Please provide definitions for Radioactivity, Radioactive elements, Half-life, Nuclear energy, and Gravitational field.

Here are the definitions from your notes:

• Radioactivity: Radioactivity is the spontaneous decay or disintegration of the nucleus of an atom of an element.

• Radioactive elements: Radioactive elements are those elements that spontaneously emit radiation from their nucleus, e.g., radium, thorium, uranium, potassium.

• Half-life: Half-life of a radioactive element is the time taken for half of the atoms initially present in the element to decay.

• Nuclear energy: Nuclear energy is defined as an energy used for generating electricity.

• Gravitational field: A gravitational field is a region in space around a mass within which the gravitational force of the mass can be experienced.