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Covalent Bonds What holds covalently bonded atoms together? What are the properties of molecular compounds? How does unequal sharing of electrons occur, & how does it affect molecules? Covalent Bond • The chemical bond formed when two atoms share electrons is called a covalent. • Except for the noble gases, nonmetals can bond to other non metals by sharing electrons in covalent bonds. • The force that holds atoms together in a covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons. Molecule • A neutral group of atoms joined by covalent bonds is called a molecule. • The number of covalent bonds a nonmetal atom can form equals the number of valence electrons needed to make a total of eight. • Some atoms share two pairs of electrons, forming a double bond. • Some atoms even form triple bonds in which their atoms share three pairs of electrons. Molecular Compound • A molecular compound is a compound that is composed of molecules. • Molecular compounds have very different properties than ionic compounds. • Compared to ionic compounds, molecular compounds have lower melting points & boiling Sodium Iodide points, and they do not conduct electricity. Polar Bonds • Atoms of some elements pull more strongly on shared electrons than do atoms of other elements. • As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges. • These charges are not as strong as the charges on ions. • A covalent bond in which electrons are shared unequally is a polar bond. Non Polar Bond • When the valence electrons are shared equally, the bond is non polar. Nonpolar Bonds • A molecule is nonpolar if it contains only nonpolar bonds, or if it has polar bonds that cancel each other out. • For ex. Carbon dioxide (CO2), the oxygen atoms attract electrons more strongly than the carbon atom does. • However, the two oxygen atoms pull with equal strength in opposite directions, so the polar bonds cancel each other out. Polar Molecules • Polar molecules are more strongly attracted to each other than are nonpolar molecules. • This difference in the attraction between molecules leads to different properties in polar & nonpolar compounds. • Polar & nonpolar molecules also do not mix. • For ex. Water, which is polar, does not mix with oil, which is nonpolar. • The nonpolar oil molecules have little attraction for the polar water molecules, while the water molecules are more strongly attrected to one another.
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