Answer: Between 1 and 2.
Explanation:
Half life is the amount of time taken by a radioactive material to decay to half of its original value.
............(1)
where,
a = amount of reactant left after n-half lives = 40
= Initial amount of the reactant = 100
n = number of half lives
Putting in the values we get:

taking log on both sides


Thus half-lives that have elapsed is between 1 and 2
double-displacement reaction
Explanation:
We have the chemical reaction:
Na₂S (aq) + Cd(NO₃)₂ (aq) → CdS (s) + 2 NaNO₃ (aq)
where:
(aq) - aqueous
(s) - solid
This is a double-displacement reaction because the reactants exchange atoms or group of atoms between themselves to form the products. To drive the reaction to the right, one of the products is a precipitate.
Generally we can express the double-displacement reaction as following:
AB + CD → AC + BD
Learn more about:
types of chemical reactions
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Silicon must have 4 single covalent bond, every single bond has 2 electrons (one pair), so 4·2=8 electrons (octet).
Sir Joseph John Thomson OM PRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered.