Subscripts cannot be changed because they are the ratio of the amount. And as we know, in a chemical equation/reaction, mass cannot be created nor destroyed. Therefore, we cannot change subscripts, however, we could change coefficients. <span />
Explanation:
To balance the reactions given, we must understand that the principle to follow is the law of conservation of matter.
Based on this premise, the number of moles of species on the reactant and product side must be the same;
Li + Br₂ → LiBr
Put a,b and c as the coefficient of each species
aLi + bBr₂ → cLiBr
balancing Li;
a = c
balancing Br;
2b = c
let a = 1;
c = 1
b = 
or a = 2, b = 1 , c = 2
2Li + Br₂ → 2LiBr
P + Cl₂ → PCl₃
Using the same method;
aP + bCl₂ → cPCl₃
balancing P;
a = c
balancing Cl;
2b = 3c
let a = 1;
c = 1
b = 
or
a = 2, b = 3, c = 2
2P + 3Cl₂ → 2PCl₃
iii,
H₂ + SO₂ → H₂S + H₂O
use coefficients a,b,c and d;
aH₂ + bSO₂ → cH₂S + dH₂O
balancing H;
2a = 2c + 2d
balancing S;
b = c
balancing O
2b = d
let b = 1,
c = 1
d = 2
a = 3
3H₂ + SO₂ → H₂S + 2H₂O
Activation energy is a thermodynamic barrier that must be overcome before products are formed in a reaction. It is the minimum amount of energy needed for a reaction to occur. The energy can be in the form of kinetic or potential energy. This concept was introduced by Svante Arrhenius, which brought about the Arrhenius equation which is a formula used to determine rate of reactions.
Table Giving Answer
Element Atomic mass % Amount
Mg_24 24 79 18.96
Mg_25 25 10 2.5
Mg_26 26 11 2.86
Total 24.32
Discussion
The method of calculation for this table, which was done in Excel (a spread sheet) is shown below. Assume that there is 100 grams of material of "pure" magnesium. What is it's mass?
<em><u>Sample Calculation</u></em>
The the sample atomic mass = 24
Mass = % * sample atomic mass
Mass = 79% * 24
Mass = (79/100) * 24
Mass = 18.96
<em><u>Note</u></em>
The other two elements are found exactly the same as the sample calculation.
Then all you do is add the 3 masses together.
Answer
The mass of Mg to 1 decimal place is 24.3 <<<< Answer.
We know that the element Z = 119 would be placed right below the Fr, in the column of the alcaline metals.
We also know that the trend in the electronegativity is to decrease when you go up-down ia group.
The known electronegativities of the elements of this group are:
Li: 0.98
Na: 0.93
K: 0.82
Rb: 0.82
Cs: 0.79
Fr: 0.70
Then the hypotetical element Z = 119 would probably have an electronegativity slightly below 0.70, for sure in the range 0.60 - 0.70.
