List of top Chemistry Questions

What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?
How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?
In a reaction \(A + B_2 → AB_2\) Identify the limiting reagent, if any, in the following reaction mixtures.
  1. 300 atoms of A + 200 molecules of B
  2. 2 mol A + 3 mol B
  3. 100 atoms of A + 100 molecules of B
  4. 5 mol A + 2.5 mol B
  5. 2.5 mol A + 5 mol B
Among the second period elements the actual ionization enthalpies are in the order 
Li < B < Be < C < O < N < F < Ne. 
Explain why 
  1. Be has higher ΔiH than B
  2. O has lower ΔiH than N and F?
Energy of an electron in the ground state of the hydrogen atom is -2.18 x 10-18  J. Calculate the ionization enthalpy of atomic hydrogen in terms of J mol-1  .
If the speed of light is 3.0×108 ms–1, calculate the distance covered by light in 2.00 ns.
What is the significance of the terms - 'isolated gaseous atom' and 'ground state' while defining the ionization enthalpy and electron gain enthalpy?
Hint: Requirements for comparison purposes.

The size of isoelectronic species - F-, Ne and Na+ is affected by

Anything that influences the valence electrons will affect the chemistry of the element. Which one of the following factors does not affect the valence shell?
Explain why cations are smaller and anions larger in radii than their parent atoms?
How does atomic radius vary in a period and in a group? How do you explain the variation?
What does atomic radius and ionic radius really mean to you?
Why do elements in the same group have similar physical and chemical properties?
Which element do you think would have been named by
(i) Lawrence Berkeley Laboratory
(ii) Seaborg's group?
Write the atomic number of the element present in the third period and seventeenth group of the periodic table.
In terms of period and group where would you locate the element with Z =114?
On the basis of quantum numbers, justify that the sixth period of the periodic table should have 32 elements.
What is the basic difference in approach between the Mendeleev's Periodic Law and the Modern Periodic Law?
Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that? 
What is the basic theme of organisation in the periodic table?
How many significant figures are present in the following? 
(i) 0.0025 
(ii) 208 
(iii) 5005 
(iv) 126,000 
(v) 500.0 
(vi) 2.0034
Round up the following upto three significant figures: 
(i) 34.216 
(ii) 10.4107 
(iii) 0.04597 
(iv) 2808
Express the following in the scientific notation: 
(i) 0.0048 
(ii) 234,000 
(iii) 8008 
(iv) 500.0 
(v) 6.0012
What do you mean by significant figures?
What is the SI unit of mass? How is it defined?