Around the year 1800, only 30 elements were known. Dobereiner in 1817 and Newlands in 1866 tried to arrange the then-known elements and framed laws which were rejected by the scientists. Even after the rejection of the proposed laws, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses.The main credit for classifying elements goes to Mendeleev for his most important contribution to the early development of a Periodic table of elements wherein he arranged the elements on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties. The formulae of their hydrides and oxides were treated as basic criteria for the classification of the elements.However, Mendeleev’s classification also had some limitations as it could not assign the position to isotopes. He also left some gaps in the periodic table.(a) State Mendeleev’s Periodic Law.(b) Why did Mendeleev leave some gaps in the Periodic table?(c) If the letter ‘R’ was used to represent any of the elements in the group, then the hydride and oxide of carbon would respectively be represented as(i) RH4, RO (ii) RH4, RO2(iii) RH2, RO2 (iv) RH2, RO(d) Isotopes are-(i) Atoms of an element with similar chemical properties but different atomic masses. (ii) Atoms of different elements with similar chemical properties but different atomic masses. (iii) Atoms of an element with different chemical properties but same atomic masses. (iv) Atoms of different elements with different chemical properties but same atomic masses.
(a) Mendeleev’s Periodic Law states that the properties of elements are the periodic functions of their atomic masses.
(b) Mendeleev left some gaps in the Periodic table to fill it with elements that were yet to be discovered.
(c) Correct Answer: (ii) RH4, RO2
Explanation: If the letter R is used to represent the element in the group, then the hydride and oxide of carbon would respectively be represented as RH4, RO2
(d) Isotopes are
Correct Answer: (ii) Atoms of different elements with similar chemical properties but different atomic masses.
Explanation: Isotopes are atoms of an element with similar chemical properties but different atomic masses.
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- According to Mendeleev’s Periodic Law, the elements were arranged in the periodic table in the order of(a) increasing atomic number(b) decreasing atomic number(c) increasing atomic masses(d) decreasing atomic masses
- Which of the following statement is correct in regard to the classification of elements ?(a) Elements in the periodic table are arranged on the basis of increasing atomic masses  (b) Elements in Mendeleev’s periodic table are arranged on the basis of increasing atomic numbers (c) In modern periodic table, the element nickel of lower atomic mass is kept before the element cobalt of higher atomic mass (d) In modern periodic table, the isotopes of chlorine having diffrenet atomic masses are kept in the same group.
- (a) Why do we classify elements?(b) What were the two criteria used by Mendeleev to classify the elements in his periodic table? (c) Why did Mendeleev leave some gaps in his periodic table? (d) In Mendeleev’s periodic table, why was there no mention of noble gases like helium, neon and argon? (e) Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.
- (a) State Mendeleev’s periodic law.(b) What chemical properties of elements were used by Mendeleev in creating his periodic table? (c) State any three limitations of Mendeleev’s classification of elements. (d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table ? (e) Which group of elements was missing from Mendeleev’s original periodic table?
- Name the Russian chemist who said that the properties of elements are a periodic function of their atomic masses.
- Why do we classify elements?What were the two criteria used by Mendeleev in creating his Periodic Table?Why did Mendeleev leave some gaps in his Periodic Table?In Mendeleev's Periodic Table, why was there no mention of Noble gases such as Helium, Neon and Argon?Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.
- Atoms of elements having the same mass number but different atomic number are called ?
- Isotopes of an element have(a) the same physical properties(b)the same chemical properties(c)different number of neutrons(d) different atomic numbers.
- In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give a reason for the same.
- (a) Explain why:(i) All the elements of a group have similar chemical properties. (ii) All the elements of a period have different chemical properties. (b) The atomic radii of three elements X, Y, and Z of a period of the periodic table are 186 pm; 104 pm, and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.
- Name two elements whose properties were predicted on the basis of their positions in Mendeleev’s periodic table.
- Which of the following are the correct characteristics of isotopes of an element?(i) same atomic mass(ii) same atomic number(iii) same physical properties(iv) same chemical properties(a) (i), (ii) and (iv)(b) (ii), (iii) and (iv)(c) (ii) and (iii)(d) (ii) and (iv)
- State whether the following statement are true or false:(a) Newlands divided the elements into horizontal rows of eight element each. (b) According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic numbers. (c) The elements in a group have consecutive atomic numbers.
- (a) Explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements(b) Why do elements in the same group show similar properties but the elements in different groups show different properties?(c) For each of the following triads, name the element with the characteristics specified below:
- What is the reason for the different atomic masses of the isotopes of an element?
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