Study Portal 11 – Chapter 11

Periodic Table and

Periodicity of Properties

 

Q1.      Fill in the blanks with suitable words.

  1. i) Triads were proposed by
  2. ii) Mendeleeve in 1869 classified the all known elements in to groups and is regarded as creator of modern periodic table.

iii)     Modern periodic table has 18 groups and 7 periods.

  1. A period is a horizontal row of elements in the periodic table.
  2. Halogen constitutes the 17 groups of periodic table.
  3. The concept of electro negativity was given by
  • Smaller atoms have more ionization energy.
  • D- Block elements are also known as transition elements.
  1. The size of atoms decreases in the periods from left to right.
  2. Noble gases constitute the group 18 of periodic table.

Q2.      Choose the correct answer.

  1. i) In a group the elements have almost same:

(a) physical characters

(b) chemical characters

(c) both physical and chemical

(d) neither physical nor chemical.

  1. ii) Group 1 members are generally known as:

(a) Alkali metals               (b) Alkaline earth metals

(c) transition metals           (d) noble gasses.

  • At ordinary temperature and pressure, hydrogen exists as (a) liquid (b) gas             (c) solid           (d) plasma.
  1. Periodicity is seen in the periodic table in

(a) groups                                      (b) periods

(c) both in groups and periods

(d) neither in groups nor in periods.

  1. The atomic size increases in:

(a) group                            (b) period

(c) in both of above          (d) none of above.

Q3.      Answer the following questions in short.

  1. i) How Dobriener classified elements? Why his classification did not received popularity?

Ans.  Dobriener in 1820 classified the elements into triads. Since very few elements could be arranged in the form of triads so, this classification did not received popularity.

  1. ii) What is the resemblance between Newlands classification elements and music?

Ans.     Newlands put forward his observations in the form of ‘Law of octaves’ He noted that there was a repetition in chemical properties of every eight element, if they were arranged by their increasing atomic masses. He compared it with musical notes.

  • Why in the history of periodic table the name of Mendeleeve is so important?

Ans.     A very important classification was presented by Mendeleeve in 1869 which was based upon the atomic masses of the elements, and after the discovery of protons, the classification based upon the number of protons built up, which finally resulted in a comprehensive periodic table and this classification make very easy the understanding the properties of elements.

  1. iv) Describe the importance of group in periodic table.

Ans.     In groups, elements show similarities in their chemical properties due to some valence shell electronic configuration how in groups physical properties changes gradually down the groups due to increase in atomic size.

  1. How many periods are there in modern periodic table?

Ans.     In the modern periodic table the rows are called periods. There are seven periods in modern periodic table.

  1. What are alkali metals?

Ans.     Elements of group 1st have one electron in there valence shell; they are given the family name of Alkali metals.

  1. Discuss importance of noble gasses.

Ans.     In periodic table the elements of group 18 is called noble gasses. This group also called zero groups. He, Ne, Ar, Kr, Xe, Rn are his members. Eae of noble gasses is close to zero.

  • What do you understand by the terms “block of periodic table” what types of blocks are there in modern periodic table?

Ans.     The name of blocks depends upon the outermost electronic arrangement of the element and is very important in determining the chemical properties of the element. The periodic table has been divided four blocks i.e. S-block, p-block, d-block and f-block.

  • What is ionization energy? How it varies across modern periodic table?

Ans.     It is the energy to remove the least bonded last electron from an atom. The energy required to remove the last electron is called 1st ionization potential, for 2nd last electron it is called 2nd ionization potential and so on. The ionization energy depends upon.

The size of atom:-  Greater the size less will be the ionization potential.

The nuclear charge:-  Higher is the nuclear greater more will be the ionization potential.

The shielding effect:- The electron in the outermost energy level experience more inter electron repulsion, so greater shielding less will be the ionization potential.

The filled and half filled orbit:-  Filled and half filled orbital have higher ionization energy.

  1. Define periodicity of elements in modern periodic table.

Ans.  The gradual changing in properties of elements is termed as periodicity. A greater advantage of periodic table is periodicity, i.e. the elements show a gradual increase or decrease in physical and chemical properties, e.g. in atomic radius, ionization energy, electron affinity and electro negativity oxides etc.

Q4.      Answer the following question with reasoning.

  1. i) What is the relation between electronic configuration and the positioning of elements in periodic table?

Ans.  As far as the electronic configuration of elements is concerned, a period in the table represents the completion of a shell. For example after the completion of second shell at neon (10Ne) the third period states with next element Sodium (11Na)

  1. ii) How atomic size is related with the nuclear charge?

Ans.  The size of atoms or their radii increases from top to bottom in a group. It is because a new shell of electrons is added up in the successive period, which decreases the effective nuclear charge.

iii)     How electron affinity varies in periodic table?

Ans.  The electron affinity is the energy change when an electron is added to a gaseous atom. Generally the element liberates energy when electron is added to them. The value of energy released greatly depends upon the size and nuclear charge of that element. Thus the atoms, whose anions are relatively more stable than neutral atoms, have smaller electron affinity energy.

  1. iv) How electro negativity is important for chemist?

Ans.  The ability of an atom to attract the shared pair of electrons towards itself in a molecule is called electro negativity. It is an important property especially when covalent type of bonding of element is under consideration. It increases in a period from left to right because higher Zeff shortened distance from the nucleus of the shared pair of electron.

  1. v) Discuss inertness of noble gasses.

Ans.  In 1894, Raleigh and Ramsay succeeded in separating a new gas from air and named it Argon. During the next ten years, five more such gasses resembling argon were discovered. These were helium, neon, krypton, xenon and radon. Each one of them was element and they did not react chemically with other elements none of their compounds could be prepared. Hence they were named inert gasses in 1962; however some compounds of xenon were prepared. Hence their name, inert gases was changed to noble gasses.

 

Q5.   Why it is necessary to classify the elements? Explain with reasoning.

Ans.  Many elements had already been discovered till the beginning of nineteenth century. Some these elements showed similarities in their characteristics. There was a need to arrange these elements based on the similarities in their properties in the beginning, some efforts were made to group the elements together on the basis of their physical states at ordinary temperature i.e. solid, liquid and gas. Similarly elements were grouped as metal and non metals. These classifications are still useful. With the increase in the number of elements which were being discovered, renewed attempts were made to group them on the basis of the similarities in their physical and chemical properties. Such grouping of elements was useful. In 1829 Dobriener classified chemically, similar elements in group of three on the basis of their atomic masses. These groups were called triads. Up to 1860 the number of elements discovered rose to about sixty.

In 1864 Newlands stated that if the elements were arranged in the ascending order of their atomic masses every eight elements will have properties similar to the first this arrangement was named of Octaves.

In 1869, Loather Meager, tried to remove the defects in Newlands’ table and plotted a graph between atomic volume and atomic mass of elements. His graph included about fifty elements and there was seen a periodicity in its curves. The curves obtained show similarity in the properties of elements after definite. This is called periodicity.

Q6.   Discuss the importance of groups and periods in the modern periodic table.

Ans.  A period is a horizontal row in the periodic table of the elements. There are 7 periods in the periodic table and the number of period shows actually the numbers of shell in that particular row of elements. It means the elements in period-1 have just one shell, and the elements in 2nd periods have two shells and so on. From the 6th and 7th periods two rows are withdrawn and are written below the others, because their properties are remarkably similar with each other, these series are termed as “Lanthanides” and “Actinides” series. In the modern periodic table the columns are called groups. There are eighteen groups in the modern periodic table. The groups are named by the numbers beginning from 1 to 18. Group “1” is called alkali metals, Group “2” is alkaline earth metals, group 17 is called Halogens and group ‘18’ is called noble gases. In a group the members show many same properties like same last electronic configuration, same valencies and same type of chemical characters.

Q.7.  What do you understand by periodicity of properties? Explain with examples.

Ans.  A great advantage of the periodic table is the periodicity, i.e. the elements show a gradual increase or decrease in physical and chemical properties, e.g. in atomic radius, ionization energy electron affinity, and electro negativity, oxides etc.

Atomic Size:- Atomic size or atomic radius is half of the distance between the nuclei of a disatomic molecule. The atomic radius is influenced by 3 main factors, (i) Energy Level (ii) Nuclear Change and (iii) Shielding effect. Atomic size decreases along a period from left to right and increases down a group.

Ionization Energy:- It is energy to remain the least bonded last electron from an atom. The energy required to remove the last electron is called 1st ionization potential (I.P), for 2nd last electron it is called 2nd ionization potential and so on the ionization energy depends upon: (i) The size of atom, (ii) The nuclear charge, (iii) The shielding effect and (iv) The filled and half filled orbits. The ionization potential decreases down the group and increases along a period.

Electron Affinity:-  The electron affinity is the energy change when an electron is added to a gaseous atom. Generally, the element liberates energy released greatly depends upon the size and nuclear charge of that elements. Electron affinity energy (Eae) generally increases across a period since the radius slightly decreases.

Shielding Effect:- The shielding effects is the effect of electrons between the nucleus and outer most valance shell electrons of an atom. The shielding effect increases down the group in the periodic table, but across a period it remains unchanged.

Electro negativity:- The term electro negativity is defined as the ability of an atom to attract shared electron pair of bond in a compound. The electro negativity of an atom is the net effect of electron affinity and first ionization potentials. In the periodic table electro negativity values decreases down the group and increases along the period.

Q.8.     Write a brief note on evaluation of periodic table?

Ans.  Chemistry deals with the study of matter or the elements, so for 118 elements have been discovered, if they are studied separately it would be a difficult job for any one so there is a need of classifying such a huge number of elements. As the number of discovered elements increased, the need to classify them also increased, because it became more difficult to remember the characteristics of the elements individually.

The early classifications were full of errors but with the passage of time, the more perceived classification was presented by various scientists. In 1820 Dobriener classified the elements into triad. In 1865 Newland presented the concept of classifying the elements based on law of Octaves. A very important classification was presented by a Mendeleeve in 1869 which was based upon the atomic masses of the elements and after the discovery of protons, the classification based upon the number of protons built up which finally resulted in a comprehensive periodic table called the modern periodic table.

Q.9.     What is modern periodic law? Explain with examples.

Ans.  In 1913 H.Moseley discovered a new property of the elements, i.e. atomic number. He observed that atomic instead of atomic mass should determine the position of element in the periodic table and accordingly the periodic law was amended as “Properties of the elements are periodic function of their atomic numbers”. Atomic number of an element is equal to the number of electrons in a neutral atom. So atomic number provides the basis of electronic configurations as well.

Q10. What do you know about the blocks of the modern periodic table?

Ans.  On the basis of completion of a particular sub shell elements with similar sub shell electronic configuration are referred as a block of elements. There are four blocks in the periodic table named after the name of the sub shell which is in the process of completion by the electrons. There are S,P,d and f blocks.

S-Block-Elements:- these are the elements in which valence electrons occupies S-sub shells. Elements of group 1 and 2 and Helium are collectively called S-block elements.

P-Blocks-Elements:-  Elements of group 13, 14, 15, 16, 17 and 18 have their valence electron in P-sub shell. There fore they are referred to as P-Block elements.

D-Block-Elements:- d-block-elements lies between S and P-block elements in period number 4, 5 and 6 in the periodic table-Elements of group 3 to group 12 are d-block elements. So, there are 10 groups of d-block elements.

F-Block-Elements:- F-block-elements lies separately at the bottom of the periodic table. There are two rows of these elements, first row is called lanthanide and the second row is called actinides. F-block-elements are collectively called as inner-transition elements.

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