Because the concordance rate for bipolar disorder among identical twins is biological causal explanations are

Bipolar disorder is a mood disorder characterized by alternating periods of mania mental illness marked by periods of great excitement, euphoria, delusions, and overactivity), and depression.

Concordance rates or the presence of bipolar disorder among identical twins is reported to be as high as 85%.  Biological causal explanations are: there is considerable overlap between the genes and imbalances in serotonin activity. Omega-3 fatty acids are reported to  provide protection from bipolar disorder

all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex