1.  Define the following terms:

(a)  Valency shell   (b)  Radioactivity

(c)  Atomic mass    (d)  Nucleons

(e)  Electronic configuration

Ans.  (a)  Valency Shell :  the outermost shell of an atom is known as its valency shell.

(b)  Radioactivity  :  The phenomenon due to which certain elements spontaneously emit highly pentraiting rays made of sub-atomic particles is known as radio activity.

(c)  Atomic mass :  The atomic mass of an element is the number of times by which the mass of its atom is heavier than 1/12th of mass of carbon atom.

(d)  Nucleons :  The protons and neutrons collectively are known as nucleons.

(e)  Electronic configuration :  The distribution of electrons in the various or bits of the atom of an element is called electronic configuration of that element.

2.  Mention briefly the salient features of Dalton's atomic theory.

Ans.  Salient features of Dalton's atomic theory:

(i)  Matter consists of very small and indivisible particles called atoms, which can neither be created nor can be distroyed.

(ii)  The atoms of an element are alike in all respects but they differ from the atoms of other elements.

(iii)  Atoms of an element combine in small numbers to form molecules of the element.

(iv)  Atoms of one element combine with atoms of another element in simple numerical ratio to form molecules of compounds.

(v)  Atoms are the smallest units of matter that can take part in a chemical reaction.

3.  Describe Rutherford's atomic model.

Ans.  In 1911, Lord Rutherfor, a scientist from New- Zealand conducted an experiment in order to find the arrangement of electrons and protons in an atom.

     Rutherford bombarded a thin sheet of gold with alpha particles in an evacuated chamber.  It lead to discovery of small, positive charged nucleus in the centre of an atom.

4.  What are the observations of the experiment done by Rutherford in order to determine the structure of an atom?

Ans.  Following were the observations made by Rutherford:

(i)  Most of the alpha particle passed straight through the foil without any deflection from their path.

(ii) A small fraction of them were deflected from their original path by small angles.

(iii)  Only a few particles bounced back.

5.  Answer the following questions:

(a)  What are isotopes?  How  does the existence of isotopes contradict Dalton's theory?

(b)  What is variable valency?  Name two elements having variable valency and state their valencies.

Ans.  (a)  Isotopes :  The atoms of an element having the same atomic number but different mass number are called the isotopes that elements.

     Each element has its unique atomic number i.e. each atom has the same number of protons in its nucleus.  But mass number of all atoms may not be the same.  Isotopes are the atoms of the same element with same atomic number but a different mass number due to the different in the number of neutrons.

(b)  Variable valency :  Some elements exhibit more than one valency.  They are said to have variable valency. eg: Iron, copper, tin, lead.

Iron         Fe     Fe²⁺  or  Fe³⁺

Copper  Cu     cu⁺   or   cu²⁺

6.  The atomic number and the mass number of sodium are 11 and 23 respectively.  what information is conveyed by this statement.

Ans.  Atomic number =11; No of protons = 11

Mass number = 23, Number of protons + Number of neutrons.

No of neutrons = 23-11=12

7.  Explain the rule according to which electrons are filled in various energy levels.

Ans (i)  The maximum number of electrons that can be present in any shell or orbit of an atom is given by the formula 2n² where is the serial number of the shell.  The first shell can accomodate not more than 2*1²=2, The second 2*2²=8 electrons.

(ii)  Another rule known as octet rule, determines the actual number of electrons present in its outermost shell.  It states that "The maximum number of electrons that the outermost shell of an electrically neutral and chemically stable atom can have is 8 but in case of hydrogen it is 2.

8.  Answer the following questions:

(a)  What are radioactive substances?  Give two examples. 

(b)  What are the three types of radioactive radiations? Mention two properties for each of them

(c)  What is a nuclear reactor?  State any two precautions taken in running a nuclear reactor.

(d)  Why can not nuclear fusion be used to generate electricity?

Ans.  (a)  Radioactive Substances :  The substances which emit radioactive rays are called radioactive substances eg. uranium,thorium.

(b)  Types of radio active radiations :

(i) Alpha rays : (1)  These rays consists of positively charged particles.

                            (2)  These are slightly affected by magnetic and electrical fields.

(ii)  Beta rays :  (1)  These rays consists of 1 unit negative charge and have mass equivalent to the                 mass of an electron.

                             (2)  The velocity of the alphs particles is equal to the velocity of light

(iii) Gama rays :  (1)  These are electromagnetic radiations

                               (2)  They have neither mass nor charge

(c)  Nuclear reactor :  Nuclear reactors are the nuclear power plants which are set up to generate electericity.

Precautions :  (i)  The nuclear reactor must be embedded in special concrrete walls.

                          (ii)  Workers in a nuclear power plant must wear lead- lined aprons and gloves.

     Nuclear fusion take place only at extremely high temperature and pressure.  We have no means to produce such high temperature and pressure.  This process is not used to generate electricity.