Adecomposition reaction is the opposite of a synthesis reaction true or false

The answer is true, not false

True, a DECOMPOSITION reaction involves breaking a larger substance into smaller parts, hence decomposes means "breaks down."
A SYNTHESIS reaction involves the opposite, a combining or synthesizing of smaller substances into a larger one.
An example of a synthesis is combining hydrogen gas (H2) with oxygen (O2) to form water (H2O): 2H2 + O2 -- > 2H2O
The opposite reaction, in reverse, is called hydrolysis, a decomposition, literally meaning "splitting water": 2H2O -- > 2H2 + O2

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.