰₋₁e or ⁰₋₁β) is emitted. A neutron transforms into a proton. The mass number remains unchanged, but the atomic number increases by 1.

Example: aAZ → aAZ+1Y + ⁰₋₁e

• γ-decay: A gamma ray (⁰₀γ) is emitted. Gamma decay does not change the mass number or atomic number; it simply releases energy from an excited nucleus.

Example: aAZ* → aAZX + ⁰₀γ (where * denotes an excited state)

Radio Waves: Communication and Broadcasting

Radio waves have the longest wavelengths in the electromagnetic spectrum, ranging from about 1 millimeter to over 100 kilometers. Their long wavelengths allow them to diffract around buildings and hills, making them ideal for long-distance communication. Key applications include radio and television broadcasting, where signals are transmitted from an antenna and received by receivers. They are also used in mobile phone communications, enabling wireless voice and data transfer across vast networks. Remote controls for various devices often use specific frequencies of radio waves.

Microwaves: Heating and Satellite Communication

Microwaves have shorter wavelengths than radio waves, typically from 1 millimeter to 1 meter. Their most common domestic use is in microwave ovens, where they cause water molecules within food to vibrate rapidly, generating heat. This efficient heating mechanism makes them popular for cooking and reheating. Beyond the kitchen, microwaves are crucial for satellite communication. Satellites orbiting Earth receive microwave signals from ground stations and retransmit them back, facilitating global communication, GPS, and weather forecasting. Radar systems also utilise microwaves to detect and track objects.

Infrared and Visible Light: Heat, Vision, and Fibre Optics

Infrared (IR) radiation is often associated with heat. All warm objects emit infrared radiation, which is why it's used in thermal imaging cameras to detect temperature differences, making them useful for night vision and building insulation surveys. Remote controls for electronic devices, like televisions, use IR to send signals over short distances. Visible light is the narrow range of the EM spectrum that the human eye can detect, allowing us to see and interpret our surroundings. It forms the basis of photography, lighting, and is critical for fibre optic communication, where light signals carry data through thin glass fibres at high speeds over long distances.

Ultraviolet, X-rays, and Gamma Rays: Specialised Applications

Ultraviolet (UV) radiation has wavelengths shorter than visible light. While excessive exposure can be harmful, it has beneficial uses such as sterilising medical equipment and water by killing microbes. Sunbeds use UV-A light to stimulate tanning. UV lamps are also used in security features to reveal hidden markings on banknotes and passports. X-rays are highly energetic waves used extensively in medicine for imaging bones and identifying fractures due to their ability to penetrate soft tissues but be absorbed by denser materials like bone. Airport security scanners also use X-rays. Gamma rays are the most energetic, produced during radioactive decay. Their high penetrating power and ability to destroy living cells make them vital for radiotherapy in cancer treatment, sterilising medical instruments, and preserving food by killing bacteria and viruses.