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21 Chemistry -- Classification of Elements and Periodic Table

1.    Name the type of bonds which are present in ice. 2.    Explain O-hydroxy Benz aldehyde and P-hydroxy Benz aldehyde, which is more soluble in water and why?3.    Lone pairs of electrons present on an atom in any molecule affect its geometry. Explain with an example.4.    Dis...

1.   
Name the type of bonds which are
present in ice.

2.   
Explain O-hydroxy Benz aldehyde
and P-hydroxy Benz aldehyde, which is more soluble in water and why?

3.   
Lone pairs of electrons present on
an atom in any molecule affect its geometry. Explain with an example.

4.   
Discuss the formation of KCl and
CCl4 molecules on the basis of electronic theory of valency. Give
any two characteristics of ionic and covalent bond.

5.   
What is a hydrogen bond? What
are the factors which result on the effective hydrogen bonding? Give one
example each of intermolecular and intramolecular hydrogen
bonding.

6.   
Define dipole moment? What is
its unit? Can its magnitude give an idea about the structure of the
molecule? Explain briefly.

7.   
What is resonance? Are the
resonance structures be the actual structure of the molecule? Draw resonance
structure of ozone (O) and Sulphur trioxide (SO).

1 Answers

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1. Name the type of bonds which are present in ice.

Ans: The type of bond present in ice are: 1. Covalent bond    2. Hydrogen bond

2. Explain O-hydroxy Benz aldehyde and P-hydroxy Benz aldehyde, which is more soluble in water and why?

Ans: P-hydroxy benzaldehyde is more soluble in water than o-Hydroxy benzaldehyde, because P-Hydroxy benzaldehyde can form hydrogen bonds with water molecules resulting in enhanced solubility.


3.   Lone pairs of electrons present on an atom in any molecule affect its geometry. Explain with an example.


Ans: If the central atom of a molecule has lone pairs along with bond pair of electrons, the ideal geometry is distorted to give different bond angles than expected to obtain minimum repulsion. This is because lone pair occupies more space around the central atom than the bond pair; since the lone pair is attracted by one atom only while bond pair is shared between two nuclei. 
Hence, repulsion of electron pairs around the central atom is in the order of  l.p--l.p > l.p--b.p. > b.p--b.p



For example: take NH3. 
Total e.p. in central atom Nitrogen=4
Expected shape                                = Tetrahedral (109.5) 
Actual shape in nature                    =Distorted Tetrahedral (107.5<109.5)
                                                              i.e Trigonal Pyramidal Shape

4. Discuss the formation of KCl and CCl4molecules on the basis of electronic theory of valency. Give any two characteristics of ionic and covalent bond.

Ans: The Electronic theory of valency states that atoms of elements(which are not noble gases) tend to acquire the nearest noble gas configuration by either transfer or sharing of electrons.

KCl is an ionic molecule formed as: 
19K= 2,8,8,1    17Cl = 2,8,7 

Potassium donates the extra valence electron and attains stable configuration of Argon, and Positive charge. Chlorine accepts the electron and also attains the stable configuration of Argon and a negative charge. 


Formation of CCl4

6C=2,4    17Cl = 2,8,7 
Carbon requires four more electrons for stable configuration. Chlorine too requires one electron in its valence shell to gain stable configuration. So, both carbon and chlorine mutually contribute one electron each, and share it between them. This way, covalent bond is formed.

Characteristics of ionic bond:
- It is the electrostatic force of attraction between cation and anion.
- It is non-directional.
Characteristics of covalent bond:
- It is the chargeless sharing of electrons.
- It is directional in nature.

5.What is a hydrogen bond? What are the factors which result on the effective hydrogen bonding? Give one example each of intermolecular and intramolecular hydrogen bonding.

Ans. Hydrogen bonding is a special type of dipole-dipole attraction between molecules. It results from the attractive force between a hydrogen atom(covalently bonded to a very E.N. atom such as a N, O, or F atom) and another very electronegative atom( N,O or F).

The factors for effective hydrogen bonding are: 
1. Electronegativity of the atom must be high.
2. Atomic size of the E.N. atom must be small. So, only F,O and N are able to make H-bond.

Intermolecular hydrogen bonding:

- Water (H₂O)
 

- Alcohol (R-OH) and water (H₂O)



Intramolecular Hydrogen Bonding:

- o-nitrophenol


6.   Define dipole moment? What is its unit? Can its magnitude give an idea about the structure of the molecule? Explain briefly.

Ans. When atoms in a molecule share electrons unequally, they create what is called a dipole moment. 
Dipole moment is a vector quantity given by μ= qxd    (partial charge in either atom x Bond length of molecule)
Its unit is  Cm / esu cm.
The larger the difference in electronegativity, the larger the dipole moment. Thus, the magnitude of the dipole moment will give the idea about the structure. 
-If the magnitude of dipole moment is high, then, the shared E.P is pulled more towards the E.N. atom. and has a bulky structure at one side. 
-If magnitude is small, it is vice versa. 
Eg:
 


7.   What is resonance? Are the resonance structures be the actual structure of molecules? Draw resonance structure of ozone (O) and Sulphur trioxide (SO).

Ans- Resonance is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by a single Lewis formula. A molecule or ion with such delocalized electrons is represented by several resonance structures.

When it is possible to write more than one equivalent resonance structure for a molecule or ion, the actual structure is the average of the resonance structures. But resonance structures represent different ways of placing electrons on the atoms in a molecule's Lewis dot structure. They do not describe different molecules and all resonance structures have the same connectivity. If you change the connectivity, you change the molecule, and that is not a resonance structure.

Resonance Structure of Molecules 

Ozone(O)


Sulphur Trioxide(SO)




       



More questions on Classification of Elements and Periodic Table

How does electronegativity differ from electron affinity? Arrange the following in the order of increasing electronegativity: M, M-, M+  .

Ans: Electronegativity and electron affinity differs from each other in the following ways:

Electronegativity Electron Affinity
  • It is the tendency of an atom of element to attract bond pairs of electrons towards itself.
  • It is amount of energy released when an isolated gaseous state atom gain an electron.
  • It is applied only to a single atom.
  • It is either applied to an atom or a molecule.
  • Electronegativity is qualitative.
  • It is quantative.

According to increasing electronegativity: M-...

Sodium and Mg+ ion have same number of electrons but removal of electron from Mg+ requires more energy than sodium. Explain. 

Ans: Though sodium andMg+ion have same number of electrons the atomic size of Mg+is smaller than Na. Since, smaller the size, the greater is the nuclear force of attraction for valance electrons due to which removal of electron from Mg+requires more energy than sodium.

 Which of the following electronic configuration would you expect to have the lowest ionization energy and why?a)1S2, 2S2, 2P6b) 1S2, 2S2, 2P5c) 1S2, 2S2, 2P6,3S1

Ans:As we know the elements having half-filled and full filled electronic configuration are stable and they need more energy to move the electron and have higher ionization energy. So, (a) and (c) have higher ionization energy because they have fulfilled and half-filled electronic configuration respectively and (b) has lowest ionization energy due to having unstable electronic configuration.

Size of the cation is smaller than the size of parent atom. Explain.

Ans: Size of cation is smaller than the size of parent atom because as parent atom loses electron and form cation where number of proton becomes more than number of electron and attractive force increases so radius decreases.

Size of the anion is larger than the size of parent atom. Explain.

Ans: The size of anion is larger than the size of parent atom because anions are formed due to gain of electrons. When the electrons increases, there are still the same number of protons. This causes the electrons to not be bound tightly to the nucleus like earlier and results in an increase atomic size.

Electron gain enthalpy of fluorine is unexpectedly less than that of chlorine. Give reason. 

Ans: Fluorine has very small atomic size so there is more inter electronic repulsion. But chlorine has larger size and repulsion between the electron of chlorine is very less. Hence, it can easily gain electron but fluorine can't. So electron gain enthalpy of fluorine is unexpectedly less than of chlorine.

 State modern periodic law. Why is modern table superior to Mendeleev's periodic table?

Ans: Modern periodic law states that, " The physical and chemical properties of elements are the periodic function of their atomic numbers."

Modern periodic table does correct the defects of Mendeleev's Periodic table. It is based on atomic number which is a more fundamental property of the atom. It removes the anomalyabout the position of isotopes in the Mendeleev's periodic table. It also removes the other anomaliesfound in Mendeleev's periodic table like grouping of chemically dissimilar...

A student reported that the radii of Cu, Cu+ and Cu++ are 96 pm, 122 pm and 72 pm respectively. Do you agree with the reported values? Justify the answer.

Ans: No, I don't agree with reported values. The measurement is made wrongly. More the positive charge more will be the nuclear attraction and outermost electron is more attracted towards the nucleus and the atomic radius is less. So Cu++< Cu+< Cu should be the order. But the measurement shows Cu+> Cu > Cu++ which can not be true.

 Electron affinity of alkaline earth metal and noble gases are almost equivalent to zero. why?

Ans: Since alkaline earth metal and noble gases are stable elements due to having full or half filled electronic configuration. So, they can't easily gain any electron. Therefore their electron affinity are almost equivalent to zero.

Define isoelectronic species.

Ans: Isoelectronic species are theatoms or ions that have same number of electrons.


Why the first ionisation energy of sodium is higher than that of magnesium?

Ionisation energy of an element depends on several factors including the atomic size, effective nuclear charge, electronic configuration, penetration of electrons and so on.

But in case of Sodium and magnesium, the first ionisation energy of Sodium is higher because as the atomic size of sodium is comparatively smaller than Magnesium so it experiences strong effective nuclear charge hence, more amount of energy is required to release valence electron in its ground state.

Is there any trend for ions describing either the increasing or decreasing hydration energy ? If yes do explain . 

Which is bigger Li+ or Na+?

SimilarlyLi+would be the smallest because it has only 2 electrons and therefore the least electron- electron repulsion. When comparing F- and Na +, we have to be careful, as both of them have 10 electron,SoNa+is smaller due to thelargerattractive force of the protons.

explain how to solve it.

For the elements with large atomic numbers, we prefer to write the electronic configuration taking the help of inert elements near to that element. And for this, we may need knowledge about every element of the periodic table which is quite difficult.

So in this context, we take the help of half-filled and full filled orbitals. I mean the electronic configuration is done by fulfilling the electrons in the orbitals either fully or half to get a stable configuration. Since f orbital can hold 14...

Electronegativity is a relative concept. Explain.

Electronegativity is a realtive concept. The first reason is that we are studying Pauling's scale of Electronegativity which is developed by taking Fluorine as the most electronegative element and that has a EN value of 4.0.

The other reason is a highly EN element - Chlorine is considered more EN if only it has been paired up with a less EN element. If it has been paired up with element equal or greater in EN value than Chlorine, Chlorine isn't considered EN at that moment.

The table below gives information about the atomic structure of some elements. Elements Protons Electron Arrangement Group in periodic table Metal and Non- metal  A 6 .................. 14 Non...

a.

Elements
 Protons Electron Arrangement Group in periodic table Metal and Non- metal
A 6 2,4 14 Nonmetal
B 19 2,8,8,1
 1 metal
C 10 2,8
 18 Nonmetal
D 9 2,7
 17 Nonmetal

b. Ans: Element B (Potassium) will react with the element D (fluorine) to form an ionic compound i.e. Potassium fluoride.

c. Ans: Elements that exist in monoatomic molecules are elements A (carbon), element B (potassium) and element C (Neon). All the elements from above table exists in a monoatomic molecule...

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