<unk>  arise from the fact that each atom within a molecule occupies a certain amount of space.  If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.
There are several types of<unk> :<unk>  hindrance or<unk>  resistance occurs when the size of groups within a  molecule prevents chemical reactions that are observed in related smaller molecules. Although<unk>  hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions<unk>  protection).<unk>  hindrance between adjacent groups can also restrict torsional bond angles.  However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the<unk>  hindrance of the small hydrogen atom is far too small.  .<unk>  shielding occurs when a charged group on a molecule is seemingly weakened or spatially shielded by less charged (or oppositely charged) atoms, including counterions in solution (Debye shielding). In some cases, for an atom to interact with<unk> ally shielded atoms, it would have to approach from a vicinity where there is less shielding, thus controlling where and from what direction a molecular interaction can take place.<unk>  attraction occurs when molecules have shapes or geometries that are optimized for interaction with one another. In these cases molecules will react with each other most often in specific arrangements.
Chain crossing — A random coil can't change from one conformation to a closely related shape by a small displacement if it would require one polymer chain to pass through another, or through itself.
Understanding<unk>  is critical to chemistry, biochemistry and pharmacology. In chemistry,<unk>  are nearly universal and affect the rates and energies of most chemical reactions to varying degrees. In biochemistry,<unk>  are often exploited in naturally occurring molecules such as enzymes, where the catalytic site may be buried within a large protein structure. In pharmacology,<unk>  determine how and at what rate a drug will interact with its target bio-molecules.
Steric effects
Sedative
Intrinsic sympathomimetic activity
Anorectic
Cytopathic effect
Antithrombotic
Neurotoxicity
Cardiotoxicity