Forces
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What are van der waals caused by?
When electrons in an atom are randomly moving around, and for a fraction of a second there are more electrons on one side of the atom than the other, giving this side a slight negative charge, causing a dipole, other side will be positive. This is an instantaneous dipole - This means the atoms electrons next to it will be repelled by the electrons being on mainly one side than the other causing electrons in this atom to move to the other side creating another dipole, this is an induced dipole as it has been caused by the instantaneous dipole -These dipoles now experince forces of attraction, van der waals Instantaneous dipoles cause dipoles to form in neighbouring molecules
Describe the features of van deer waals
• Weak and easily broken • Strength of these depend on the number of electrons, atoms with more van deer waals have a higher boiling point
How are permanent dipole dipole interactions formed?
When molecules with a permanent dipole ( when one element has a higher electronegativity than the other attracting an electron density causing a S-, causing a dipole) E.g HCL When two of these molecules interact closely enough, their permanent dipoles cause these dipole dipole interactions
How are van deer waals used to explain changes in boiling points of molecules
The more electrons in the molecule the higher the chance a temporary dipole will form This makes the van der waals stronger between molecules so bp is higher E.g down group 7 atoms get bigger meaning they have more electrons meaning there is more of a chance of temporary dipoles meaning the van der waals are stronger, meaning boiling points increase
When do hydrogen bonds happen?
In compounds where a hydrogen atom is attached to an atom with a higher electronegativity, for example F,Cl,N, which have an avainable lone pair of electrons Large electronegativity difference between the H and individual elements The slightly positive hydrogen atom from one polar molecule is attracted to the lone pair of electrons on the slightly negative electronegative atom on another molecule This causes a hydrogen bond
Why is the solid form of water less dense than the liquid form?
Water molecules randomly move around next to eachother in liquid form, sometimes they are close and sometimes they are far apart Hydrogen bonds are constantly being formed or broken As we cool water down molecules move more slowly When it reaches freezing point, water molecules form a stable structure stablised by hydrogen bonds In ice the water molecules are further away than in liquid water, making it less dense and floating onm surface