Bond energy When a bond is broken, energy is absorbed. When a bond is formed, energy is released. Example Cl Kcal Cl + Cl (endothermic, ΔH is positive) Cl + Cl Cl Kcal (exothermic, ΔH is negative) Notice that in the potential diagram for the above reactions the molecule Cl 2 is more stable than the 2 atoms of Cl. This is why chlorine can be found in nature as a diatomic molecule. Example H 2 (g), N 2 (g), O 2 (g), F 2 (g), Cl 2 (g), Br 2 (l), and I 2 (s)

Electronegativity Electronegativity indicates how strongly an atom of an element attracts electrons in a chemical bond. Electronegativity values are assigned according to arbitrary scales. Example In the molecule HCl the atoms are bonded with 2 electrons. Cl attracts electrons in the chemical bond more strongly than H because the electronegativity of Cl is 3.2 while H is 2.1. Therefore the electrons are closer to Cl than H.

Octet rule Noble gases have stable valence configurations and tend not to bond. A stable valence configuration consist of 8 valence electrons. This is called the octet rule. There are exceptions such as helium that has only 2 valence electrons. Noble gases, elements in group 8A, are inert or unreactive gases and are found in nature as monatomic molecules (1 atom per molecule). Example He(g) and Kr(g)

Electron-dot diagrams (Lewis structures) Electron-dot diagrams (Lewis structures) can represent the valence electron arrangement in elements, compounds, and ions. Lewis structure of some elements Example Ne, He, Si, and O

Lewis structure of some ions Example I -, K +, Mg 2+, and Br 5+

Lewis structure of some compounds Example Cl 2 (a molecular or covalent compound) and NaCl (an ionic compound)

Lewis structure of some compounds Atoms attain a stable valence electron configuration by bonding with other atoms. Example Cl 2 and NaCl

Atomic and ionic radius When an atom gains one or more electrons, it becomes a negative ion and its radius increases. Example

Atomic and ionic radius When an atom loses one or more electrons, it becomes a positive ion and its radius decreases. Example