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Instruction 3-1

Covalent, Metallic or Ionic Bonds | Chemical Bonds in Molecular Atoms | Salt Crystals | How Atoms and Molecules Move in Liquid? | Lewis Dot Structure and Molecular Shape | Electronegativity, Ionization, and Bond Formation | Van Der Waals Forces

Covalent, Metallic or Ionic Bonds
CCSTD HS Chemistry 2.a..

A chemical bond is the “glue” that holds atoms together. Chemical bonds can be weak or strong, and everything in between!

One thing that all chemical bonds share is that they are just an atom’s attempt to complete its outer (valence) shell of electrons. The closer an atom’s valence shell comes to being completely full, then the closer that atom comes to resemble an inert, or noble, gas.

It turns out that all atoms strive to have the electronic configuration of an inert gas atom! Why is that such a desirable state? Inert gases are extremely stable. In fact, inert gases are so stable that they don’t bond, ever, to anything. They are perfectly content to remain just as they are.

 

Covalent Bonds
http://en.wikipedia.org/wiki/Covalent_bond

Let’s take a look at the smallest chemically bonded molecule; that of Hydrogen gas, or H2. Each Hydrogen atom has one proton and one neutron in its nucleus, and one electron in its outer shell. The only way a Hydrogen atom can fill its outer shell, which would then resemble the outer valence shell of Helium, is to take on one electron. However, the addition of an electron would give a resultant charge of –1. A solution is to have two atoms share one electron, and that’s exactly what a covalent bond does!

If we consider a covalent bond to be co-valent, then the term itself describes its own character. In other words, a covalent bond between two atoms is the case where two atoms share a single valence shell. Sharing a shell means that electrons within that shell travel around both nuclei. The two atoms in molecular, or diatomic, Hydrogen has a shared pair of electrons that provides each hydrogen atom with two electrons in its valence shell.

If you think of the electron as a cloud of probability, rather than a particle, then it’s easier to imagine a Hydrogen molecule as having a shape that resembles a sausage. That sausage-shaped cloud displays the probability that the electron will be found at any one location within the “sausage”.

Generally, the covalent bond has properties of:

  • High stability (it’s not easy to break a bond of shared electrons),
     
  • High energy (when a covalent bond is broken, it releases a large quantity of energy), and
     
  • No net charge.

 

Ionic Bonds
http://en.wikipedia.org/wiki/Ionic_bond

Another type of chemical bond, the ionic bond, involves a somewhat different arrangement of electrons within a molecule. Ionic bonds form when a “give and take” situation exists between atoms. One atom within the molecule gives an electron (to leave behind a positively charged atom), and another atom takes an electron (to result in a negatively charged atom).

You can just imagine what will happen if you put a positively charged atom (cation) next to a negatively charged atom (anion)! They are attracted to each other and we call this type of attraction an ionic bond.

Think of an ionic bond as one valence shell cloud with a positive charge being attracted to another valence shell cloud with a negative charge. When those two clouds are next to each other, they “stick” together and the resultant molecule has no net charge.

Table salt, or Sodium Chloride (NaCl), is a compound that has an ionic bond (see Figure 3.1.2). The Sodium within the molecule donates its electron and the Chloride accepts the electron. This condition gives the Sodium atom the electronic configuration of a Neon atom (an inert gas), and the Chloride atom the electronic configuration of an Argon atom (another inert gas!).

 

An ionic bond can involve the loss of more than one electron, with the other atom in the ionic compound gaining the same number of electrons. Magnesium Chloride (MgCl2) is an example of a doubly ionic compound. Once again, Magnesium Chloride has no net charge, and in this case, one Magnesium atom has lost two electrons (achieving the electronic configuration of Neon), with each of two Chloride atoms accepting one electron.

Ionic bonds have distinctive properties of:

  • When in solid form, all ionic compounds are crystalline solids at room temperature (the alternating positive and negative ions are arranged in a crystal lattice structure),
     
  • Strong ionic bonding forces making the solid structures hard with high melting and boiling points,
     
  • Multiple charges within the ionic bond lead to stronger bonding forces (i.e. the bonding of MgCl2 is stronger than the bonding of NaCl), and
     
  • Most ionic compounds are soluble in water (once they are fused (melted), or dissolved in water, ionic compounds conduct electricity because the anions are then free to move freely).

Most often, ionic compounds are combinations of cations of atoms from the furthest left of the periodic (groups IA and IIA), with anions of atoms from the right of the periodic table (group VIIA), immediately preceding the inert gases.

A helpful discussion of Ionic bonds can be found at: http://www.wpbschoolhouse.btinternet.co.uk/page04/4_72bond.htm  - transfer

 

Metallic Bonds

The metallic bond is actually a special case of an ionic bond. Think of metallic bonding as a more or less static arrangement of positive ions within a moving array of mobile electrons.

This unique arrangement of cations and electrons give metals their characteristic properties. Metallic properties include:

  • Heat conductivity (mobile electrons can carry the kinetic energy of heat),
     
  • Shiny appearance (the rapidly moving electrons emit energy in the form of light),
     
  • Electricity conductors (electricity is the flow of electrons), and
     
  • Malleability (ability to be easily shaped into flat sheets or drawn into wires).


Figure 3.1.3 allows you to see an idealized arrangement of atoms within a metallic bond. For additional descriptions and images of the metallic bond with its delocalized electrons, go to: http://www.chemguide.co.uk/atoms/bonding/metallic.html
or to: http://dl.clackamas.cc.or.us/ch104-08/metallic.htm

 

Reading List

 
  Pauling, Linus C.: The Nature of the Chemical Bond and the Structure of Molecules and Crystals; An Introduction to Modern Structural Chemistry.

 

for Students, Parents and Teachers

Now let's do Practice Exercise 3-1 (top).

 

Next Page:  Chemical Bonds in Molecular Atoms (top)