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प्रश्न
The atomic masses of three elements A, B and C having similar chemical properties are 7, 23 and 39, respectively.
- Calculate the average atomic mass of elements A and C.
- Compare the average atomic mass with atomic mass of B.
- What could the elements A, B and C be?
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उत्तर
(a) The average atomic mass of elements A and C = `(7+39)/2=46/2=23`
(b) The average atomic mass of elements A and C is equal to the atomic mass of element B.
(c) A is lithium, B is sodium and C is potassium.
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संबंधित प्रश्न
Given below are some elements of the modern periodic table. Atomic number of the element is given in parentheses.
A(4), B(9), C(14), D(19), E(20)
(a) Select the element that has one electron in the outermost shell. Also, write the electronic configuration of this element.
(b) Which two elements amongst these belong to the same group? Give reasons for your answer.
(c) Which two elements amongst these belong to the same period? Which one of the two has bigger atomic radius?
The position of eight elements in the Modern Periodic Table is given below where atomic numbers of elements are given in the parenthesis.
| Period No. | ||
| 2 | Li(3) | Be(4) |
| 3 | Na(11) | Mg(12) |
| 4 | K(19) | Ca(20) |
| 5 | Rb(37) | Sr(38) |
(i) Write the electronic configuration of Ca.
(ii) Predict the number of valence electrons in Rb.
(iii) What is the number of shells in Sr?
(iv) Predict whether K is a metal or a non – metal.
(v) Which one of these elements has the largest atom in size?
(vi) Arrange Be, Ca, Mg and Rb in the increasing order of the size of their respective atoms.
An element 'X' has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of 'X'. Also write group number, period number and valency of 'X'.
An element 'M' with electronic configuration (2, 8, 2) combines separately with (NO3)–, (SO4)2– and (PO4)3– radicals. Write the formula of the three compounds so formed. To which group and period of the Modern Periodic Table does the element 'M' belong? Will 'M' form covalent or ionic compounds? Give reason to justify your answer.
The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.
Based on the group valency of elements state the formula lot the following giving justification for each :-
(i) Oxides of 1st group elements,
(ii) Halides of the elements of group 13, and
(iii) Compounds formed when an element of group 2 combines with, an element of group 16.
Define the term : Atomic size
Write any one difference in the electronic configurations of group-1 and group-2 elements ?
An element with the atomic number 19 will most likely combine chemically with the element whose atomic number is ______.
How do atomic structures (electron arrangements) change in a period with increase in atomic numbers from left to right?
The electronic configuration of the atom of an element X is 2, 8, 4. In modern periodic table, the element X is places in :
(a) 2nd group
(b) 4th group
(c) 14th group
(d) 8th group
Given alongside is a part of the periodic table. As we move vertically downward from Li to Fr:
| Li | Be |
| Na | |
| K | |
| Rb | |
| Cs | |
| Fr | Ra |
What happens to their metallic character?
Where would you locate the element with electronic configuration 2, 8 in the modern periodic table?
On moving from left to right in a period of the periodic table, the atomic number of elements increases. What happens to the size of atoms of elements on moving from left to right in a period?
(a) increases
(b) decreases
(c) remain the same
(d) first increases then decreases
How does the number of:
valence electrons and
How do the following change on moving from left to right in a period of the periodic table?
Give examples in support of your answer.
Chemical reactivity of elements ?
An element X belong to 3rd periods and group II of the periodic table state:
the valency,
F, Cl and Br are the elements each having seven valence electrons. Which of these (i) has the largest atomic radius, (ii) is most reactive? Justify your answer stating reason for each.
Taking into consideration the period of the elements given below, answer the following questions:
| Elements | Atomic Radius (pm) |
| O | 66 |
| B | 88 |
| C | 77 |
| N | 74 |
| Be | 111 |
| Li | 152 |
- Arrange the above elements in decreasing order of their atomic radii.
- State the period to which the above elements belong.
- Why this arrangement of elements is similar to the above period of modern periodic table?
- Which of the above elements have the biggest and the smallest atom?
- What is the periodic trend observed in the variation of atomic radius while going from left to right within a period?
Study the extract of the Periodic Table given below and answer the questions that follow. Give the alphabet corresponding to the element in question. DO NOT repeat an element. 
Which is an inert gas?
How many groups and periods are there in the modern periodic table?
State whether the following statement is true or false
Similar electronic configuration is repeated after intervals of 2, 8, 8, 18 and 32.
Arrange the following in increasing order of property indicated
SiO2, P2O5, SO3, Cl2O7 (acidic nature)
Select the correct answer
Number of shells in potassium are
The following table represents the first period of the modern periodic table. Study the table and answer the questions that follow:
- Write the formula of the sulphate of the element with atomic number 13.
- What type of bonding will be present in the oxide of the element with atomic number 1?
- Which feature of the atomic structure accounts for the similarities in the chemical properties of the elements in group VIIA of the periodic table?
- Name the element which has the highest ionization potential.
- How many electrons are present in the valence shell of the element with atomic number 18?
- What is the name given to the energy released, when an atom in its isolated gaseous state accepts an electron to form an anion?
- What is the electronic configuration of the element in the third period which gains one electron to become an anion?
- Fill in the blanks:
The atomic size ______ as we move from left to right across the period, because the ______ increases, but the ______ remains the same.
Choose the most appropriate answer from the following list of oxides which fit the description.
A basic oxide.
Choose the most appropriate answer from the following list of oxides which fit the description.
An amphoteric oxide: Al2O3 (shows both acidic and basic properties)
Arrange the following as per instruction given in the bracket.
Mg, Cl, Na, S, Si (decreasing order of atomic size)
The position of elements A, B, C, D and E in the periodic table are shown below:
|
Group 1 |
Group 2 |
Group 17 |
Group 18 |
|
|
|
|
D |
|
|
B |
C |
|
|
A |
|
|
E |
Which is larger in size (i) D or E (ii) B or C
An element belongs to the third period and Group IIIA (13) of the periodic table. State: the valency,
Name and state the following with reference to the elements of the first three periods of the periodic table.
Valency of elements in Group 14 and 15.
State the nature of compounds formed when group 17 elements combine with (i) metals (ii) non-metals.
Moving down in the second group, number of valence electrons ______.
Name the elements in Period 1.
Name or state following with reference to the element of the first three periods of the periodic table.
The valency of elements in group 1 [I–A].
Name or state following with reference to the element of the first three periods of the periodic table.
The non-metallic element present in period 3 other than sulphur and chlorine.
There are three elements E, F, G with atomic numbers 19, 8 and 17 respectively.
Classify the above elements as metals and non-metals.
