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Ionic and covalent bonds

  • Thread starter bjoyful
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1. Homework Statement

NaCl is an ionic compound, but chlorine and silicon are joined by covalent bonds in SiCl4

A. Use Table 5.4 (page 229 of your text) to determine the electronegativity difference between chlorine and sodium, and between chlorine and silicon. What values did you calculate in each case?
B. What general correlations can be drawn about the electronegativity difference between bonded atoms and their tendency to form ionic or covalent bonds?
C. How can you explain on the molecular level the conclusion you reached in part B?

3. The Attempt at a Solution

A. Cl (3.0) – Na (0.9) = 2.1. Cl (3.0) – Si(1.8) = 1.2

Other than question a, I am not sure what I am doing.
 

Answers and Replies

75
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For B, you need to look at SiCl4; it's a tetrahedral molecule, and has those four chlorine atoms all pulling in equal but opposite directions, causing their pull to cancel out. That's why it's left covalent. Whereas NaCl is an incredibly lop-sided molecule where the sodium has all but had its electron ripped out of it and gobbled up by the chlorine atom.

So that's what makes the difference between ionic and covalent molecules; ionic happens when there's a great electronegativity difference in the two. Covalent can either mean there's a small difference in electronegativity, or the molecule is arranged in such a way that it's dipole is canceled by opposite by equal pulls by its atoms.
 

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