How to know if there's a dipole moment???
You look at two things.
Are any of the bonds polar? If they are not the molecule will not be polar. Example H-H, Cl-Cl, etc. If there are polar bonds, go to #2.
#2 is the symmetry. If the molecule is symmetrical in three dimensions it will not be polar. Most students don't understand this part and it's difficult to explain on a forum like this. Here are some example.
CH4. There is a small difference in EN of C and H; therefore, the 4 C-H bonds are slightly polar so CH4 could be polar if the 3D part is ok. Go to part 2.
When the CH4 structure is drawn you have 4 C-H bonds in the shape of a tetrahedral molecule. That makes it symmetrical and the polarity of the molecule is zero (not polar) because there is no net dipole moment.
You can see this easier on a molecule that is polar;i.e., NH3.
The N-H bond is slightly polar. You have three of them PLUS two unpaired electrons. In a 3D arrangement. You have 3H atoms at the points of a tetrahedral molecule BUT the fourth point (responsible for canceling the polarity of the CH4 molecule) is NOT another H atom. That makes it unsymmetrical so NH3 has a net dipole moment and NH3 is polar.
H2O is another example. You have two H atoms at two corners of the tetrahedral molecule and two sets of unpaired electrons at the other two points so H2O is a polar molecule. CO2 is a non-polar molecule. It has two C-O bonds that are polar BUT it is a linear molecule so the polarity of the left C-O bond is canceled by the polarity of the right C-O bond and the molecule as a whole has no net dipole moment so it is non-polar. Hope this helps.
THANK YOU! if there are many unpaired electrons, will it means its polar?
No. Draw the Lewis structure for Cl-Cl and you will find 3 sets of unpaired electrons around each Cl atom but the Cl2 molecule is non-polar (mostly because there is no difference in electronegativity between one Cl atom and the other so the bond is not polar).
For molecules in which there is a polarity on the bonds, the presence of an unpaired set of electrons almost always leads to a polar molecule no matter what the 3D arrangement is. But if there are TWO sets of unpaired electrons those COULD BE diametrically opposed so as to balance each other and the molecule would not be polar.
To determine if a molecule has a dipole moment, you need to consider the molecule's geometry and the polarity of its individual bonds. Follow these steps to evaluate if a molecule has a dipole moment:
1. Determine the molecule's geometry: Start by identifying the atom arrangement within the molecule. This can be done by drawing its Lewis structure or examining the molecule's molecular formula. The molecular geometry can be determined using the VSEPR theory (Valence Shell Electron Pair Repulsion theory), which predicts the arrangement of electron pairs around a central atom.
2. Identify the electronegativity difference: Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond. The electronegativity difference between the atoms participating in a bond determines the bond's polarity. Generally, the higher the electronegativity difference between two atoms, the more polar the bond.
3. Consider the molecular symmetry: A molecule's symmetry is another critical factor in determining if it has a dipole moment. If a molecule has a symmetric structure (such as when the polar bonds cancel each other out), the dipole moments of each bond may cancel, resulting in a nonpolar molecule.
4. Calculate the vector sum of the bond dipole moments: For asymmetrical or non-symmetric molecular structures, calculate the net dipole moment by summing the individual bond dipole moments. The dipole moment of each bond can be obtained by multiplying the bond length by the magnitude of the electronegativity difference between the atoms in that bond.
5. Consider the molecular shape and dipole moments: For symmetrical structures, the molecular shape can affect the presence of a dipole moment. In some cases, even if individual bonds have dipole moments, the symmetry of the overall molecule may result in a cancelation of the dipole moments, making the molecule nonpolar.
By following these steps and considering the molecular geometry, bond polarity, symmetry, and vector sum of the bond dipole moments, you can determine whether a molecule has a dipole moment or not.