Ch_2_SG_012408 - BIO 105 Ch 2(Molecules of cells 2.1 Basic...

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BIO 105 Ch. 2 (Molecules of cells) 01-24-08 2.1 Basic Chemistry Matter- substance which occupies space Composes all living and nonliving things Can exist in solid, liquid, or gaseous form An e lement - pure substance considered a building block of matter 92 naturally occurring elements organized into the periodic table Based on number of subatomic particles Biologically significant elements: C, H, N, O, S Elements and atoms Composed of atoms Atom- smallest unit of an element Atomic structure Central nucleus Protons- positive charge Neutrons-negative charge Electrons Circle nucleus Arranged in energy “shells” Inner shell-lower energy Outer shells-higher energy Model of an atom Fig. 2.2 Elements and atoms cont’d. Electron shells The inner shell holds 2 electrons The outer shells can hold 8 electrons each Atomic number and mass Atomic number = number of protons an atom has Atomic mass = number of protons + neutrons An electrically neutral atom has # protons = # electrons Carbon atom Fig. 2.3 Periodic table first letter of name of atom is used as symbol atomic number is placed above the atomic symbol atomic mass is placed below the atomic symbol arranged horizontally by increasing atomic number arranged vertically by the number of electrons in the outermost shell Periodic table of the elements Isotopes -atoms of the same element that have the same atomic number but different numbers of neutrons
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atomic mass represents an average for atoms of each element Some isotopes are unstable when they decay they emit radioactive particles Radiation can be both harmful ( carcinogenic) and useful Uses for radioactive isotopes include Carbon dating Radioactive tracers in medical tests Medical imaging Radiation therapy Molecules and compounds Definitions Molecule- formed by the bonding of 2 or more atoms Compound- a molecule composed of atoms of 2 or more different elements Ions and ionic bonds An atom which has lost or gained electrons in a chemical reaction Atoms react with other atoms in order to attain a full outer electron shell Electrostatic interaction between a positive ion (cation) and a negative ion (anion) is called an ionic bond Formation of an ionic compound Molecules and compounds, cont’d. Covalent bonds Formed by sharing of electrons between atoms Each atom contributes one electron to the shared pair Results in a stable outer shell for both atoms More than one pair of electrons can be shared Double covalent bond- 2 pairs shared Triple covalent bond-3 pairs shared Formation of covalent compounds Molecules and compounds, cont’d.
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