Atoms, Isotopes, Ions, And Molecules: The Building Blocks

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• 1 -
• 2 - The Structure of the Atom
• 3 - Atomic Number and Mass
• 4 - Isotopes
• 5 - The Periodic Table
• 6 - Electron Shells and the Bohr Model
• 7 - Electron Orbitals
• 8 - Chemical Reactions and Molecules
• 9 - Ions and Ionic Bonds
• 10 - Covalent Bonds and Other Bonds and Interactions
• 11 - Section Summary
• 12 - Art Connections
• 13 - Review Questions
• 14 - Free Response
• View all as one page

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At its most fundamental level, life is made up of matter. Matter is any substance that occupies space and has mass. Elements are unique forms of matter with specific chemical and physical properties that cannot break down into smaller substances by ordinary chemical reactions. There are 118 elements, but only 98 occur naturally. The remaining elements are unstable and require scientists to synthesize them in laboratories.

Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols derive from their Latin names. For example, the symbol for sodium is Na, referring to natrium, the Latin word for sodium.

The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). In the nonliving world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth as a whole, as Table shows. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the earth’s crust, although it contains oxygen and a small amount of hydrogen, has little nitrogen and carbon. In spite of their differences in abundance, all elements and the chemical reactions between them obey the same chemical and physical laws regardless of whether they are a part of the living or nonliving world.

Approximate Percentage of Elements in Living Organisms (Humans) Compared to the Nonliving World
Element	Life (Humans)	Atmosphere	Earth’s Crust
Oxygen (O)	65%	21%	46%
Carbon (C)	18%	trace	trace
Hydrogen (H)	10%	trace	0.1%
Nitrogen (N)	3%	78%	trace

Previous 1 of 14

• 1 -
• 2 - The Structure of the Atom
• 3 - Atomic Number and Mass
• 4 - Isotopes
• 5 - The Periodic Table
• 6 - Electron Shells and the Bohr Model
• 7 - Electron Orbitals
• 8 - Chemical Reactions and Molecules
• 9 - Ions and Ionic Bonds
• 10 - Covalent Bonds and Other Bonds and Interactions
• 11 - Section Summary
• 12 - Art Connections
• 13 - Review Questions
• 14 - Free Response
• View all as one page

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