Atoms

An atom is the smallest unit of ordinary matter that forms a chemical element. Atoms are made up of three subatomic particles: protons, neutrons, and electrons.

Structure of an Atom

Atoms consist of a nucleus, which contains protons and neutrons, and electrons that orbit the nucleus in energy levels or shells.

Protons

Protons have a positive electrical charge and are located in the nucleus of the atom. The number of protons in an atom determines its atomic number, which defines the element.

Neutrons

Neutrons have no electrical charge and are also found in the nucleus of the atom. They contribute to the mass of the atom but do not affect its chemical properties.

Electrons

Electrons have a negative electrical charge and orbit the nucleus in specific energy levels or shells. They are involved in chemical bonding and determine the reactivity of the atom.

Atomic Number and Mass Number

The atomic number of an atom is equal to the number of protons it contains. The mass number is the sum of the protons and neutrons in the nucleus. It is represented by the letter 'A' and is written as a superscript to the left of the element's symbol.

Study Guide

1. What are the three subatomic particles that make up an atom?
2. Where are protons and neutrons located within an atom?
3. What is the electrical charge of an electron?
4. How do electrons contribute to the reactivity of an atom?
5. Define atomic number and mass number.

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