Some bonds between different elements are only minimally polar, while others are strongly polar. Ionic bonds can be considered the ultimate in polarity, with electrons being transferred rather than shared. To judge the relative polarity of a covalent bond, chemists use electronegativity, which is a relative measure of how strongly an atom attracts electrons when it forms a covalent bond.
There are various numerical scales for rating electronegativity. The polarity of a covalent bond can be judged by determining the difference in the electronegativities of the two atoms making the bond. The greater the difference in electronegativities, the greater the imbalance of electron sharing in the bond.
Although there are no hard and fast rules, the general rule is if the difference in electronegativities is less than about 0. If the difference in electronegativities is large enough generally greater than about 1. An electronegativity difference of zero, of course, indicates a nonpolar covalent bond. A popular scale for electronegativities has the value for fluorine atoms set at 4.
Describe the electronegativity difference between each pair of atoms and the resulting polarity or bond type. In short, the molecule itself is polar. In a nonpolar covalent bond, the distribution of electrical charge is balanced between the two atoms. Figure 3. A nonpolar covalent bond is one in which the distribution of electron density between the two atoms is equal. The two chlorine atoms share the pair of electrons in the single covalent bond equally, and the electron density surrounding the Cl 2 molecule is symmetrical.
An example would be a bond between chlorine and bromine. Figure 4. In the polar covalent bond of HF, the electron density is unevenly distributed. There is a higher density red near the fluorine atom, and a lower density blue near the hydrogen atom. A bond in which the electronegativity difference between the atoms is between 0.
A polar covalent bond is a covalent bond in which the atoms have an unequal attraction for electrons and so the sharing is unequal. In a polar covalent bond, sometimes simply called a polar bond, the distribution of electrons around the molecule is no longer symmetrical. The atom with the greater electronegativity acquires a partial negative charge, while the atom with the lesser electronegativity acquires a partial positive charge.
The delta symbol is used to indicate that the quantity of charge is less than one. A crossed arrow can also be used to indicate the direction of greater electron density. Take the quiz on the right-hand side of this page:. Skip to main content. Covalent Bonding.
Now one puppy has two electron bones and one puppy has none. Because the electron bones in our analogy have a negative charge, the puppy thief becomes negatively charged due to the additional bone.
The puppy that lost its electron bone becomes positively charged. Because the puppy who lost his bone has the opposite charge of the thief puppy, the puppies are held together by electrostatic forces, just like sodium and chloride ions!
In our analogy, each puppy again starts out with an electron bone. Some covalently bonded molecules, like chlorine gas Cl2 , equally share their electrons like two equally strong puppies each holding both bones. Other covalently bonded molecules, like hydrogen fluoride gas HF , do not share electrons equally. The fluorine atom acts as a slightly stronger puppy that pulls a bit harder on the shared electrons see Fig.
Even though the electrons in hydrogen fluoride are shared, the fluorine side of a water molecule pulls harder on the negatively charged shared electrons and becomes negatively charged. The hydrogen atom has a slightly positively charge because it cannot hold as tightly to the negative electron bones.
Covalent molecules with this type of uneven charge distribution are polar. Molecules with polar covalent bonds have a positive and negative side. In this analogy, each puppy represents an atom and each bone represents an electron. Water H2O , like hydrogen fluoride HF , is a polar covalent molecule. When you look at a diagram of water see Fig. The unequal sharing of electrons between the atoms and the unsymmetrical shape of the molecule means that a water molecule has two poles - a positive charge on the hydrogen pole side and a negative charge on the oxygen pole side.
We say that the water molecule is electrically polar. Each diagram shows the unsymmetrical shape of the water molecule. In part c , the polar covalent bonds are shown as electron dots shared by the oxygen and hydrogen atoms.
0コメント