Atoms are the basic building blocks of matter and the smallest unit of an element that retains the properties of that element. They are composed of three subatomic particles: protons, neutrons, and electrons.

1. Protons: Protons are positively charged particles found in the nucleus of an atom. They have a relative mass of 1 and a charge of +1.

2. Neutrons: Neutrons are neutral particles found in the nucleus of an atom. They have a relative mass of 1, similar to protons, but they have no electrical charge.

3. Electrons: Electrons are negatively charged particles that orbit the nucleus of an atom in specific energy levels or shells. They have a much smaller mass compared to protons and neutrons, and their charge is -1.

The number of protons in the nucleus determines the element’s identity and is called the atomic number. For example, hydrogen atoms have one proton, carbon atoms have six protons, and oxygen atoms have eight protons.

The total number of protons and neutrons in the nucleus determines the atom’s mass number. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different mass numbers.

Atoms are typically electrically neutral because they have an equal number of protons and electrons. However, ions are formed when atoms gain or lose electrons, resulting in a net positive or negative charge.

Atoms can combine to form molecules through chemical bonds, such as covalent bonds and ionic bonds, which involve the sharing or transfer of electrons between atoms, respectively.

In summary, atoms are the fundamental units of matter, consisting of protons, neutrons, and electrons. They combine to form elements, which in turn combine to create the vast diversity of substances found in the universe.