Answer the following:
1. How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?
2. In the modern periodic table, which are the metals, non-metals and metalloids among the first twenty elements?
3. What are the demerits of Mendeleev’s periodic table?
4. Define atomic size. How does it vary in a period and in a group?
Answer the following:
1. How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?
2. In the modern periodic table, which are the metals, non-metals and metalloids among the first twenty elements?
3. What are the demerits of Mendeleev’s periodic table?
4. Define atomic size. How does it vary in a period and in a group?
1. Mendeleev was unable to give a regular arrangement of elements according to atomic mass and isotopes in the periodic table.
• In the Mendeleev’s periodic table the elements were arranged in increasing order of their atomic masses. The atomic mass does not increase regularly. Therefore, it was impossible to predict the number of elements between two elements.
The Modern periodic table has been made according to the increasing atomic number (Modern Periodic Law states that the properties of an element are the periodic function of its atomic number). The atomic number gives us the number of protons in the nucleus of an atom. The atomic number increases by one as we go from one element to the next. Thus, this makes it easy to ascertain how many undiscovered elements may be there between two known elements.
• Mendeleev’s periodic table was made according to increasing atomic masses. We know that an element can have the same chemical properties and atomic number but different atomic masses. So, the concept of isotopes cannot be satisfied.
The Modern periodic table is according to increasing atomic numbers. Therefore, the problem of isotopes is easily dealt with.
2. Classification of first twenty elements into metals, non-metals and metalloids:
Metals: Lithium (Li), Beryllium (Be), Sodium (Na), Magnesium (Mg), Aluminium (Al), Potassium (K), Calcium (Ca)
Non-metals: Hydrogen (H), Helium (He), Boron (B), Carbon (C ), Oxygen (O), Nitrogen (N), Fluorine (F), Phosphorous (P), Sulphur (S), Chlorine (Cl)
Metalloids: Silicon (Si)
3. Demerits of Mendeleev’s periodic table
In spite of the above advantages, Mendeleev’s periodic table suffered the following defects:
• The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens in their properties.
• In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).
• Isotopes were not given separate places in the periodic table although Mendeleev 's classification is based on the atomic masses.
• Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.
• Mendeleev could not explain the cause of periodicity in the elements.
• The position for lanthanides and actinides were not included in this table.
4. Atomic size: Atomic size is generally determined using atomic radius. Atoms are generally regarded as spherical in shape. Atomic radius is therefore the distance between the centre of the nucleus and the outermost shell of an isolated atom.
For example, when we say that the atomic radius of a hydrogen atom is 37 pm (picometre, 1 pm = 10−12m), it means that the distance between hydrogen nucleus and the outermost shell is 37 pm. The greater the atomic radius, the size of the atom is increased.
Atomic size in a period: When we move from left to right across a period, the numbers of shells in the atom remain the same but the number of valence shell electrons increase. This means that the effective nuclear charge on the electrons also increase resulting in atoms of a smaller size.. For example, take the case of period 2 elements, Li, Be, B, C, N, O, F, Ne. The number of orbitals (shells) in all these elements is same (i.e., 2 shels) but the number of valence electrons increase as we move from left to right. This results in an increase in nuclear charge on the outermost shell (L shell) of the atoms moving from left to right. Hence, the atomic size decreases.
Atomic size in a group: When we move down the group the atomic size increases because the new shell is added ,this increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the nuclear charge.
1. Mendeleev was unable to give a regular arrangement of elements according to atomic mass and isotopes in the periodic table.
• In the Mendeleev’s periodic table the elements were arranged in increasing order of their atomic masses. The atomic mass does not increase regularly. Therefore, it was impossible to predict the number of elements between two elements.
The Modern periodic table has been made according to the increasing atomic number (Modern Periodic Law states that the properties of an element are the periodic function of its atomic number). The atomic number gives us the number of protons in the nucleus of an atom. The atomic number increases by one as we go from one element to the next. Thus, this makes it easy to ascertain how many undiscovered elements may be there between two known elements.
• Mendeleev’s periodic table was made according to increasing atomic masses. We know that an element can have the same chemical properties and atomic number but different atomic masses. So, the concept of isotopes cannot be satisfied.
The Modern periodic table is according to increasing atomic numbers. Therefore, the problem of isotopes is easily dealt with.
2. Classification of first twenty elements into metals, non-metals and metalloids:
Metals: Lithium (Li), Beryllium (Be), Sodium (Na), Magnesium (Mg), Aluminium (Al), Potassium (K), Calcium (Ca)
Non-metals: Hydrogen (H), Helium (He), Boron (B), Carbon (C ), Oxygen (O), Nitrogen (N), Fluorine (F), Phosphorous (P), Sulphur (S), Chlorine (Cl)
Metalloids: Silicon (Si)
3. Demerits of Mendeleev’s periodic table
In spite of the above advantages, Mendeleev’s periodic table suffered the following defects:
• The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens in their properties.
• In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).
• Isotopes were not given separate places in the periodic table although Mendeleev 's classification is based on the atomic masses.
• Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.
• Mendeleev could not explain the cause of periodicity in the elements.
• The position for lanthanides and actinides were not included in this table.
4. Atomic size: Atomic size is generally determined using atomic radius. Atoms are generally regarded as spherical in shape. Atomic radius is therefore the distance between the centre of the nucleus and the outermost shell of an isolated atom.
For example, when we say that the atomic radius of a hydrogen atom is 37 pm (picometre, 1 pm = 10−12m), it means that the distance between hydrogen nucleus and the outermost shell is 37 pm. The greater the atomic radius, the size of the atom is increased.
Atomic size in a period: When we move from left to right across a period, the numbers of shells in the atom remain the same but the number of valence shell electrons increase. This means that the effective nuclear charge on the electrons also increase resulting in atoms of a smaller size.. For example, take the case of period 2 elements, Li, Be, B, C, N, O, F, Ne. The number of orbitals (shells) in all these elements is same (i.e., 2 shels) but the number of valence electrons increase as we move from left to right. This results in an increase in nuclear charge on the outermost shell (L shell) of the atoms moving from left to right. Hence, the atomic size decreases.
Atomic size in a group: When we move down the group the atomic size increases because the new shell is added ,this increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the nuclear charge.
