They are kind of opposite processes. Chemical synthesis is execution of chemical reactions to make products. Like you take two substances, you put them together, they react and you have new a substance. For example.

here Sodium and Chlorine react to make a new substance which is Sodium Chloride.
Chemical decomposition is process of seperating a substance to different substances. There is a substance by a reaction this substance becomes two substances, Like : XY -> X+Y For example :
Answer:
Dark matter makes up 85% of the mass of the universe. Dark matter is not directly observable because it doesn't interact with any electromagnetic wave. In the development of the universe, without dark matter, the universe will not function, move or rotate as it does now (this speculation led to the quest to find the anomaly of mass and energy in the known universe, eventually leading to the idealization of dark matter) and will not have enough gravitational force to hold it together. After the big bang,<em> the presence of dark matter and energy ensured that the newly formed universe didn't just float away, rather, it provided enough gravitational force to hold the universe while still allowing it to expand sufficiently</em>.
The development of the universe would have been different without the universe in the sense that the young universe won't have enough mass to hold it together, and the universe would have simply floated apart. The behavior of the universe would have been different from what we observe now, and some physical laws that applies now will not apply to the universe.
Answer:
Reference image attached
Explanation:
Please see the attached image.
Answer:

Explanation:
Alkaline earth metals are in group 2. The number of valence electrons of elements in group 2 is 2.