The number above the symbol is the atomic mass (or atomic weight). This is the total number of protons and neutrons in an atom. The number below the symbol is the atomic number and this reflects the number of protons in the nucleus of each element's atom. Every element has a unique atomic number.
Elements in group 1-2, 13-18, the number of valence electrons is related to the group number. For example, in the first group, the alkali metals there is one valence electron, however in group 13, there are 3 valence electrons. Valence electrons are also known as the outershell electrons.
I think the correct form of the equation is given as:
a = a0 * (0.9)^t
where t is an exponent of 0.9 since this is an
exponential decay of 1st order reaction
Now to solve for the half life, this is the time t in
which the amount left is half of the original amount, therefore that is when:
a = 0.5 a0
Substituting this into the equation:
0.5 a0 = a0 * (0.9)^t
0.5 = (0.9)^t
Taking the log of both sides:
t log 0.9 = log 0.5
t = log 0.5 / log 0.9
t = 6.58 years
Answer:
half life = 6.58 years
