Atom

An atom is the basic unit of matter. It is composed of a nucleus containing protons and neutrons, with electrons orbiting around the nucleus.

Structure of an Atom

An atom consists of three main subatomic particles:

• Protons: Positively charged particles found in the nucleus.
• Neutrons: Neutral particles found in the nucleus.
• Electrons: Negatively charged particles that orbit the nucleus in energy levels or shells.

Atomic Number and Mass Number

The atomic number of an atom is equal to the number of protons in its nucleus. The mass number is the sum of the number of protons and neutrons in the nucleus.

Electron Shells

Electrons are arranged in energy levels or shells around the nucleus. The first shell can hold up to 2 electrons, while the second and third shells can hold up to 8 electrons each.

Valence Electrons

The outermost shell of an atom is called the valence shell, and the electrons in this shell are called valence electrons. The number of valence electrons determines an atom's reactivity and its ability to form chemical bonds.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This results in variations in their mass numbers.

[Atom] Related Worksheets and Study Guides:

• Properties of atomsScience • Eighth Grade
• Atomic Structure - Set IIChemistry • High School