The Periodic Table of the Elements categorizes elements primarily by the number of protons in their nucleus (atomic number) and secondarily by the characteristics they exhibit.

The electromagnetic spectrum covers all possible wavelengths and frequencies of radiation. From lowest frequency (longest wavelength) to highest frequency (shortest wavelength) radiation: radio waves → microwaves → infrared waves → visible light → ultraviolet light → X-rays → gamma rays.

Momentum is a measure of how difficult it is for a moving object to stop and is calculated by multiplying the object's mass by its velocity: \(\vec{p} = m\vec{v}\). Like velocity, momentum is a vector quantity as it expresses force applied in a specific direction.

The metric system is a number system that designates one base unit for each type of measurement. For example, the base unit for length is the meter and the base unit for mass is the gram.

The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are called terrestrial (Earth-like) planets because, like the Earth, they're solid with inner metal cores covered by rocky surfaces.