Metallic and Molecular Bonds

Metallic Bonds

Bond that keeps metal atoms together is called metallic bond. In metals;

  • Number of valence electrons is smaller than number of valence orbitals. So, they have many empty valence orbitals.
  • Their ionization energies are small and they are weakly attracted by nucleus.

Valence electrons of metals can jump to the other atom's valence orbitals. This free movement of electrons makes metals good conductor of electricity and heat. All valence electrons of metals can behave like this.

  • In periodic table, as we go from top to bottom in metal group, strength of metallic bond and melting point decrease.
  • In periodic table, as we go from left to rşght in same period, strength of metallic bond and melting point increase.

 

Molecular Bonds

Atoms bonded with covalent bonds produce molecules and these molecules attract each other and form secondary bonds. Molecular bonds determine physical properties like melting point, boiling point of matters. Moreover, they also determine physical states of matters.We examine these bonds under three titles, Vander Waals Bond, Dipole-Dipole Interactions and Hydrogen Bond.

1) Van der Waals Bonds:

We see these bonds in molecules having % 100 nonpolar bonds like, I2, Cl2, H2 and molecules having polar bonds but nonpolar molecules like CO2. When nonpolar molecules like CO2 are get closer,they repel each other and symmetry of molecule is broken. And then, positive and negative dipoles are formed in molecule. An instant attraction of these dipoles each other is called Van der Waals bonds. Increasing molar mass increases strength of van der waals bonds.

2) Dipole-Dipole Interaction:

This types of bond is seen between polar molecules in solid and liquid phases such as, HCl, SO2, H2S, PH3. Since there is no symmetry in polar molecules, there are poles having negative and positive charges. Attraction between these poles in molecule is called dipole-dipole interaction. These forces are not temporary as in the case of va der waals bonds. Thus, dipole-dipole interactions are stronger than van der waals bonds. Molecules having closer molar masses have different boiling points. Polar molecules have higher boiling point than nonpolar molecules because of strengths of dipole-dipole interactions.

3) Hydrogen Bonds:

Chemical bonds formed between H and atoms having high electronegativity like F, N O, are stronger than dipole-dipole interactions. We can explain this ;

Since O has high electronegativity, it attracts H in H-O bond strongly and these bonds are called hydrogen bond. In other words, attraction between H atom of one molecule and O atom of another molecule is called hydrogen bond. We can show schema of hydrogen bond in water as given below;

hydrogen bond

 

Compounds including hydrogen bonds have higher boiling and melting points than compounds having van der waals bonds and dipole-dipole interactions.

Bonds in solid State:

We can analyze bonds in solids under four categories.

1) Ionic Solids:

Solids that are bonded with ionic bonds are called ionic solids. They ;

  • have high melting points
  • do not conduct electricity in solid phase but in liquid phase and solutions they conduct electric current
  • are hard and fragile and they can not be shaped
  • can dissolve in polar solvents like water but can not dissolve in nonpolar solvents like CCl4.

2) Covalent Solids:

They are huge molecules formed by covalent bonds. diamond, graphite, SiC are examples of covalent solids.They;

  • have too high melting points
  • are too hard and fragile
  • do not conduct electric current
  • do not dissolves any matter polar or nonpolar

3) Molecular Solids:

They are solids including  van der waals bonds, dipole-dipole interactions or hydrogen bonds. They;

  • have low melting points
  • can easily sublimate
  • are fragile
  • do not conduct electric current in solid and liquid phases but when they dissolve in water they produce ions and conduct electricity
  • have different solubility changing with the bonds of solvent, polarity of solvent etc.

4) Metallic Solids:

Metals including metallic bond are produce this group. This solidification can be seen in all pure metals and alloys. Fe, Au, Ag, Cr are some common examples of metallic solids. They;

  • are good conductors of heat and electric current
  • are ductile
  • have low melting points
  • can only dissolve in other metals

 

Chemical Bonds Exams and Problem Solutions


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