The shape of a molecule, and so the bond angle, is based on a fundamental principle regions of high electron density repel one another as far apart as possible.
The regions of high electron density have the same negative charge, so they repel one another. A bond angle of 104.5* keeps the regions of high election density as far apart as possible
How does the shape of water molecules (i.e. its 104.45º angle) relate to its polarity?
a polar bond can be envisioned as something in between the most ideal ionic and covalent bond. In reality, many covalent bonds are not ideally covalent, the electron density in covalent bonds tend to be attracted towards one side of the atoms involved in bonding due to electronegativity differences.
Electronegativity results from different electrostatic interactions between different atoms containing different protons with the electrons involved in covalent bonding. This creates an unequal sharing of electron density known as polarisation, leading to what we call polar bonds.
However, molecular symmetry is important in determining overall bond polarity.
whereas in linear molecule like CO2, despite electronegativity differences between C and O, the dipole moments exerted due to this cancel each other out - they are vector quantities - directions matter (like force)
in H2O, the molecule is bent, the dipoles are exerted along the bent symmetry, and they do not cancel out - i.e. there is a net dipole moment - therefore water is a polar molecule.
in CH4, yes each C-H bond is polar, but due to the tetrahedral symmetry and all are C-H bonds, then the net dipole is cancelled out - CH4 is non polar. See this trend though,
CH4 - non polar
CH3Cl - polar
CH2Cl2 - polar
CHCl3 - polar
CCl4 - polar
Hope you get the ideas. I think with time, and experiencing and practising more examples, you'd get better.
a polar bond can be envisioned as something in between the most ideal ionic and covalent bond. In reality, many covalent bonds are not ideally covalent, the electron density in covalent bonds tend to be attracted towards one side of the atoms involved in bonding due to electronegativity differences.
Electronegativity results from different electrostatic interactions between different atoms containing different protons with the electrons involved in covalent bonding. This creates an unequal sharing of electron density known as polarisation, leading to what we call polar bonds.
However, molecular symmetry is important in determining overall bond polarity.
whereas in linear molecule like CO2, despite electronegativity differences between C and O, the dipole moments exerted due to this cancel each other out - they are vector quantities - directions matter (like force)
in H2O, the molecule is bent, the dipoles are exerted along the bent symmetry, and they do not cancel out - i.e. there is a net dipole moment - therefore water is a polar molecule.
in CH4, yes each C-H bond is polar, but due to the tetrahedral symmetry and all are C-H bonds, then the net dipole is cancelled out - CH4 is non polar. See this trend though,
CH4 - non polar
CH3Cl - polar
CH2Cl2 - polar
CHCl3 - polar
CCl4 - polar
Hope you get the ideas. I think with time, and experiencing and practising more examples, you'd get better.
Water has two polar bonds that can be resolved in two dimensions x and y (horizontal and vertical). It only has three atoms and so can lie on a two dimensinal plane.
With the oxygen atom at the top the vectors cancel out in a horizontal direction, but add up in the vertical. Here's the diagram: