Questions

Given Equation is 

Mg + O2 → MgO

Balancing the Chemical Equation is nothing but Conservation of Matter. As Matter is Neither created nor Destroyed, it must be conserved.

Here we have 1 Mg and 2 O on the left side and 1 Mg and 1 O on the right side

To balance that, add 1 Mg on the left side so that it becomes 2 Mg and 2 O on the left side

i.e,

2 Mg + O2 → 2 MgO

Here it has 2 Mg & 2 O on the left side and 2 Mg and 2 O on the right side and the Chemical equation is Balanced.

Bioaccumulation is defined as the increase in concentration of a substance(s) in an organism or a part of that organism.

The affected organism has a higher concentration of the substance than the concentration in the organism's surrounding environment.

For the second statement: Microorganisms eat pesticide infected things. Invertebrates eat hundreds of the microorganisms and plants infected with the pesticides, accumulating a pretty large amount of the toxin in them.

The invertebrates are generally then eaten by larger invertebrates or amphibians, which ingest hundreds of similar organisms, accumulating an even higher concentration of toxins in their bodies. Then the fish eat hundreds of these organisms, accumulating the highest concentration of toxins in themselves.

In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC).