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.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 2.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 3.
What is gram-atomic mass of an element?
Q 4.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 5.
Define 'formula mass' of a compound.
Q 6.
What are polyatomic ions? Give examples?
Q 7.
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Q 8.
What are the postulates of Dalton's atomic theory?
Q 9.
Which of the following statements is NOT true about an atom?
(a) Atoms are the building blocks from which molecules and ions are formed.
(b) Atoms cannot exist independently.
(c) Atoms are neutral in nature
(d) Atoms combine together to form matter that we can see, feel or touch.
Q 10.
Give four examples of diatomic molecules.
Q 11.
Fill in the blanks:
Atomic radius is measured in __________.
Q 12.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 13.
What is the chemical symbol for iron?
Q 14.
What are the building blocks of matter ?
Q 15.
(a) What is meant by 'a mole of carbon atoms' ?
(b)(b) Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.
(Atomic masses : A1 = 27 u ; Fe = 56 u)
Q 16.
What is the law of conservation of mass?
Q 17.
Who proposed Law of Definite Proportions (or Law of Constant Composition)?
Q 18.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 19.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 20.
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q 21.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 22.
Define mole. What is its significance?
Q 23.
Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.
Q 24.
An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?
Q 25.
How many atoms are there in 0.25 mole of hydrogen ?
Q 26.
Fill in the blanks:
Atoms can be observed using ____________ Microscope.
Q 28.
What are ionic compounds?
Q 29.
What is meant by atomicity ? Explain with two
Q 30.
Work out the formula for magnesium hydrogencarbonate.
Q 31.
Calculate the number of molecules in 4 g of oxygen.
Q 32.
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 33.
Fill in the blanks:
According to Dalton's atomic theory, atoms of different elements differ in ______, size and chemical properties.
Q 34.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 35.
Calculate the number of molecules of sulphur (S
8) present in 16 g of solid sulphur.
Q 36.
What is meant by the symbol of an element ? Explain with examples.
Q 37.
Calculate the molecular masses of the following compounds :
(a) Methanol, CH
Q 38.
Calculate the molecular mass of hydrogen bromide (HBr).
(Atomic masses: H=1; Br= 80 U)
Q 39.
What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.
Q 40.
What is the numerical value of Avogadro number ?
Q 41.
How many atoms are present in one gram atomic mass of a substance ?
Q 42.
Calculate the mass of 3.011 x 10
atoms of carbon
Q 43.
(a) Define gram atomic mass of a substance.How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?
(i)Al
(ii) co
2(iii) C1
(iv) H
(p)A1
Q 44.
(a) What is meant by the 'molar mass'of a substance ? State the unit in which molar mass usually expressed.
(b) Calulate the molar masses of the following substances. Write the results with proper units
(1) Ozone molecule, O
Q 45.
Fill in the blanks:
Mass of 2 Hydrogen atoms is __________.
Q 46.
Name the international organization who approves names of elements.
Q 47.
Name the element having following Latin names
(i) Stibium
(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
Q 48.
Define molecule. What are its important properties?
Q 49.
Fill in the blanks:
An element has only one type of __________.
