Science [3 Mark Questions]

1. Electronic configuration of Ca (20): 2, 8, 8, 2
2. Rb belongs to Group 1, and all Group 1 elements have one valence electron.
3. Sr belongs to Period 5, and thus, it has five shells.
4. K is a metal with electronic configuration 2, 8, 8, 1. Thus, it will donate its one electron to acquire the noble gas configuration.
5. The atomic size increases down the group and decreases across a period. Rb is the element which has the largest atomic size.
6. Be < Mg < Ca < Rb.

In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
As we move down the group, the electrostatic attraction between the nucleus and the outer-most electron decreases due to increase in the distance between them. This happens because, on moving down the group, a new shell is added. So the valence electron can be easily lost by the element. As we know, metallic character is characterized by the ease of loss of an electron, thereby, metallic character increases on moving down a group in the Modern Periodic Table.
When we move across a period, the number of electrons in the same shell increases. This leads to greater electrostatic attraction between the nucleus and the outer-most electron. This increased attraction pulls the outer-most electron closer to the nucleus, thereby decreasing the atomic size.

Electronic configuration of element with atomic no. 16 is 2, 8, 6.
Since it has 3 shells, the period no. will be 3.
Since the no. of valence electrons is 6, the group no. will be 10 + 6 = 16.
Valency of the element will be 8- valence electrons ie 8 – 6 = 2.

a. All three elements have valency 2 . Thus, they should be present in group 2 of the periodic table.
b. On going down a group in the periodic table, the reactivity of an element increases because of decrease in the effective nuclear charge and increase in the atomic size; this makes the removal of valence electrons easy. Thus, among the given elements, 4 Be is the least reactive element.
c. On going down a group in the periodic table, the atomic size of elements increases because of the addition of new shells. Thus, among the given elements, 20Ca has the largest atomic size.

Vertical columns of the periodic table are known as groups.

1. The number of valence electrons remains constant when we move down the group.
2. The number of occupied shells increases down the group.
3. The size of atoms increases down the group.
4. The metallic character of elements increases down the group.
5. The effective nuclear charge decreases down the group.

1. Aluminium (Al) has the highest valency. Valency of Na is 1, Mg is 2, and Al is 3.
2. Sodium (Na) has the largest atomic radius. As we move from left to right in a period, the atomic radius decreases as increase in nuclear charge pulls the electrons closer to the nucleus reducing the atomic size.
3. Sodium (Na) has the maximum chemical reactivity. Reactivity decreases on moving from left to right in a period.

Atomic number of X = Mass number of X – No of neutrons = 35 – 18 = 17.
Therefore Electronic configuration of X = 2,8.7.
Group number =17.
Period no = 3.
Valency = 8-7 = 1.

1. Element, E will form only covalent compounds.
2. Element D have 2 valency.
3. B is non-metal, having 3 valency.
4. D has bigger atomic radius than E. Both belong to third period. In a period, the atomic radius decreases from left to right
5. C and F belong to noble gas family.

Elements	Atomic No.	Electronic configuration	Valence electrons
P	12	2, 8, 2	2
Q	13	2, 8, 3	3
R	14	2, 8, 4	4
S	15	2, 8, 5	5

1. Valency of Q is 3 as Q loses 3 electrons to obtain inert gas configuration.
2. P and Q are metals as these elements lose 2 and 3 electrons respectively to obtain inert gas configuration and those elements which lose electrons are metals.
   R and S are non-metals as these elements either share or gain electrons for attaining inert gas configuration and those metals which accept electrons either by sharing or gaining are non-metals.
3. Element P will form the most basic oxide because P is the most metallic element among the 4 given elements and thus forms the most basic oxide.

1. Recurrence of similar properties of elements after certain fixed interval of atomic number is known periodicity.
2. They have similar properties because:

1. They all have same number of valence electrons.
2. They all have same valency.

3. The tendency to gain electrons increases due to increase in the nuclear charge. Due to this, atomic size decreases and a large number of valence electrons are held towards the nucleus and tendency to gain electrons increases.

• Atomic number is more important parameter than atomic mass as atomic number determines the number of valence electrons which decide the chemical properties of an atom of an element.
• Metallic character decreases from left to right in a period, because the tendency to lose electrons decreases due to increased attraction between nucleus and valence electrons.
• Metallic character increases down the group, as the tendency to lose electrons increases, due to decreased attraction between nucleus and valence electrons because outermost electrons are farther away.

1. On moving down a group, the number of shells increases at each succeeding element. As a result, the valence electrons lie farther away from the nucleus. Therefore, the force of attraction between nucleus and the electrons decreases and tendency to lose electrons increases down the group.
2. As we move left to right in a period, number of shells remain same but number of valence electrons increases. As a result, effective nuclear charge increases and the tendency to lose electrons decreases.

1. The electronic configuration is 2, 8, 2.
2. It is a metal because it has two valence electrons. That is electropositive in nature.
3.  

1. The compound formed when X reacts with Y is XY.
2. The compound formed when X reacts with Z is XZ 2.

The main aim of classifying elements is to follow periodicity based on the octet configuration. The elements are arranged in increasing order of their atomic number.Basic property of elements used in the development of modern periodic law:

1. Same physical and chemical elements.
2. The extreme left side is metals,
3. The extreme right side is non-metals.
4. The between are metalloids probably they seems to be right.

1. 19 K has one electron in its outermost shell.
2. 4 Be and 9 F r elements belonging to the same group.

X(2, 8, 7) is chlorine and the element formed by 19K and 17Cl is KCl and the nature of the compound is ionic.

There are 7 periods in the modern periodic table.
The valency of an element is determined by the number of valence electrons present in the outermost shell of the atom. The valence electrons increases in s and p blocks as one move from left to right. As we move along left to right, valency changes from positive to negative.
As we move from left to right, the nuclear charge on valence shell electron increases. So, tendency to lose electrons decreases. Hence, metallic character decreases.
On moving down the group, the valence shell electrons remain the same. Therefore, valency of the atom does not change in a group.
As we go down the group, a new shell is added. So, the atomic size increases.

1. Na will have the largest atomic radius as atomic size goes on decreasing along a period from left to right. It is due to increase in nuclear charge (number of protons in the nucleus) which pulls the electrons towards it, i.e., a force of attraction between nucleus and valence electrons increases, therefore atomic size decreases.
2. Al is least reactive because the reactivity of an element depends upon the ability of its atoms to donate or accept electrons. Tendency to lose electrons along a period generally decreases with decrease in atomic size, i.e., the force of attraction between the valence electrons and the nucleus increases, therefore electrons cannot be removed easily.

1.  

1. Valency: The combining capacity of an element called its valency.

• The valency an element is determined by the number of valence. Electrons present in the atom of the element.
• The number of electrons lost or gained or shared by one atom of an element to achieve the nearest inert gas configuration gives the valency of the element.

2. Atomic size: The atomic size of an atom is the distance between the centre of the nucleus and outermost electron shell of an isolated atom.

2.  

• On moving from left to right along a period in the modern periodic table, the valency of element from 1to4 and then decreases to zero.
• On moving from left to right in a period of the periodic table, the size of the atoms decreases.

1. Position of the elements in the periodic table:

Element	Period	Group
A	3	17
B	4	1

2.  

So, the formula of the compound formed when elements A and B combine is BA.

3.  

In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
As we move down the group, the electrostatic attraction between the nucleus and the outer-most electron decreases due to increase in the distance between them. This happens because, on moving down the group, a new shell is added. So the valence electron can be easily lost by the element. As we know, metallic character is characterized by the ease of loss of an electron, thereby, metallic character increases on moving down a group in the Modern Periodic Table.
When we move across a period, the number of electrons in the same shell increases. This leads to greater electrostatic attraction between the nucleus and the outer-most electron. This increased attraction pulls the outer-most electron closer to the nucleus, thereby decreasing the atomic size.

1. The atomic number of calcium is 20. Its electronic configuration is 2, 8, 8, 2. The valency of calcium is +2. Hence it is a metal belong to the group 2.
2. The radius of calcium is smaller than potassium because both elements lie in the third period of the periodic table. Potassium lies to the left of calcium in the period, and atomic size decreases, going left to right due to increase in the force of attraction by the nucleus.
3. The oxide of calcium is calcium oxide and its chemical formula is given as CaO.

The modern periodic table consists of 18 groups and 7 periods.

1. Down the group, the atomic size and metallic character of elements increase.
2. On moving from left to right in a period, the atomic size and metallic character of elements decreases.

In the modern periodic table there are total of 7 periods. The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. On moving from left to right in a period the valence electrons increase in s and p-blocks. In going from left to right the valency changes from positive to negative.
On moving from left to right in a period metallic character decreases as nuclear charge on valence shell electrons increases so, tendency to lose electrons decreases and hence metallic character decreases.
Down the group, valency remains the same as it depends upon valency shell electrons which remain the same on moving down the group and atomic size increases because a new shell is added.

1. The number of valence electrons determines the group of the element. F, Cl and Br have valence electrons 7 and belong to group 17. As we go down the group, new atomic shells are added. So, atomic size increases. Therefore, Br has the largest atomic radius as compared to Cl and F which has 3 and 2 shells respectively.
2. F is most reactive due to smaller size, high effective nuclear charge and high electro-negativity.