A phosphorus atom needs to gain ---electrons to achieve a full octet

Three (3).

Explanation:

The octet rule is a rule in chemistry that states that, chemical elements would either gain or loss electrons in their outermost shell in order to attain a stable electronic configuration i.e having eight (8) valence electrons in their outermost shell such as chemical elements found in group 8 (2-8-8) known as noble gas.

A phosphorus atom needs to gain three (3) electrons to achieve a full octet because the atomic number of phosphorus is 15 and as such it has the electronic configuration of 2-8-5 and needs three electron in order to become a stable element.

all of the answers are correct.

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explanation:

  the grams     of oxygen gas   that can be produced     is 20.6   grams of o₂

calculation

2liclo₃ →   2 licl + 3 o₂

step 1:   find the moles of liclo₃

moles = mass÷ molar mass

from periodic table the molar mass of liclo₃ =6.9 +35.5   +(16 x3) =90.4 g/mol

moles=38.7 g÷90.4 g/mol=0.4281   moles

step 2:   use the   mole ratio   to calculate the moles of o2

from equation above, liclo₃: o₂   is 2: 3  

therefore the moles of   o₂=0.4281 moles x3/2 =0.6422   moles

step 3: find mass   of o₂

mass= moles x molar mass

from periodic table the   molar mass of   o₂ = 16 x2= 32 g/mol

mass=0.6422   moles   x   32 g/mol = 20.6 grams of o₂