Q1. 3, 1, 2 electrons are in valence shells of X, Y, Z elements. From this information, state the group in which they belong and write their valencies.
Answer
| Valence shell electron(s) |
Group | Valency |
| 3 | 13 | 3 |
| 1 | 1 | 1 |
| 2 | 2 | 2 |
Updated on: 2026-03-31 | Author: Rahul Patil
| Valence shell electron(s) |
Group | Valency |
| 3 | 13 | 3 |
| 1 | 1 | 1 |
| 2 | 2 | 2 |
Bromine is in liquid form in the halogen family.
Atomic radius is the distance between the nucleus of the atom and its outermost shell.
L is the outermost shell for elements of period 2.
According to Dobereiner’s triads rule, three elements in an increasing order of atomic mass shows the atomic mass of the middle element in each triad approximately equal to the mean of the atomic masses of the other two elements.
According to Mendeleev’s periodic law properties of elements are a periodic function of their atomic masses.
According to Newland’s octave rule, chlorine shows similarity with fluorine. Because chlorine is the eighth element from fluorine.
All the elements in the halogen family are gases- Wrong
The element with an electron configuration of 2, 8, 2 belongs to the third period of the periodic table. The period number corresponds to the number of electron shells or energy levels occupied by electrons in an atom. Since this configuration indicates electrons in three shells (2 electrons in the first shell, 8 in the second, and 2 in the third), the element is in the third period.
The atomic number of this element is 12.
The number of electrons in the element's outermost shell is two. This means it has a valency of 2. As a result, it belongs to Group 2.
The element would resemble beryllium (Be).
Explanation:
The given element has an electron configuration of 2, 8, 2, indicating that it contains two electrons in its outermost shell. It resembles beryllium (Be), which likewise possesses two valence electrons. Elements with the same number of valence electrons have similar chemical characteristics.
Thus, the provided element resembles beryllium.
Element X has more metallic character than element Y.
Valency:
i. Both elements X and Y belong to the period 3.
ii. While going from left to right in a period, atomic number increases. So, positive charge on the nucleus increases.
At the same time, electrons get added in the same outermost shell.
iii. This increases effective nuclear charge. As a result, electrons are attracted closer to the nucleus decreasing the size of the atom.
Na > Mg > Al > Si > Cl
At the time of Mendeleev 63 elements were known.
Atomic radius is expressed in the unit picometer.
Beryllium : alkaline earth metal : : sodium: Alkali metal
Beryllium and calcium are alkaline earth metals- Right
Cl : halogen group : : Ar : zero group
S- Nonmetal
Mg- Metal
Al- Metal
P- Nonmetal
N- Nonmetal
Na- Metal
| Shell | n | 2n2 | Electron holding capacity |
| K | 1 | 2 × 12 | 2 |
| L | 2 | 2 × 22 | 8 |
| M | 3 | 2 × 32 | 18 |
| N | 4 | 2 × 42 | 32 |
The temperature at which the equilibrium vapour pressure of a liquid becomes equal to the atmospheric pressure is called the boiling point of that liquid.
The merits of Mendeleev’s periodic table are followed:
Atomic masses of some elements were revised so as to give them proper place in the periodic table in accordance with their properties.
For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value of 9.4, and beryllium was placed before boron.
Mendeleev kept vacant places in the periodic table for elements not discovered till then. Three of these unknown elements were given the names eka-boron, eka-aluminium and eka-silicon from the known neighbours and their atomic masses were indicated as 44, 68 and 72, respectively. Not only this but their properties were also predicted.
Later on these elements were discovered and named as scandium (Sc), gallium (Ga) and germanium (Ge) respectively. The properties of these elements matched well with those predicted by Mendeleev.
There was no place reserved for noble gases in Mendeleev’s original periodic table. However, when noble gases such as helium, neon and argon were discovered towards the end of nineteenth century, Mendeleev created the ‘ zero’ group without disturbing the original periodic table in which the noble gases were fitted very well.
| Group 17 | Group 18 | |
| 1. | Elements of group 17 are called halogens. | Elements of group 18 are called inert gas elements or noble gases. |
| 2. | They have seven valence electrons. | They have eight valence electrons except helium which contains 2 electrons. |
| 3. | Their valency is 1. | Their valency is zero. |
| 4. | They form diatomic molecules with general formula (X2). | They are monoatomic gases under normal conditions. |
| Group | Period | |
| 1. | The vertical columns of elements in the modern periodic table are called groups. | The horizontal rows of elements in the modern periodic table are called periods. |
| 2. | There are 18 groups. | There are 7 periods. |
| 3. | The properties of all the elements in a group show similarity and gradation. | The properties of elements change slowly from one end to the other in a period. |
| 4. | While going from top to bottom within any group, the number of shells goes on increasing | While going from left to right within a period, the number of shells occupied by electrons remains the same. |
| Mendeleev’s periodic table | Modern periodic table | |
| 1. | Elements are arranged in increasing order of their atomic masses. | Elements are arranged in increasing order of their atomic numbers. |
| 2. | It is not divided into any blocks. | It is divided into four blocks, namely s-block, p-block, d-block and f-block. |
| 3. | There are 8 groups. | There are 18 groups. |
| 4. | Isotopes could not be placed properly. | Isotopes occupy the same position as the element. |
| s-block | p-block | |
| 1. | The s-block contains groups 1 and 2. | The p-block contains groups 13 to 18. |
| 2. | All the elements of s-block are metals. | p-Block elements include metals, nonmetals and metalloids. |
| 3. | s-block elements are placed to the extreme left side in the modern periodic table. | p-block elements are placed to the extreme right side in the modern periodic table. |
| 4. | There is no zig-zag line to separate different types of elements in the s-block of the periodic table. | A zig-zag line can be drawn to separate different types of elements (metals, metalloids and nonmetals) in the p-block of the periodic table. |
Dobereiner laid down the triad rule.
Eka boron : Scandium : : Eka Aluminum : Gallium
Electronegativity is the metallic property of an element- Wrong
Atomic number of element A is 11.
Element A belongs to group 1.
Electropositivity means the tendency of an atom of an element to form cation by losing its valence electrons.
Element A and B follows Newland’s octaves rule and six elements are there in between A and B.
The modern periodic table consists of seven horizontal rows called the periods and eighteen vertical columns called the groups.
The arrangement of the periods and groups results in the formation of boxes, where each box corresponds to the position of an element.
Atomic numbers are serially indicated in the upper part of these boxes.
In addition to these seven rows, lanthanide and actinide series are shown separately at the bottom of the periodic table. The first row is lanthanide series and the second row is actinide series.
There are 118 boxes in the periodic table including the two series. vi. The entire periodic table is divided into four blocks: s-block, p-block, d-block and f-block.
s-block: contains group 1 and 2 elements
p-block: contains group 13 to 18 elements
d-block: contains group 3 to 12 elements (transition elements)
f-block: contains lanthanide and actinide series
A zig-zag line is drawn in the p-block of the periodic table. Metals lie on the left side while nonmetals lie on the right side of this line. Metalloids lie along the border of this line.
Beryllium
Explanation-
Beryllium is an alkaline earth metal while others are noble gases.
Gallium
Explanation-
Gallium is a metal while others are metalloids.
Calcium
Explanation-
Calcium is a s-block element while others are p-block elements.
Potassium
Explanation-
Potassium is placed in group 1 while others are placed in group 2 of the modern periodic table.
Magnesium
Explanation-
Magnesium is placed in period 3 while others are placed in period 2 of the modern periodic table.
Electric motor
Explanation-
Electric motor is a device which converts electrical energy into mechanical energy whereas other devices are used to measure current and potential difference in a circuit.
Moseley
Explanation-
Moseley arranged elements in increasing order of their atomic numbers whereas others arranged elements based on their atomic masses.
K
Explanation-
K is placed in group 1 while others are placed in group 17 of the modern periodic table.
Boron
Explanation-
Boron is placed in group 13 while others are placed in group 17 of the modern periodic table.
Germanium
Explanation:
Germanium is a metalloid while others are metals.
Magnesium
Explanation-
Magnesium is an alkaline earth metal while others are alkali metals.
4Be- Electronic configuration is (2, 2). It is placed in period 2.
17Cl- Electronic configuration is (2, 8, 7). It is placed in period 3.
2He- Electronic configuration is (2). It is placed in period 1.
13Al- Electronic configuration is (2, 8, 3). It is placed in period 3.
Group 1 and 2 : s block : : group 13 and 18 : p Block
Group 13 to 18 : p block : : Group 3 to 12 : d block
| Electronic configuration |
Period | Block | |
| i. | 2, 8, 2 | 3 | s - Block |
| ii. | 2, 8, 7 | 3 | p - Block |
| iii. | 2, 1 | 2 | s - Block |
This statement is wrong.
Explanation:
Dobreiner's triads, three elements appear to be arranged in ascending order of their atomic masses.
Electronic configuration of beryllium is (2, 2).
Electronic configuration of calcium is (2, 8, 8, 2).
Among beryllium and calcium elements, calcium has the least effective nuclear charge exerting attractive force on valence electrons.
This is because of the electrons in the inner shells, which lie in between the valence shell and the nucleus.
In Mendeleev’s periodic table eka- silicon was later named as germanium.
In Modern Periodic Table the number of columns and periods are respectively 18 and 7.
Element A is lithium (Li).
Element B is neon (Ne).
Atomic number of element A is 3.
Atomic number of element B is 10.
Iodine : solid : : bromine : liquid
| Elements of 2nd row of modern periodic table | ||||||||
| Name | Lithium | Beryllium | Boron | Carbon | Nitrogen | Oxygen | Fluorine | Neon |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
| Atomic number | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|
Electronic |
2, 1 | 2, 2 | 2, 3 | 2, 4 | 2, 5 | 2, 6 | 2, 7 | 2, 8 |
Elements in ascending order of atomic mass:
Lithium < Beryllium < Boron < Carbon < Nitrogen < Oxygen < Fluorine < Neon
| Column A | Column B |
| 1) Alkaline earth metals | c) Group 2 |
| 2) Alkali metals | d) Group 1 |
| 3) Halogen | b) Group 17 |
| 4) Noble gas | a) Group 18 |
| Column A | Column B |
| 1) Eka-aluminium | b) Gallium |
| 2) Eka-Silicon | c) Germanium |
| 3) Eka- boron | a) Scandium |
| Column A | Column B |
| 1) s -block | c) Group 1, 2 |
| 2) p -block | d) Group 13 to 18 |
| 3) d -block | b) Group 3 to 12 |
| 4) f -block | a) Lanthanides and actinides |
| Column A | Column B |
| 1) Triads | b) Dobereiner |
| 2) Octave | e) Newland |
| 3) Atomic number | d) Moseley |
| 4) Atomic mass number | a) Mendeleev |
| Column A | Column B |
| Electronic configuration |
Valency |
| 1) 2, 2 | b) 2 |
| 2) 2, 8, 1 | a) 1 |
| 3) 2, 5 | c) 3 |
Mendeleev's periodic table : atomic mass : : Modern periodic table : atomic number
Moving from left to right, the size of the atom decreases- Right
Nanometer unit is used to measure atomic radius- Wrong
Newlands’ Law of Octaves is applicable upto calcium.
Elements A and B are in the same row. Similarly, elements C and D are also in the same row.
Elements A and C have the same column. Similarly, elements B and D also have the same column.
Elements B and D are in the 18th column.
Element C is more reactive than element A.
Element A will form compounds as it can easily lose its valence electron.
The elements in the third period are Sodium, Magnesium, Aluminium, Silicon, Phosphorus, Sulphur, Chlorine and Argon.
Their valencies are 1, 2, 3, 4, 3, 2, 1 and 0 respectively.
Sodium (Na), Magnesium (Mg) and Aluminium (Al) are metals. Silicon (Si) is a metalloid.
Phosphorus (P), Sulphur (S) and Chlorine (Cl) are nonmetals. Argon (Ar) is an inert gas element and is not classified as a metal, nonmetal or metalloid.
Nonmetals belong to p-block.
Two metalloid elements: Silicon and Germanium.
On moving down the group metallic character increases- Right
On moving from left to right in a periodic table, the size of the atom decreases.
Mendeleev organized the period table on the basis of atomic masses of elements.
When he arranged the 63 elements known at that time in the increasing order of their atomic masses, he found that the chemical and physical properties of elements showed repetition after certain fixed interval.
On the basis of this finding, he stated that ‘properties of elements are a periodic function of their atomic masses’. This is known as Mendeleev’s periodic law.
p-block is composed of groups 1 and 2- Wrong
Element is A is Non-metal.
Outermost shell of element B is M.
Element C is bromine (Br) and it exists in a liquid state.
Silicon is a metallic element- Wrong
Decreasing order of atomic radii: Cs > Rb > K > Na > Li
Cesium (Cs) has the biggest atom and lithium (Li) has the smallest atom.
In each triad, when the elements are placed according to the increasing order of their atomic masses, the atomic mass of the middle element in each triad was approximately equal to the mean of the atomic masses of the other two elements.
Mendeleev’s periodic law states that "The properties of elements are a periodic function of their atomic masses."
When the elements are arranged in an increasing order of their atomic masses, the properties of the eighth element are similar to the first.
Modern periodic law states that "The physical and chemical properties of all elements are a periodic function of their atomic numbers".
The d-block elements are called as transition elements.
The elements beryllium, magnesium and calcium are in group 2 and their valency is 2.
The elements from the zero group are called noble gases.
The elements lithium and beryllium are in the same period because their valencies are similar- Wrong
The groups 1 and 2 constitute the s block.
The K and L shells of the elements Na and Mg contain electrons- Right
The metallic character of the element decreases moving across a period from left to the right- Right
The modern periodic table has 1 to 7 periods- Right
The modern periodic table is divided into s-block, p-block, d-block, f-block.
The modern periodic table shows the molecular masses of the elements in each frame- Wrong
The number of electrons in the outermost shell of an atom determines the valency of an element.
The number of shells decreases in a group from top to bottom- Wrong
The similarity between the properties of the first and eighth elements is called the octave rule- Right
The size of an atom depends on the number of valence electrons- Wrong
The size of an atom is indicated by its radius.
The tendency of an element to form anion is the nonmetallic character of that element.
The tendency of an element to form cation is the metallic character of that element.
The valency of the elements in group 2 is 1- Wrong
To the left of the zig-zag line in the periodic table are the metal elements- Right
Newland’s octave rule is applicable only up to calcium.
Two elements in the same position : demerit of Newland's octaves : : place for Isotopes : Demerits of Mendeleev’s periodic table
Demerits of Mendeleev’s periodic table:
The challenges faced by Mendeleev while following periodic law were:
Mendeleev had to arrange the elements with the thought that the information available till then was not final but it could change.
He had to revise the atomic masses of some elements so as to give them a proper place in the periodic table in accordance with their properties. For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was placed before boron.
He had to keep some vacant places in the periodic table for elements not discovered till then. However, he predicted the atomic masses and properties of some these unknown elements.
There was no place reserved for noble gases in Mendeleev’s original periodic table. So, when noble gases were discovered, he had to include them. He did this without disturbing the original periodic table by creating ‘zero’ group.
He had to indicate a question mark before the symbol of the element due to uncertainty in the names of some of the elements.
There was ambiguity regarding the sequence of elements having a whole number atomic mass such as Co and Ni.
It was difficult to place isotopes of elements as they have the same chemical properties but different atomic masses.
It was not possible to predict how many elements could be discovered between two heavy elements as the rise in atomic mass is not uniform. In addition to this, it was difficult to decide the position of hydrogen in the periodic table.
The chemical reactivity of an element is determined by the number of valence electrons in it and the shell number of the valence shell.
The physical and chemical properties of elements that Mendeleev considered while compiling periodic table were:
The molecular formulae of hydrides and oxides of the elements
melting points, boiling points and densities of the elements and their hydrides and oxides.
All known elements could not be classified into groups of triads on the basis of their properties.
Dobereiner could find only three triads; .i.e total of 9 elements only.
However, the total number of elements was more than that of those encompassed in Dobereiner’s Triad.
Na
K
18 vertical columns are known as groups.
Valency
11, 19
Cobalt (59Co) and nickel (59Ni) show an ambiguity regarding their sequence in Mendeleev’s periodic table.
While designing the periodic table, Mendeleev considered the chemical and physical properties of the elements- Right
While going from top to bottom in a group the atomic radius increases.
Atomic radius is a measure of an atom’s size and changes predictably across periods and groups in the periodic table.
Electronegativity is the tendency of an atom to attract the shared pair of electrons towards itself in a chemical bond. Elements with high electronegativity strongly pull electrons, while those with low electronegativity tend to lose electrons easily. Electronegativity decreases down a group due to increased atomic size, and increases across a period due to increasing nuclear charge.
Electropositivity is the tendency of an atom to lose one or more electrons and form a positive ion (cation). Elements that easily donate electrons are called electropositive elements. Electropositivity increases down a group due to an increase in atomic size, and decreases across a period due to an increasing nuclear charge.
In the modern periodic table, a group is a vertical column of elements. All elements in the same group have the same number of valence electrons, which gives them similar chemical properties and valency. As we move down a group, the number of shells increases, leading to a gradual change in physical properties such as atomic size and metallic character, while chemical properties remain similar.
A period is a horizontal row in the periodic table where elements have the same number of electron shells and show a gradual change in properties across the row.
| Atomic Number | Electronic configuration | Group | Period | Element |
| 10 | 2, 8 | 18 | 2 | Neon (Ne) |
| 20 | 2, 8, 8, 2 | 2 | 4 | Calcium (Ca) |
| 7 | 2, 5 | 15 | 2 | Nitrogen (N) |
| Element | Symbol | Atomic number | Electronic configuration |
| Lithium | Li | 3 | 2, 1 |
| Beryllium | Be | 4 | 2, 2 |
| Boron | B | 5 | 2, 3 |
| Carbon | C | 6 | 2, 4 |
| Nitrogen | N | 7 | 2, 5 |
| Oxygen | O | 8 | 2, 6 |
| Fluorine | F | 9 | 2, 7 |
| Neon | Ne | 10 | 2, 8 |
Fluorine (F)
Electronic configuration of boron is (2, 3). Electronic configuration of oxygen is (2, 6).
Both boron and oxygen have electrons in the two shells, K and L.
The elements with the same number of shells occupied by electrons belong to the same period.
The valency of an element is determined by the number of valence electrons in the outermost shell of an atom of an element.
For example, the elements of group I contain only one valence electron, the valency of elements of group I is one. Similarly for group II, the valency is two.
While going down from the top to bottom in a group, a number of shells increases.
The outermost electrons go farther and farther from the nucleus, extending the radius and ultimately increasing the size of the atom even though the nuclear charge increases.
Boron and oxygen are placed in the same period. However, their atomic numbers are different.
Atomic number of boron is 5 and that of oxygen is 8.
In a period, as the atomic number increases from boron to oxygen, the positive charge on the nucleus increases. At the same time, electrons get added in the same outermost shell. This increases effective nuclear charge.
As a result, electrons are attracted closer to the nucleus decreasing the size of the oxygen atom.
Electronic configuration of lithium is (2, 1). Electronic configuration of sodium is (2, 8, 1).
Both lithium and sodium have single electron in their outermost shell.
Their valency is 1.
In Mendeleev’s periodic table, the elements are arranged in increasing order of their atomic masses such that chemically similar elements are placed together in a group.
However, while arranging the elements, Mendeleev kept vacant places in the periodic table for elements not discovered till then.
Three of these unknown elements were given the names eka-boron, ekaaluminium and eka-silicon from the known neighbours.
The atomic masses of these elements were indicated as 44, 68 and 72, respectively. He also predicted their properties.
While going down a group a new shell gets added, resulting in an increase in the distance between the nucleus and the valence electrons.
This results in lowering the effective nuclear charge and thereby lowering the attractive force on the valence electrons.
As a result of this, the tendency of the atom to form cation by losing electrons increases.
The metallic character of an atom is its tendency to lose electrons.
While going from left to right in a period, electrons are added to the same outermost shell. At the same time, protons get added in the nucleus increasing the nuclear charge.
As a result, the ability of an atom to become anion by accepting outside electrons increases.
The non-metallic character of an atom is its tendency to gain electrons.
Hydrogen resembles alkali metals. It has one electron in its outermost shell like alkali metals. Its valency is 1 like alkali metals.
It forms oxides (H2O) similar to alkali metals (Na2O). However, hydrogen also resembles halogens. It is a diatomic molecule (H2) like halogens (F2, Cl2).
It is one electron deficient to achieve stable electronic configuration similar to halogens.
Due to this unique behaviour of hydrogen, it could not be decided whether the correct position of hydrogen is in the group of alkali metals or in the group of halogens.
Thus, there was ambiguity about the correct position of hydrogen in Mendeleev's periodic table.
Isotopes were discovered long time after Mendeleev developed the periodic table.
Isotopes have same chemical properties but different atomic masses.
characteristic of Dobereiner’s triads - In each triad, the elements were placed according to the increasing order of their atomic masses and, the atomic mass of the middle element in each triad was approximately equal to the mean of the atomic masses of the other two elements.
| Element | Molecular formula |
| Na | Na2O |
| Si | SiO2 |
| Ca | CaO |
| C | CO2 |
A halogen from period 4- Bromine (Br)
Name: Fluorine
Symbol: F
Highest electropositive element- Francium (Fr)
Magnesium (Mg) is an element having electronic configuration 2,8,2.
The noble gas having 2 electrons in its valence shell is helium.
The atom having smallest atomic radius from group 1- Hydrogen (H)
The atom having the biggest atomic size from period 2- Lithium (Li)
The atom having the smallest atomic radius from zero group- Helium (He)
The element having stable electron configuration from the third period- Argon (Ar)
The family of elements having zero valency- Noble gases/Inert gases
The family of group 1 elements- Alkali metals
The group of elements having electronic configuration 2, 2- Group 2
Two elements having two orbits- Carbon (C), oxygen (O)
X and Y are two elements having similar properties which obey Newlands’s Law of Octaves. The minimum and maximum number of elements in between X and Y respectively are 6 and 13.
Explanation:
It is assumed that X and Y are two elements with comparable characteristics that adhere to Newland's Law of Octaves.
This suggests that there are eight groupings of elements, with the ninth member having attributes identical to the first.
As a consequence, the lowest and maximum number of elements between X and Y are 6 and 13 respectively.
Element X is placed in group 1 and period 3.
Element Y is placed in group 17 and period 3.
Element X is a metal while element Y is a nonmetal.
Element X belongs to s-block and element Y belongs to p-block.
Electronic configuration of element X is (2, 8, 1). Its valency is 1.
Electronic configuration of element Y is (2, 8, 7). Its valency is 1.