Element Element
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18) Element Rand Element Q have the same number of valence electrons. These elements both have similar chemical behavior, but Element Rhas fewer energy levels than Element
Which statement best describes the positions of the two elements in the periodic table?
A) The two elements are in the same period, with Element R the first element in the period and
Element Q the last element.
B) The two elements are side by side in the same period, with Element Q to the left of Element R.
C) The two elements are in the same group, with Element R just above Element Q
D) The two elements are in the same group, with Element Q at the top of the group and Element Rat the bottom
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the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2