ATOMS AND MOLECULES

Atom – An atom is the smallest particle of an element that can take part in a chemical reaction.

-         Atoms are the building blocks of all the matter around us.

-         Atoms are so small in size, that they cannot be seen even under the most powerful light microscope.

-         The size of an atom is indicated by its radius, which is called atomic radius.

-         Atomic radius is measured in nanometers (nm). 1 nm = 10^-9 m

Element-Any substance that contains only one kind of an atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P₄) or sulfur (S₈) cannot be broken down into simpler substances by these reactions.

Compounds-Elements combine to form chemical compounds that are often divided into two categories.(i)ionic (ii)Covalent

Atomic mass – According to Dalton’s atomic theory, each element has a characteristic atomic mass.

One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12^th) the mass of one atom of carbon – 12.

 The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12^th the mass of one carbon-12 atom.

 Molecule – A molecule is the smallest particle of an element or a compound that is capable of an independent existence and show all the properties of that substance.

 The number of atoms constituting a molecule is known as its atomicity. Thus the atomicity of hydrogen is 2, sulphur is 8, ozone is 3, phosphorus is 4, etc.

Molecular mass – The relative molecular mass of a substance is the average mass of its molecule as compared with the mass of a carbon-12 atom taken as 12 units.

-         The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

 Formula unit mass – An ionic compound is made up of an extremely large number of cations and anions. e.g. Sodium chloride is made up of a large (but equal) number of Na⁺ and Cl^- ions so the actual formula of sodium chloride should be (Na⁺Cl^-)_n but for simplicity we represent it by NaCl. So NaCl is the simplest formula of sodium chloride and not its actual formula.

The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound.

-         e.g. formula unit mass of NaCl is 23 x 1 + 35.5 x 1 = 23 + 35.5 = 58.5u.

A mole is defined as the amount of a substance that contains as many particles (atoms, molecules or ions) of the substance as there are atoms in exactly 12 g of carbon-12.

In other words, a mole of a substance is that amount of the substance that contains 6.022 x 10²³particles of the substance.

-         The atomic mass of an element expressed in grams is called gram atomic mass of the element. 

-         The molecular mass of a substance expressed in grams is called gram molecular mass of the substance.

-         Gram atomic mass or gram molecular mass also represents molar mass of the substance.

-         The mass of 1 mole of a substance is called molar mass of the substance.

           1 mole = 6.022 x 10²³ particles = Gram atomic mass (or gram molecular mass)

Q1 State law of conservation of mass.

Ans The mass can neither be created nor destroyed in a chemical reaction.

Q2  State law of constant proportion.

Ans A pure chemical compound always consists of the same elements that are combined together in a fixed proportion by mass.

Q3  Write name and symbol of first 20 elements of the periodic table.

Ans   Atomic number       Name of the element      Symbol

                        1                          Hydrogen                        H

        2                          Helium                             He

        3                          Lithium                            Li

        4                          Beryllium                         Be

                        5                          Boron                               B

                        6                          Carbon                             C

                        7                          Nitrogen                           N

                        8                          Oxygen                             O

                        9                          Fluorine                             F

                        10                         Neon                                Ne

                        11                        Sodium                             Na

                        12                       Magnesium                       Mg

                        13                        Aluminium                       Al

                        14                        Silicon                              Si

                        15                        Phosphorous                    P

                        16                        Sulphur                            S

                        17                        Chlorine                           Cl

                        18                        Oxygen                            O

                        19                        Potassium                         K

                        20                        Calcium                            Ca

Q4  Define atomic mass unit.

Ans: The mass of one twelfth (1/12) of the mass of one atom of carbon taken as 12.

Q5   What are molecules?

Ans:  Molecules represents a group of two or more atoms(same or different) chemically  bonded to each other and held tightly by strong attractive forces.

Molecules are of two types

(a) Molecules of elements

(b) Molecules of compound

Q6 What do you mean by molecular mass?

Ans: The average relative mass of the molecule as compared to the mass of carbon takrn  as 1u.

Q7  Write the relationship between number of moles and atomic mass.

Ans:     Number of moles=given mass/gram atomic mass

Q8 Why are chemical reactions according to law of conservation of mass?

Ans:  In all chemical reactions, there is only exchange of reactants taking place when  products are formed. Since there is no loss or gain of mass, the chemical reactions are  according to law of conservation of mass.

Q9 What is basic difference between atoms and molecules?

Ans:  Atoms except those of noble or inert gas elements cannot exist of their own.  However ,all molecules can have independent existence.

Q10 The atomic mass of an element is in fraction .What does it mean?

Ans:  If the atomic mass of an element is in fraction, this mean that it exists in the form of  isotopes. The atomic mass is the average atomic mass and is generally fractional.

Q11 What is the difference between the mass of molecule and molecular mass?

Ans:  Mass of a molecule is that of a single molecule also known as its actual mass. But  molecular mass is the mass of Avagadro’s number of molecules.

Q12 Where do we use the words mole and mol?

Ans:  In the text part we use the word mole while as a  unit ,we call it mol.

Q13 How many moles are present in 11.5 g of sodium?

Ans:  Gram atomic mass of Na =23g

         23g of Na represents = 1 mol

11.5g of Na represents= (1mol)*(11.5g)/23.5g)=0.5 mol

Q14 Explain why the  number of atoms in one mole of hydrogen gas is double the  number of atoms in one mole of helium gas?

Ans: Hydrogen gas is a diatomic in nature(H2) while helium gas is monoatomic (He).As a  result, the number of atoms in one mole of hydrogen(2*NA )are expected to be double as  compared to number of atoms in one mole of helium(NA)

Q15An element Z forms an oxide with formula Z₂O₃. What is its valency?

Ans:Valency of Z=3                                                                                             

Q16The valency of an element A is 4 . Write the formula of its oxide.

Ans:  The formula of its oxide is A₂O₄ or AO₂.

Q17  An element X has valency 3 while the element Y has valency 2.Write the formula  of the compound between X and Y .

Ans:  X₂Y₃

Q18 What are ions?

Ans:  Ions are of two types

(a) cations  - positively charged

(b) anions  -  negatively charged

Q19  Which postulate of Dalton’s Atomic theory is the basis of law of conservation of  mass?

Ans:  “ Atoms can neither be created nor destroyed during a physical or a chemical  change”

Q20 Write the formulae of sodium oxide and aluminium chloride

Ans:   Sodium oxide = Na₂O          Aluminium chloride =AlCl₃

Q21  Find out the ratio by mass of the combining elements in the following  compounds(a) MgCO₃     (b)  CH₃OH     (c) CaCl₂         

Ans: (a) MgCO3  ------   Mg :C:O  = 24:12:48 = 2:1:4

(b) CH3OH    ------ C : H : O =12:4:16  =3:1:4

(c) CaCl2          ----------- Ca :Cl = 40:71

Q22  Define atomicity of the molecule.

Ans:  Atomicity of the molecule is the number of atoms present in the molecule.

Q23  What is the atomicity of  oxygen,ozone,neon and sulphur?

Ans :    Oxygen O2  = 2 

            Ozone O3  = 3

Neon  Ne  =1

Sulphur S8 =8

Q24 What is wrong with the statement '1 mol of hydrogen'

Ans:  The statement is not correct.We must always write whether hydrogen is in atomic  form or molecular form. The correct statement is :   1mole of hydrogen atoms or one mole of hydrogen molecules .

Q25 The atomic mass of an element is in fraction. What does it  mean?

Ans:  If the atomic mass of an element is in fraction, this mean that it exists in the form of  isotopes. The atomic mass is the average atomic mass and is generally fractional.

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