Secondary Bonds

Secondary Bonds:

Secondary bonds are much weaker than primary bonds. They often provide a "weak link" for deformation or fracture. Example for secondary bonds are:

Hydrogen Bonds

Van der Waals Bonds

Hydrogen Bonds

Hydrogen bonds are common in covalently bonded molecules which contain hydrogen, such as water (H2O). Since the bonds are primarily covalent, the electrons are shared between the hydrogen and oxygen atoms. However, the electrons tend to spend more time around the oxygen atom. This leads to a small positive charge around the hydrogen atoms, and a negative charge around the oxygen atom. When other molecules with this type of charge transfer are nearby, the negatively charged end of one molecule will be weakly attracted to the positively charged end of the other molecule. The attraction is weak because the charge transfer is small.

Figure 6. Hydrogen bonds.

Van der Waals Bonds

Van der Waals bonds are very weak compared to other types of bonds. These bonds are especially important in noble gases which are cooled to very low temperatures. The electrons surrounding an atom are always moving. At any given point in time, the electrons may be slightly shifted to one side of an atom, giving that side a very small negative charge. This may cause an attraction to a slightly positively charged atom nearby, creating a very weak bond. At most temperatures, thermal energy overwhelms the effects of Van der Waals bonds.

Van Der Waals bonding is a secondary bonding, which exists between virtually all atoms or molecules, but its presence may be obscured if any of the three primary bonding types is present. Secondary bonding forces arise from atomic or molecular dipoles. In essence, an electron dipole exists whenever there is some separation of positive and negative portions of an atom or molecule. When an electron cloud density occurs at one side of an atom or molecule during the electron flight about the nucleus, Van Der Waals forces are generated. This creates a dipole wherein one side of the atom becomes electrically charged and the other side has deficiency of electrons and is considerably charged positive. The atom is distorted as shown in Figure 7.

Figure 7. Van der Waals Bond

Lattice Structures

Atoms are the building blocks of all materials. They are put together in a great variety of ways and bonded or "held together" by cohesive forces in a manner characteristic of a particular material. In a liquid state the atoms of metal are said to be in somewhat random arrangement, having short-range order. At times several unlike atoms will arrange themselves in the characteristic pattern of a particular metal. However, this is a probability event. Since the forces are weak and there is much activity taking place, they soon separate and re-form again. This phenomenon of random grouping, scattering, and regrouping for short periods of time is characteristic of the liquid state. As the random grouping mechanism becomes less frequent and the atomic movement of unlike atoms become more agitated, the material may become a gas.

As the energy input decreases, the random movement of the unlike atoms becomes less frequent, the bonding becomes stronger, and ordered arrays of atoms form lattices.

A crystal is a repeating array. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell). The most fundamental property of a crystal lattice is its symmetry. In three-dimensions, unit cells stack like boxes, filling the space, making the crystal. The different colors are just to show the separate boxes - each unit cell is identical.

Figure 1. Cubic Lattice Structure	Figure 2. Hexagonal Lattice Structure

If we take a unit cell and stack it, we produce a lattice. Notice that once we begin stacking the unit cells, we never change the orientation of any subsequent unit cells as they stack. In other words, once the orientation of a unit cell is determined, all unit cells within that lattice have the same orientation.

Figure 3. Orientation of the unit cells in a lattice.

Unit Cell: When a solid has a crystalline structure, the atoms are arranged in repeating structures called unit cells, which are the smallest units that show the full symmetry of a crystal.

Lattice: The three dimensional array formed by the unit cells of a crystal is called lattice.

When a crystalline solid starts to form from the molten or gaseous state, these unit cells will tend to stack in a three-dimensional array, with each cell perfectly aligned, and they will form a crystal. If crystals are growing in a melt at the same time, the crystals will eventually meet and form grains. The junction of the grains is called grain boundaries.

The majority of metals have one of three well-packed crystal structures:

Face-centered cubic (F.C.C.)

Body-centered cubic (B.C.C.

Hexagonal-close-packed (H.C.P.)

Figure 4. Face Centered Cubic (F.C.C) Lattice Structure

To view the FCC crystal structure visit the following links:

Figure 5. Body Centered Cubic (B.C.C) Lattice Structure

To view the BCC crystal structure visit the following links:

Figure 6. Closed Packed Hexagonal (C.P.H) Lattice Structure To view the CPH lattice structure visit the following links: