Magnetism and magnetic fields Study Guide

Within every atom, electrons are not randomly scattered. Instead, they occupy specific energy levels or shells at varying distances from the central nucleus. These shells can be thought of as orbits, with electrons in shells closer to the nucleus having lower energy, and those further away having higher energy. The arrangement of electrons in these shells is known as the electron configuration of an atom. This configuration is unique for each element and largely determines its chemical properties.

Atoms can undergo changes in their electron arrangements by interacting with electromagnetic radiation. When an atom absorbs electromagnetic radiation, such as light or X-rays, an electron can gain energy and move from a lower energy shell to a higher energy shell. This atom is then said to be in an 'excited state'. Conversely, when an electron in an excited state falls back to a lower energy shell, it emits electromagnetic radiation, typically as light, with a specific frequency and wavelength corresponding to the energy difference between the shells. This process is fundamental to how elements produce unique line spectra.

Atoms can also lose or gain electrons, leading to the formation of ions. An ion is an atom or molecule that has a net electrical charge due to the loss or gain of one or more electrons. If an atom loses one or more of its outermost electrons, it becomes a positively charged ion (a cation) because it then has more protons than electrons. This often occurs when atoms, particularly metals, try to achieve a stable electron configuration, like that of a noble gas. The number of electrons lost determines the magnitude of the positive charge. The loss of outer electrons requires energy input to overcome the electrostatic attraction between the electrons and the positively charged nucleus. This process is called ionisation.

Different substances interact with electromagnetic (EM) waves in unique ways, depending on the material's properties and the specific wavelength of the EM wave. This interaction can involve absorption, transmission, refraction, or reflection. Absorption occurs when a material takes in the energy of the EM wave, converting it into another form, often thermal energy. For example, dark-coloured objects absorb more visible light, leading to a temperature increase. Transmission is when EM waves pass through a material. Transparent materials like glass transmit visible light, while opaque materials do not. The degree of transmission can depend heavily on wavelength; for instance, materials that transmit visible light might absorb or reflect ultraviolet or infrared radiation.

Refraction is the bending of EM waves as they pass from one medium to another, caused by a change in wave speed. The extent of refraction is dependent on the refractive index of the materials involved and the wavelength of the EM wave. This is why a prism can split white light into its constituent colours (dispersion), as different wavelengths (colours) are refracted by slightly different amounts. Reflection is the bouncing back of EM waves from a surface. The angle of incidence equals the angle of reflection for smooth surfaces (specular reflection). Different materials reflect different wavelengths to varying degrees; mirrors, for instance, are designed to reflect most of the visible light spectrum. The specific atomic and molecular structure of a substance dictates how it will interact with different parts of the electromagnetic spectrum, making these properties crucial in fields from medical imaging (X-ray absorption) to telecommunications (radio wave transmission).