B ond energy

1. Bond energy When a bond is broken, energy is absorbed. When a bond is formed, energy is released.  Example Cl2 + 58 Kcal Cl + Cl    (endothermic, ΔH is positive) Cl + Cl Cl2 + 58 Kcal    (exothermic, ΔH is negative) Notice that in the potential diagram for the above reactions the molecule Cl2 is more stable than the 2 atoms of Cl. This is why chlorine can be found in nature as a diatomic molecule. Example H2(g), N2(g), O2(g), F2(g), Cl2(g), Br2(l), and I2(s)

2. 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.

3. 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)

4. 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

5. Lewis structure of some ions • Example • I-, K+,  Mg2+, and Br5+

6. Lewis structure of some compounds • Example • Cl2 (a molecular or covalent compound) and NaCl (an ionic compound)

7. Lewis structure of some compounds • Atoms attain a stable valence electron configuration by bonding with other atoms.  • Example • Cl2 and NaCl

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

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