subject: Thinking About What Are Chemical Bonds & Why Do They Form? [print this page] This example demonstrates the gain, loss and sharing of electrons by atoms for possession of the same number of electrons as the noble gas in closest proximity on the periodic table. With eight valence electrons 2,6, all noble gases are chemically stable in a phenomenon called the octet rule But certain exceptions are possible Among them, one group of atoms has less than eight electrons like hydrogen with only one BeH2 has just four valence electrons around Be, with Beryllium contributing two electrons and each hydrogen with one The second exception applies in elements in periods 4 and above Their atoms can have over four surrounding valence pairs in certain compounds
Types of Chemical Bonds
Familiarity with three types of chemical bonds is required for the SAT II Chemistry exam ionic bonds, covalent bonds and metallic bonds
Ionic Bonds
An electrostatic attraction between ions with opposite charges cations and anions causes ionic bonds. They usually involve metals and nonmetals as elements active in ionic bonds are mostly from opposite ends of the periodic table with an electronegativity difference exceeding 1.67 Being very strong ionic bonds in compounds increase melting points and take a solid form in normal conditions Finally an electron in an ionic bond is transferred from the less electronegative atom to the more electronegative element A prime example of an ionic bond content molecule is NaCl or table salt
Covalent Bonds
Atoms may share electrons instead of transferring them from atom to atom, resulting in covalent bonds But the sharing is rarely ever equal due to the difference in electronegativity value of each atom The only exception is the bond between two atoms of the same element. Covalent bonds are said to be non polar when the difference in electronegativity of two atoms ranges from 0 to Polar refers to electronegativity difference between 0,4 and 1,6,7 For both non polar and polar covalent bonds, higher electronegativity in an element results in stronger attraction of the electron pair Carbon dioxide CO2 molecules have two bonds which are covalent bonds
