Chemical Bonds & Lattice Energy

Transcription

1 Chemical Bonds & Chemical bonds form when individual atoms become attached to other atoms. This happens when the electrons in each atom s outer energy level become filled. This is known as a stable octet. An octet is eight (8). So a stable octet means that the outer energy level has eight electrons in it. Na 1s 2 2s 2 2p 6 3s 1! Cl 1s 2 2s 2 2p 6 3s 2 3p 5 outer energy level = 1! outer energy level = 7 For sodium and chlorine to become stable, Na must lose 1 electron and! Cl must gain 1 electron. Na! 1s 2 2s 2 2p6 3s 1! Cl! 1s 2 2s 2 2p6 3s 2 3p 5 Na 1+ 1s 2 2s 2 2p 6! Cl 1-! 1s 2 2s 2 2p6 3s 2 3p 6 What must happen for Na and Cl to become stable? Na is now a positive ion and Cl is a negative ion. This transfer of an electron is how a chemical bond is formed.

2 There are three (3) basic types of chemical bonds: Ionic!! Polar Covalent! Covalent An Ionic bond is formed by the complete transfer of one or more electrons from a metal to a nonmetal. A Covalent Bond is formed by the complete sharing of electrons. A Polar Covalent Bond is somewhere in-between a pure Ionic and a pure Covalent bond. For the main group elements, the s and p block members, the electrons available for bonding, the valence electrons are the outer shell s and p electrons (except those of the noble gases!) In forming compounds from these elements, only these electrons will be used, in two ways: They may be transferred to form ions so that incomplete subshells are completed or removed; They may be shared so that two atoms together have complete subshells Ionic Bond Ionic bond: the electrostatic force that holds ions together in an ionic compound. Li + F Li + F - 1s 2 2s 1s 12 2s 2 2p 5 [He] 1s [Ne] 2 2s 2 2p 6 LiF Li Li + + e - e - + F F - Li + + F - Li + F -

3 Covalent Bond A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? F + F F F 7e - 7e - 8e - 8e - Lewis structure of F 2 single covalent bond F F F F single covalent bond Lewis structure of water single covalent bonds Predicting Bond Type H + O + H H O H or H O H 2e8e - 2e - How do you predict which type of bond will be formed? Double bond two atoms share two pairs of electrons O C O or O C O double 8e - 8ebonds - - double bonds Triple bond two atoms share three pairs of electrons N N or N N triple 8e 8e - bond - triple bond You use the Electronegativity values that are listed in your Periodic Table for each element in the compound.

4 Electronegativity Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond. Electron Affinity - measurable, Cl is highest X (g) + e - X - (g) Electronegativity - relative, F is highest Predicting Bond Type Bond Type Ionic Bond Difference 1.7 or more Polar Covalent Covalent 0.0 What would be the bond type for each of these substances? NaF BeN ! =! 3.04! bond is: Ionic ignore ! subscripts =! 1.31!bond! is: Polar AlCl ! =! 1.37!bond is: Polar Electronegativity Polar Bonding What does it mean that one end of the molecule has a higher electronegativity? The electrons are distributed unevenly.

5 Polar Bonding What does it mean that a molecule is polar? One end of the molecule has a higher electronegativity than the other end does. And as a result the electrons are pulled unevenly. Lattice energy is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. E = k Q + Q - r Lattice energy increases as Q increases and/or as r decreases. E is the potential energy Q + is the charge on the cation Q - is the charge on the anion r is the distance between the ions Compound (kj/mol) MgF 2 MgO LiF LiCl Q: +2,-1 Q: +2,-2 r F - < r Cl - Lattice energy is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. Now it s time to Practice! The magnitude of the lattice energy depends upon the nature of the ions involved: The greater the charge on the ions, the greater the electrostatic attraction and hence the greater the lattice energy and vice versa. The larger the ions, then the greater the separation of the charges and the lower the lattice energy, and vice versa.