An atom is the smallest unit of matter with unique chemical properties. Atoms are the chemical units of cell structure. They consist of a central nucleus with protons and neutrons and orbit(s) of electrons. A proton carries a +1 positive charge, while a neutron has no charge. Thus the nucleus has a net positive charge. Electrons carry a –1 negative charge and are consequently attracted to the positive nucleus. In general, the number of protons usually equals the number of electrons. Recall that atoms have unique (individual) chemical properties, and thus each type of atom is called a chemical element, or just element.
Atomic number refers to the number of protons in an atom, while atomic weight refers to the number of protons and neutrons in an atom, measured in daltons. It is possible for elements to exist in multiple forms, called isotopes; the only difference is the number of neutrons in the nucleus, while protons and electrons always stay the same as the original element.
The human body depends upon four major elements for form and function: Hydrogen (H), Oxygen (O), Carbon (C), and Nitrogen (N).
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