Chapter 2 - Chapter 2-Atoms Molecules and Life An...

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Chapter 2—Atoms, Molecules, and Life An understanding of chemistry is important because the interaction between atoms is one of the lowest levels of biological organization. The basic unit of all forms of matter is the atom —the smallest unit of matter that retains the physical and chemical properties of the element. o Atoms are made up of subatomic particles. The most stable subatomic particles are: Neutrons —uncharged particles found at the core, or nucleus, of the atom. Protons —positively charged particles found at the core, or nucleus, of the atom. Electrons —light, negatively charged particles that orbit around the nucleus in electron shells. A collection of atoms of the same type form an element —a substance that can’t be broken down into other substances by chemical reactions. The atomic weight of an element is the number of protons plus the number of neutrons. o The atoms of a particular element may have different numbers of neutrons. This difference results in isotopes —atoms of the same element which have different atomic weights. Electron Orbitals o Electron orbitals are the three-dimensional space around the nucleus of an atom where an electron will be found. o The electron orbitals of an atom are arranged in electron shells based on their energy level. o The chemical behavior of an atom is determined by the number of electrons in the outermost electron shell. An atom with an incomplete (partially full) electron shell is reactive whereas an atom with a full electron shell is inert , or not chemically active. If an atom has a vacancy in the outer electron shell, it will try to fill that vacancy or get rid of the “extra” electrons so the outer shell is full. A reactive atom will try to fill the valence shell by interacting with other atoms. This interaction may result in two atoms forming a chemical bond —an attraction that holds two atoms together. o A molecule is two or more atoms held together by chemical bonds. The smallest unit that retains the physical and chemical properties of a compound.
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Chapter 2 - Chapter 2-Atoms Molecules and Life An...

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