Science [2 Mark Questions]

Electronic Configuration:
C - 2, 4
O - 2, 6
Ne - 2, 8
Na - 2, 8,1
Si - 2, 8, 4
Elements in which electrons are filled in first 2 shells: C, O and Ne. So these elements lie in the same period.
Elements in which electrons are filled in first 3 shells: Na and Si. So these elements lie in the same period.
Elements with same number of 4 valence electrons: C, Si. So, these elements lie in the same group.

1. Na has the largest atomic radius as compared to Mg and Al because going along the period atomic radius decreases due to an increase of electrons and nuclear charge tends to pull the electrons closer to the nucleus and hence decrease the atomic radius.
2. Al is smaller in size as compared to Na and Mg so it has lesser tendency to lose electron due to high effective nuclear charge.

1. On moving from up to down in a group there is no change in valency
2. On moving from left to right in a period at first velency is increased from 1 to 4 and then decrease from 4 to 0.

The electronic configuration of calcium (20) = 2, 8, 8, 2.
The electronic configuration of other elements
Atomic number 12 = 2, 8, 2
Atomic number 19 = 2, 8, 8, 1
Atomic number 21 = 2, 8, 8, 3
Atomic number 38 = 2, 8, 18, 8, 2
The element having atomic number 12 and 38 resemble physical and chemical properties with calcium (20). This is because the valence electron of in these element is same as calcium (e = 2) and the physical and chemical properties depend on the number of valence electron present in the outermost shell.

1. The valency of an element is determined by the number of electrons present in the last shell.

If a number of valence electrons are less than or equal to 4 then, valency = a number of valence electrons.

If, the number of valence electrons greater than 4 then, valency = 8 - number of valence electrons.

2. The electronic configuration of element X with atomic number 9 is 2, 7

Valency of X = 8 - 7 =1

Hence, the valency of the element with atomic number 9 is 1.

Electronic configuration of an element tells us the no. of electron in it which is used in finding the valence electron and the atomic number of the element. The no. of electron in the outer most shell specifies the position of an element in the periodic table.
Ex. The no. of electron present in Na, K, Li, is 1 so they all are placed in group 1.

1. The valency of an element is determined by the number of electrons present in the last shell.

If a number of valence electrons are less than or equal to 4 then, valency = a number of valence electrons.

If, the number of valence electrons greater than 4 then, valency = 8 - number of valence electrons.

1. The electronic configuration of element X with atomic number 9 is 2, 7

Valency of X = 8 - 7 =1

Hence, the valency of the element with atomic number 9 is 1.

Modern periodic law states that the physical and chemical properties of an element in the periodic table are the periodic function of the atomic number of that element. Electronic configuration of the elements plays an important role in the placement of elements in the modern periodic table. The valence shell electron of an element decides its position in a particular group or period.
For example, if the electronic configuration of an element is 2, 1, it means that the element belongs to group I and 2nd period (valence electron is present in the 2nd shell). Similarly, if the electronic configuration of an element is 2, 8, 1, it means that the element belongs to group I and 3rd period (valence electron is present in the 3rd shell).
Thus, the modern periodic table is based on the electronic configuration of the different elements.

If the object distance is greater than 25cm the image is always formed on the retina as the focal length of the human eye lens gets adjusted depending on the object distance. This special property of human eye is called as power of accommodation. Hence, image distance will remain same if object distance changes from 10m to 1m.

Name - sodium
Symbol - Na
Electronic configuration - 2, 8, 1

1. The electronic configuration of the given element is 2, 8, 4. Since, the number of valence electrons for the given element is 4. Hence, the element belongs to group 14 (10 + 4) and the 3rd period.
2. The atomic number of the given element is 14. Hence, the given element is Silicon. Silicon is a metalloid.

1. Eka - boron.
2. Eka - aluminum.
3. Eka - silicon.

The no. of valence electrons in the atoms of elements decides which element will be the 1st element in a period and which will be the last in a period.

1. In a group, all the elements have the same valency.
2. In a period, on moving from left to right, the valency of elements first increases from 1 to 4 and then decreases to zero.

1. B and C.
2. A and C.

1. A in group 14; B in group 17.
2. Covalent bond.
3. AB 4.

The noble gases are placed in a separate group because they are chemically very inert or unreactive (having completely filled outermost electron shells).

1. Neon (2, 8)
2. Magnesium (2, 8, 2)
3. Silicon (2, 8, 4)
4. Boron (2, 3)
5. Carbon(2, 4).

No fixed position could be given to hydrogen in Mendeleev's periodic table because:1. Hydrogen resembles alkali metals as it forms compounds with halogens that have similar formulae and alkali metals are present in group 12.Hydrogen resembles halogens as just like halogens hydrogen can form covalent bonds with metals and non- metals. Halogens are present in group 17 So there is an anomaly about the position of hydrogen.

1. Non - metal.
2. 6.
3. 2.
4. Oxygen.
5. Na 2 Y.

1. As we go left to right in a period atomic radius decreases as new electrons added in the same shell.
2. As we go down the group, atomic radius increases as new electrons are added in new shells.

The similarity in the atoms of lithium, sodium and potassium is that all of them have 1 valence electron each.

Atomic number of q = 13
Electronic configuration = 2, 8, 3
Since 3 electrons are present in valence shell, therefore valency of q is 3

The elements having similar chemical properties form oxides and hydrides having similar formulae. Mendeleev used these properties for creating his periodic table.

1. 2.
2. 2.
3. Metal.
4. Magnesium.

1. Sodium (Na), magnesium (Mg) and aluminium (Al) belong to the same (third) period of the periodic table.
2. Lithium (Li), sodium (Na) and potassium (K) belong to the same group (group 1) of the periodic table.

1. A is a non-metal.
2. C is less reactive than A, as reactivity decreases down the group in halogens.
3. C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
4. A will form an anion as it accepts an electron to complete its octet.

1. The atomic size increases gradually from lithium to francium.
2. The metallic character increases from lithium to francium.

1. X and Z. X and Z have zero valency hence they belong to same group: noble gases.
2. Y and Z. Y: 2,4 and Z: 2,8 so, both of them belong to second period with two shells filled.

Mendeleev left some gaps in his periodic table because he believed that in future more elements will be discovered. He predicted the existence of some elements and named them by prefixing a Sanskrit numeral, Eka (one).

Position of hydrogen:Hydrogen has been placed at the top of group 1, above the alkali metals in the modern periodic table because the electronic configuration of hydrogen is similar to those of alkali metals. Both, have 1 valence electron each.

• Group 1: Hydrogen and lithium.
• Group 2: Beryllium and calcium.
• Group 16: Oxygen and sulphur.
• Group 17: Fluorine and chlorine.

On going down in a group of the periodic table, the atomic size increases. When we move from top to bottom in a group, a new shell of electrons is added to the atoms at every step due to which the size of atom increases.

Yes, Dobereiner's triads also exist in the columns of Newlands' Octaves. For example, the second column of Newlands' classification has the elements lithium (Li), sodium (Na) and Potassium (K), which constitute a Dobereiner's triad.

The main characteristic of last elements in a period is that they all have 8 valence electrons in their atoms except helium. Such elements are called noble elements.

Mendeleev’s periodic law:The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev’s periodic law which was based on atomic mass.

In Mendeleev's periodic table, the reasons for placing cobalt (Co) before nickel (Ni) though its atomic mass is higher than that of nickel, are:

1. Cobalt was placed in the group of rhodium (Rh) and iridium (Ir) because it exhibits the properties similar to them.
2. Similarly Nickel was placed with elements like palladium (Pd) and platinum (Pt) having properties similar to it.

Position of Cobalt and Nickel:The atomic number of cobalt is 27 and that of nickel is 28. According to modern periodic law, the elements are arranged in order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

The fundamental difference between the electronic configuration of group 1 and group 2 elements is that group 1 elements have 1 valence electron in their atoms whereas group 2 elements have 2 valence electrons in their atoms.

	Elements	Least atomic radius	Chemically least reactive
(i)	F, Cl, Br	F	Br
(ii)	Li, Na, K	Li	Li

The horizontal rows of elements in a periodic table are called periods.Characteristics:

1. The elements in a period have consecutive atomic numbers.
2. The no. of elements in period is fixed by the maximum no. of electrons which can be accommodated in various shells. The vertical columns in a periodic table are called groups.

Characteristics:

1. The elements in a group do not have consecutive atomic numbers.
2. All the elements in a group have similar electronic configurations and show similar properties.​​​​​​​

In order to make sure that the elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table.

The real significance of atomic number in the modern periodic classification is that it relates the periodicity in the properties of elements to the periodicity in their electronic configurations.
Example: The atomic number increases from 3 in lithium to 11 in sodium, there is a repetition of electronic configuration from 2, 1 to 2, 8, 1 (both having 1 valence electron).