Science [5 marks Questions]

1. To make the understanding of properties of elements and compounds simpler/To make order out of chaos.
2. The criteria used by Mendeleev in creating his periodic table:

1. He arranged the elements on the basis of their increasing atomic masses.
2. He put elements of similar properties in a group.

3. To make sure that elements with similar properties lie in the same group, Mendeleev left some gaps in the periodic table. These gaps were left for subsequent inclusion of the elements not known at that time. The elements were discovered later.
4. There was no mention of noble gases like helium, neon and argon as they were not known at that time and were discovered much later.
5. Same slot because their chemical properties are same.

The two reactive metals are spdium (Na) and potassium (K). Sodium reacts with chlorine.

Electrovalent bond and Ionic compounds.
Physical properties of such compounds are:

1. They are good conductors of electricity.
2. They are generally soluble in water and insoluble in organic solvents.
3. They are solids, hard, brittle.
4. Crystalline solid at room temperature.
5. Ionic compounds have high melting and boiling points.

1. The criteria used by Mendeleev in creating his periodic table:

1. He arranged the elements on the basis of their increasing atomic masses.
2. He put elements of similar properties in a group.

2. He gave a law known as periodic law. The law states that the physical and chemical properties of elements are periodic function of their atomic weights.
3. Hydrogen resembles like both the alkali metals and the halogens. Hence, it was placed in I A group VI B group and could not be given a fixed position in Mendeleev’s Periodic Table.

4.  

1. When we move from left to right in periodic table no. of shells remain same but the no. of electron increases due to which the size of an atom decreases.
2. When we move down a group the no. of shells goes on increases and hence the size of atom also increases.

1.  

1. Dobereiner’s triad:

Advantage: The three elements of the triad possess similar properties.
Limitation: Some elements which are not similar were grouped into a triad.

2. Newland’s octave:

Advantage: The properties of every eighth element are similar to the properties of the first element.
Limitation: To fit the existing element arrangement, Newlands placed two elements in the same position which differed in their properties.

3. Mendeleev’s Periodic Table:

Advantage: The properties of elements in a particular period show regular gradation from left to right.
Limitation: The position of isotopes could not be explained. Isotopes are atoms of the same element having similar chemical properties but different atomic masses. If elements are arranged according to atomic masses, then isotopes should be placed in different groups of the periodic table.

2. Mendeleev showed that atomic number of an element is a more fundamental property than its atomic mass.
3. Modern periodic law: Properties of elements are a periodic function of their atomic number.

1. Element E is Silicon. It will form a covalent bond only. The outermost orbit of silicon has four electrons and it needs four more electrons to become stable.
2. Non-metal with valency 2 is B, which is Oxygen. Oxygen has a high electronegativity, due to which it has a higher tendency to attract electrons rather than donating them.
3. Element D is a metal with valency 2. Element D is magnesium, due to its low electronegativity it has a very high tendency to donate electrons.
4. Element F has the largest atomic size. According to the electronic configuration of element F which is argon, it occupies 3 energy shells compared to elements H and C which occupies one and two energy shells and due to this its atomic radius is the largest.
5. Elements H, C, and F belong to Group number eighteen, which means according to their outer electronic configuration their octet is complete and thus these elements are stable. They have a very low tendency to react with other elements. Group eighteen elements belong to the noble gas family.

Merits of modern periodic table:

1. The modern periodic table is based on the atomic numbers of elements which is the most fundamental property of elements.
2. It helps us to understand why elements in a group show similar properties but elements in different groups show different properties.
3. It explains the reasons for the periodicity in properties of elements.
4. It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements.

1. The symbols of elements in increasing order of their atomic numbers are:

H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca.

2. The symbols of elements in increasing order of their group numbers are as follows.

1. No. This is because the elements Na, Si and Cl do not have similar properties even though the atomic mass of middle element Si is almost equal to the average atomic mass of first element Na and third element Cl.
2. Yes. This is because the elements Be, Mg and Ca have similar properties and the atomic mass of middle element Mg is almost equal to the average atomic mass of first element Be and third element Ca.

1. X is in Group 17, period 3 and Y is in group 2, period 4.
2. X is a non-metal, While Y is a metal.
3. Oxide of element Y would be basic. This compound is formed by ionic bond.
4.  

1. A (size decreases from left to right in a period).
2. K (valency of k = 3; valency of o = 1).
3. I (metallic character decreases from left to right in a period).
4. G (non - metallic character increases from left to right in a period).
5. B (or j).
6. F (or n).

1. The available information suggests that the element X is nitrogen (N) and exists in diatomic form as N, Electronic configuration of N(Z = 7); 2, 5. It has five valence electrons.
2.  

3.  

The arrangement of given elements in the Modern Periodic table is listed below.

Group/ Period	1	2	13	14
3		Mg
4	K	Ca	Ga	Ge

We know that metallic character decreases from left to right in the periodic table. In the given elements, excluding Mg, remaining elements are placed in same period so the metallic character will decrease from K to Ge and order of metallic character would be;
Ge < Ga < Ca < KNow, as we move down in a group, metallic nature of elements increases. So, Ca is more metallic than Mg. Similarly, Mg will be less metallic than K because in period, metallic nature decreases from left to right. Hence the increasing order of metallic character would be;
Ge < Ga < Mg < Ca < K

1. Newlands' law of octaves is illustrated by the above arrangement of elements.
2. John Newlands is the scientist who proposed this classification of elements.
3. This classification is compared with a characteristic of the musical scale because the repetition of the properties of elements is just like the repetition of the eighth note in an octave of music.

4. Limitation:

Newlands' law of octaves could only be applied up to the element calcium.

1. On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases.

Example: In the 3rd period of elements, sodium is a very reactive element, magnesium is less reactive whereas aluminium is still less reactive. Silicon is the least reactive in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive whereas chlorine is very reactive.

2. On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases.

Example: In the 3rd period of the periodic table, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminium and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic whereas chlorine oxides are highly acidic in nature.

Advantages of the periodic table:

1. It has made the study of chemistry systematic and easy.
2. It is easier to remember the properties of an element if its position in the periodic table is known.
3. The type of compounds formed by an element can be predicted by knowing its position in the periodic table.
4. It is used as a teaching aid in chemistry in schools and colleges.

1. 2.

For groups 1 and 2, the number of valence electrons is equal to the group number.

2. 2.

Valency is determined by the number of valence electrons present in the atom of the element.

3. 5.

For groups 13 to 18, the number of valence electrons is equal to (group no. – 10).

4. 3.

The number of electrons lost or gained by one atom of an element to achieve the nearest inert gas configuration, gives us the valency.

1. d.
2. c.
3. e.
4. Covalent bond.
5. Ionic bond.

1. Element with 3 as atomic number, i.e. Lithium.
2. Element with 9 as atomic number, i.e. Fluorine.
3. Fluorine.
4. Carbon, valency 4.

Inert gases could be placed in Mendeleev’s Periodic Table without disturbing the original order.
Noble gases; like Helium, Neon and Argon had been mentioned by various scientists even before Mendeleev. But these gases could be grouped together as noble gases much later than Mendeleev’s discovery. Mendeleev used 63 elements till his periodic table because these were the only known elements till his time. However, he was bold enough to leave gaps in his periodic table. He had predicted that that more elements would be discovered in times to come.
Mendeleev’s Periodic Table was the first comprehensive attempt to classify elements on the basis of their chemical properties. This is the main reason that new elements could be easily placed in Mendeleev’s Periodic Table without disturbing the original order.

1. Element E will from only covalent compounds,
2. Element D will have valency 2 which is metal,
3. Element B is a non metal having valency 3, and Element D has larger radius because as move right of the periodic table, atomic radius decreases.
4. Electrons come closer to the nucleus due to opposite charges of electron and nucleus.
5. The elements C and F are called as Halogens.

1. Sodium (Group 1 and Period 3) or Potassium (Group 1 and Period 4).
2. Phosphorus (Group 15 and Period 3). It shows variable valencies 3 and 5 and is stored under water.
3. Carbon (Group 14 and Period 2).
4. Helium (Group 18 and Period 1).
5. Aluminium (Group 13 and Period 3).

1. 7.
2. It is a non-metal.
3. Bromine.
4. HBr(Hydrogen bromide).
5.  

Across:

1. Magnesium.

3. Tin.
4. Iodine.

Down:

2. Sodium.

5. Lithium.
6. Neon.
7. Astatine.
8. Iron.
9. Boron.

Mendeleev purposed the relation between atomic masses of elements and their properties. He examined that physical and chemical properties of elements are periodic function of their atomic masses. He arranged all known 63 elements according to their atomic masses.
He examined the compounds formed by elements hydrogen and oxygen. He used these elements because they readily form different compounds with other elements like hydride and oxides.
The chemical properties of hydrides and oxides were used to determine the chemical properties of different elements and that again used for their classification. He recorded properties of all 63 elements and paired the elements which exhibit same chemical properties. Later he arranged elements on the basis of their similarities in chemical properties and found that most of the elements were arranged in order of their increasing atomic masses.
In this arrangement, he also observed the periodic recurrence of elements with similar physical and chemical properties. He formulated the periodic law which states that the properties of elements are the periodic function of their atomic masses.

1. Period no: 3, Group no: 17.
2. The atomic no of this element (Chlorine) is 17.
3. It is a non-metal because it accepts electron to complete its octet.
4. The element is Chlorine(Cl) and it has valency = 1.
5. F(9).

Various anomalies of Mendeleev's Periodic Table removed as follows in the Modern Periodic Table:

1. Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
2. In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.
3. Elements having same valence electron are kept in same group.
4. Elements having same number of shells were put under the same period.
5. Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.

Mendeleev's periodic table	Modern periodic table
Elements are arranged in the increasing order of their atomic masses.	Elements are arranged in the increasing order of their atomic numbers.
There are 8 Groups.	There are 18 Groups.
Each groups are subdivided into sub group 'a' and 'b'	Groups are not subdivided into sub-groups.
Groups for Noble gas was not present as noble gases were not discovered by that time.	A seprate group is meant for noble gases.
There was no place for isotopes.	This problem has been rectified as slots are determined according to atomic number.

The valency of first 20 elements is as follows:

1. Hydrogen(H) = 1
2. Helium(He) = 0
3. Lithium(Li) = 1
4. Beryllium(Be) = 2
5. Boron(B) = 3
6. Carbon(C) = 4
7. Nitrogen(N) = 3
8. Oxygen(O) = 2
9. Fluorine(F) = 1
10. Neon(Ne) = 0
11. Sodium(Na) = 1
12. Magnesium(Mg) = 2
13. Aluminium(Al) = 3
14. Silicon(Si) = 4
15. Phosphorus(P) = 3, 5
16. Sulphur(S) = 2
17. Chlorine(Cl) = 1
18. Argon(Ar) = 0
19. Potassium(K) = 1
20. Calcium(Ca) = 2

The valencies of first 20 elements you remember their atomic numbers and find the valencies of them by using their electronic configuration.