Question:
Can atoms of an element exist independently? Give examples of elements which exist in atomic form. Give examples of elements that do not exist in atomic form.
Answer:
Atoms of most elements do not to exist independently. There are few elements like Helium (He), Argon (Ar), Krypton(Kr), Radon(Rn), Neon(Ne) and Xenon(Xe) - collectively called noble gases, exist at atomic form. Atoms of other elements combine together and form molecules and
ions. These molecules or ions aggregate in large numbers to form the matter. e.g.
- Hydrogen (H2), Oxygen(O2), Nitrogen(N2) etc. atoms of the same element combine together and form molecules.
- Water (H2O), Carbon Dioxide(CO2), Common Salt(NaCl) etc. atoms of different elements combine together and form molecules.
- Na+, Cl-, Cu2+ etc. atoms of elements form ions in aqueous solutions.
Atoms and Molecules
Q 1.
Define 'formula mass' of a compound.
Q 2.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 3.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 4.
Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
Q 5.
What do we call those particles which have more or less electrons than the normal atoms ?
Q 6.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 7.
Who proposed Law of Definite Proportions (or Law of Constant Composition)?
Q 8.
What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na
2SO
3)?
Q 9.
What is gram-atomic mass of an element?
Q 10.
Convert 12 g of oxygen gas into moles.
Q 11.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 12.
What is the atomic mass unit?
Q 13.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 14.
Give one major drawback of Dalton's atomic theory of matter.
Q 15.
How many atoms are there in 0.25 mole of hydrogen ?
Q 16.
What is the law of conservation of mass?
Q 17.
What is the chemical symbol for iron?
Q 18.
Based on type of substance, how molecules are classified?
Q 19.
Give four examples of diatomic molecules.
Q 20.
Name one element each which forms diatomic and tetra atomic molecule.
Q 21.
Find out number of atoms in 15 moles of He.
Q 22.
What is the mass of 0.2 mole of oxygen atoms ?
Q 23.
If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.
Q 24.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 25.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 26.
Fill in the blanks:
The atomicity of sulphur S
8 is ________.
Q 27.
Based on atomicity, how molecules are categorized?
Q 28.
What is the molecular mass of a substance?
Q 29.
What are ionic compounds?
Q 30.
Name one element which forms diatomic and triatomic molecule.
Q 31.
Define mole. What is its significance?
Q 32.
Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.
Q 33.
Calculate the molecular masses of the following :
(a) Hydrogen, H
Q 34.
State whether the following statements are true or false :
(a)A sodium ion has positive charge because it has more protons than a neutral atom
(b)A chloride ion has negative charge because it has more electrons than a neutral atom.
Q 35.
What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.
Q 36.
How many atoms are present in one gram atomic mass of a substance ?
Q 37.
Calculate the mass of 3.011 x 10
atoms of carbon
Q 38.
State Law of constant proportions. Explain with an example.
Q 39.
Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Q 40.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 41.
Name the element having following Latin names
(i) Stibium
(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
Q 42.
What is the significance of a chemical symbol?
Q 43.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 44.
Define molecule. What are its important properties?
Q 45.
What are polyatomic ions? Give examples?
Q 46.
Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(
Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Q 47.
Name the unit in which the radius of an atom is usually expressed.
Q 48.
Calculate the molecular mass of hydrogen bromide (HBr).
(Atomic masses: H=1; Br= 80 U)
Q 49.
What is the numerical value of Avogadro number ?
Q 50.
What name is given to the number 6.022 x 10
