Content Writer
The classification of the elements is necessary to understand the similarty between the properties of elements. However, the initial theories - Newland’s law of octaves, and Dobereiner’s Triads were based on the elements which existed during the time of identification of theories and did not predict about the newly discovered elements. This was solved by the Modern Periodic table, in which the classification of elements is done based on the properties exhibited by similar elements.
Read Also: Electrochemistry
Need of Periodic Classification of Elements
[Click Here for Sample Questions]
The need for the periodic classification of elements arose when the number of elements went up from 31 initially, to 94 in the second half of the 1800s. Presently, there are about 118 elements, out of which some are even man made elements. The study of the elements initially could be done individually, as there were fewer elements discovered. Now, as the number has increased to 118, it might be very difficult to study all the elements individually, and hence they have been classified into Periods depending upon the similarity in their behaviors / characteristics.
Early Models of Periodic Table
[Click Here for Sample Questions]
-
Dobereiner’s Triads
Dobereiner, a German Chemist, in the year 1817 identified a group of 3 elements with similar chemical and physical properties in the order of aggregate atomic masses and called it a triad. He showed that the atomic mass of the central element is approximately the arithmetic mean of the two. Some of the examples of Dobereiner’s triads are:
Dobereiner’s Triads
Read More: First 20 Elements in the Periodic Table
-
Limitations of Dobereiner’s theory:
- The new elements could not satisfy the condition specified by Dobereiner
- A total of only 5 triads could be identified.
- The then existing elements also could not satisfy the conditions.
Looking forward to these limitations, other methods were founded to classify the elements.
Read More: Anode and Cathode
Newlands’ Law of Octaves
[Click Here for Sample Questions]
According to Newland’s law, when the elements are managed according to aggregate atomic masses, the physical and chemical properties of every 8th element are the same as that of the first. Newlands associated these octaves with the series of 8th notes of a musical scale.
| sa (do) | re (re) | ga (mi) | ma (fa) | pa (so) | da (la) | ni (ti) |
| H | Li | Be | B | C | N | O |
| F | Na | Mg | Al | Si | P | F |
| Cl | K | Ca | Cr | Ti | Mn | Fe |
| Co and Ni | Cu | Zn | Y | In | As | Se |
| Br | Rb | Sr | Ce and La | Zr | ______ | _____ |
Limitations of Newland’s Law:
- Some of the existing elements could not fit in the classification.
- A few elements having dissimilar properties were found to be grouped together.
- The elements that were found after the law, could not fit the classification.
Mendeleev’s Periodic Table and Law
[Click Here for Sample Questions]
Madeleev’ law stated that the chemical properties and physical properties of elements are periodic purposes of their atomic masses. On the basis of this Mendeleev’s Periodic table is formed.
Mendeleev’s Periodic table
Features of Mendeleev’s Periodic Table
- 12th horizontal rows, which were reduced to 7, known as periods.
- 8th vertical columns are known as groups.
- Group 1st to 7th subdivided into A and B subgroups.
- Group 8th does not have any contains and subgroups three elements in each row.
- Elements in the same group exhibit equal properties.
Achievements of Mendeleev’s Periodic Table
A step-by-step study of elements: Elements with equal properties were grouped, which made the study of their physical and chemical properties easier.
Correction of atomic masses: Placement of elements in Mendeleev’s periodic table useful in correcting the atomic masses of certain elements.
Prediction of properties of yet to be searched elements: eka-aluminum, eka-boron, and eka-silicon were the names provided to yet to be searched elements. The properties of these elements could be predicted correctly from the elements that belonged to the equal group.
Placement of noble gasses: When searched, they were placed perfectly in a new group called zero groups of Mendeleev’s table.
Limitations of Mendeleev’s Periodic Law
- Mendellev was unable to locate the position of hydrogen in the periodic table.
- Increase in atomic mass was not fixed on moving from one element to another. Hence, the position of elements which will be discovered in future was not predictable.
- The isotopes of elements were found later on and they violated the Mendeleev’s periodic law.
Modern Periodic Table
[Click Here for Sample Questions]
In 1913, Henry Mosely stated the Modern Period Law. He showed that the elements can be more fundamentally arranged on the basis of their atomic number that their atomic masses. According, Mandeleev’s Periodic Law was modified and the Modern Periodic Table was formed. The Modern Periodic Law states that- “ Properties of elements are a periodic function of their atomic number.”
In the Modern Periodic Table, the elements are characterised in the form of Periods and Groups.
Modern Periodic Table
Periods in Modern Periodic Table
Elements present in the same period have an equal number of shells which is the same as the period number.
Number of Elements in a Period
- The horizontal classification of the elements in the modern periodic table is known as a Period.
- The 1st period contains only two elements 1 H and 2He and is known as the direct period.
- The 2nd period (3Li to 10Ne) and the 3rd period (11Na to 18Ar) contain 8th elements each and are known as short periods.
- The fourth period (19K to 36Kr) and the fifth period (37Rb to 54Xe) contain 18 elements each and are called long periods.
- The sixth period contains 32 elements (55Cs and 86Rn) and is also known as the longest period.
- The 7th period is incomplete.
- When we consider an element moving from left to right in the modern periodic table, the number of valence electrons increases in the period, whereas the number of shells remains the same.
Groups in Modern Periodic Table
The modern periodic table includes 18 vertical columns identified as groups.
- The first group elements are known as alkali metals.
- The second group elements are known as alkaline earth metals.
- The elements of group 3 to group 12 are called transition elements.
- Fifteenth group elements are known as pnicogens.
- Sixteenth group elements are known as chalcogens.
- Seventeenth group elements are known as halogens.
- Eighteenth group elements are known as noble gases.
There is a special classification of elements after Lanthanum (57), and these elements are called Lanthanides (up to 71), and the elements after Actinium 89 are called Actinides. These elements are classified separately due to the electronic configuration of their electrons.
Read More:
| Related Topics: | |
|---|---|
| Nernst Equation | Electrolytic Cells and Electrolysis |
| Fuel Cells | Electrolytic galvanic cells |
Trends in the Modern Periodic Table
The trends of the metallic / nonmetallic characteristics, valency, variation in the atomic size in the modern periodic table have been discussed here.
Metallic character
The metals are electropositive as they form a connection by losing electrons. In general cases, oxides of metals are basic.
Nonmetallic character
Along the period, the non metallic character trend increases, as the tendency to gain electrons by an element increases because of the increase in the nuclear charge. Whereas going down the group, the trend decreases.
Metalloids
The elements which show the properties of both nonmetals and metals are called metalloids. For example – Silicon, Boron, arsenic, germanium
Variation of Atomic Size
Atomic size of radii: It is clear as the distance from the center of the nucleus to the valence shell of the atom.
Atomic radius reduces because the effective nuclear charge is produced by one unit and it pulls valence electrons cloud closer to the nucleus.
Atomic radius increases because new shells are additional, the distance between the nucleus and valence electrons or the electron cloud increases.
Variation of Metallic Properties
A metallic character is reduced because the tendency to lose valence electrons is reduced due to increasing nuclear charge.
As the distance between the nucleus and outermost electrons increases, the nuclear pull is reduced. The tendency of an atom to lose valence electron, growth metallic character increases.
Electronegativity
The trend of electronegativity increases along the period, whereas it decreases on moving down the group.
Previous Year’s Questions
Ques. The position of three elements A, B and C in the Periodic Table are shown below:
Giving reasons, explain the following:
(a) Element A is non-metal.
(b) Atom of element C has a larger size than the atom of element A.
(c) Element B has a valency of 1. (Delhi 2014, 5 Marks)
Answer:
(a) Element A is a non-metal because it belongs to Group VII. All the elements of Group VII have 7 electrons in its outermost shell. They need one electron to acquire the nearest inert gas configuration and form a monovalent -ve ion.
(b) Atom of element C has a larger size than that of element A because C belongs to the 3rd period therefore it has three shells whereas A belongs to the 2nd period which means it has two shells.
(c) Element B has a valency of 1 because it belongs to Group I and has one valence electron. To complete its outermost shell, it loses one electron and forms a monovalent + ve ion.
Ques. Explain the trends in the Modern Periodic Table of various properties like valency, atomic size, metallic and non-metallic properties of the atoms of elements. (All India 2013, 5 Marks)
Answer:
(i) Valency- Elements that belong to the same group have the same number of valence electrons and thus they have the same valency.
Valency in a particular period from left to right first increases as positive valency and then decreases as negative valency.
Example, In elements of 2nd period: Li has 1+ valency, then Be2+, Be3+, C4+ covalency, N3- valency, then O2- and F- valency.
(ii) Atomic size- The atomic size or atomic radius increases as we move down in a group and it decreases as we move from left to right in a period. Atomic size increases down a group due to increase in the number of shells. Atomic size decreases along a period due to increase in the nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
(iii) Metallic and non-metallic properties- In the modern periodic table metals are on the left side and nonmetals are on the right side. A zigzag line of metalloids separates the metals from nonmetals.
Metallic character decreases from left to right in a period and increases while going down in a group. Non-metallic character increases from left to right in a period and decreases from top to bottom in a group.
Ques. The elements of the third period of the Periodic Table are given below:
(a) Which atom is bigger, Na or Mg? Why?
(b) Identify the most (i) metallic and (ii) non-metallic element in Period 3.
(c) Which is more non-metallic, S or Cl?
(d) Which has higher atomic mass, A1 or Cl? (Delhi 2013, 5 Marks)
Answer:
(a) Na is bigger than Mg because on moving from left to right in a period, the atomic number of elements increases. The increase in the atomic number of the elements means that the number of protons and electrons in the atom also increases.
(b) (i) Most metallic element is Na and the most non-metallic element is Cl.
This is because on moving from left to right in a period the nuclear charge on the element increases thus the valence electrons are pulled more strongly by the nucleus and it becomes more and more difficult for the atoms to lose electrons. So the tendency of the atoms to lose electrons (i.e., metallic character) decreases on moving from left to right in a period.
On the other hand, due to increase in the nuclear charge, it becomes easier for the atoms to gain electrons. So the tendency to gain electrons (i.e. non-metallic character) increases on moving left to right in a period.
(c) Cl is more non-metallic than S because on moving from left to right in a period, nuclear charge increases which means that the tendency to gain electrons increases. (i.e., non-metallic character).
(d) Cl has higher atomic mass because on moving from left to right in a period, atomic number increases. As a result atomic mass also increases.
Ques. An element P (atomic number 20) reacts with an element Q (atomic number 17) to form a compound. Answer the following questions giving the reason.
Write the position of P and Q in the Modern Periodic Table and the molecular formula of the compound formed when P reacts with Q. (Delhi 2017, 3 Marks)
Answer: Element P has the electronic configuration: 2, 8, 8, 2. So, element P belongs to 4th period and group 2, as it has four shells and 2 valence electrons.
Element Q has the electronic configuration: 2, 8, 7. So element Q belongs to 3rd period and group 17, as it has three shells and seven valence electrons.
Molecular formula of the compound will be PQ2. As valency of P is 2 and valency of Q is one.
Ques. Write the names given to the vertical columns and horizontal rows in the Modern Periodic Table. How does the metallic character of elements vary on moving down a vertical column? How does the size of atomic radius vary on moving left to right in a horizontal row? Give reason in support of your answer in the above two cases. (Delhi 2017, 3 Marks)
Answer: Vertical columns are called Groups and the horizontal rows are called Periods.
Metallic character of an element increases on moving down in a group. The reason behind this is the ability of the elements to lose electrons increases on moving down the group due to increase in distance between the nucleus and the valence electrons. There is a decrease in the attraction between the nucleus and the valence electrons.
Atomic radius decreases as we move from left to right in a period. The reason behind this is that the nuclear charge increases on moving from left to right in a period resulting in an increase in the attraction between the nucleus and the valence electrons.
Ques. An element ‘X’ belongs to 3rd period and group 13 of the Modern Periodic Table. (All India 2016, 3 Marks)
(a) Determine the valence electrons and the valency of ‘X’.
(b) Molecular formula of the compound formed when ‘X’ reacts with an element ‘Y’ (atomic number = 8).
(c) Write the name and formula of the compound formed when ‘X’ combines with chlorine.
Answer: X belongs to the 3rd period and the 13th group.
(a) Since X belongs to the 13th group, therefore the number of valence electrons in it= 3.
So the valency of element X = 3
(b) When ‘X’ reacts with element ‘Y’:
∴ Molecular formula of compound formed = X2Y3.
(c) Electronic configuration of the element X= 2, 8, 3
Which means X is Aluminium (Al)
Atomic number of Chlorine = 17
Electronic configuration of chlorine = 2, 8, 7
No. of valence electrons = 7
Which means Valency of Chlorine = 8 – 7 = 1
So the compound formed is Aluminium chloride and the molecular formula of compound is AlCl3.
Ques. An element ‘X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of ‘XAlso write group number, period number and valency of ‘X’. (All India 2016, 3 Marks)
Answer: Mass number of element ‘X’ = 35
No. of neutrons = 18
So, the atomic number of X = Mass number – Number of neutrons = 35 – 18 = 17
Thus, electronic configuration of X = 2, 8, 7
Group number = 17thPeriod number = 3rd
Valency = 8 – 7 = 1
Ques. Name any two elements of group one and write their electronic configurations. What similarity do you observe in their electronic configurations? Write the formula of oxide of any of the aforesaid elements. (All India 2016, 3 Marks)
Answer: Two elements of group 1
Na = Sodium
K = Potassium
Electronic configuration of the elements:
Na = 2,8,1
K = 2,8,8,1
Similarity: Both have one electron in their outermost shell. Thus both have one valence electron and have the valency one.
Oxide of Sodium ⇒ Na2O
Oxide of Potassium ⇒ K2O
Ques. Taking the example of an element of atomic number 16, explain how the electronic configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number. (All India 2015, 3 Marks)
Answer: Atomic number of the given element = 16
Electronic configuration =
Since this element has 3 shells and the period number is equal to the number of shells. So, the element will belong to the 3rd period. No. of valence electrons = 6
∴ The group number will be 10 + 6 = 16
The valency of an element is determined by the number of valence electrons present in the outermost shell.
Valency of the element will be = 8 – valence electrons = 8 – 6 = 2
Ques. How many groups and periods are there in the modern periodic table? How do the atomic size and metallic character of elements vary as we move:
(i) down a group and
(ii) from left to right in a period? (Delhi 2015, 2 Marks)
Answer: There are 18 groups and 7 periods in the modern periodic table.
(i) Atomic size and metallic character of the elements increases down a group.
(ii) From left to right in a period-
- Atomic size and metallic character of elements decreases from left to right in a period.
- Metallic character of the element is decreased.
Ques. An element ‘M’ has atomic number 11:
(a) Write its electron configuration.
(b) State the group to which ‘M’ belongs?
(c) Is ‘M’ a metal or a non-metal?
(d) Write the formula of its chloride (Delhi 2012, 2 Marks)
Answer: Element ‘M’ has atomic number = 11
(a) Electronic configuration = 2, 8, 1
(b) ‘M’ belongs to the 1st group as it has one valence electron.(c) ‘M’ is a metal as atoms with valence electrons 1, 2, 3 are metals.
(d) Formula of its Chloride-
Sample Questions
Ques: What was the process used by Mendeleev in making his Periodic Table?(1 mark)
Answer: Mendeleev focussed on many compounds formed by the elements with Oxygen and Hydrogen. Among Physical properties. He experimented with the relationship between the atomic masses of many elements while making his periodic table.
Ques: Why do you contemplate the noble gasses are placed in a distinct group?(1 mark)
Answer: Noble gases are placed in a distinct group because of their inert nature and low focus in our atmosphere. They are kept in a separate group called the Zero group so that they do not disturb the existing order.
Ques: Name the second element you would like to show chemical reactions equal to magnesium. What is the foundation for your choice?(1 mark)
Answer: Beryllium and Calcium are equal to Magnesium because all the three elements belong to the equal group and have 2 valence electrons in their outer shell.
Ques: In the Modern Periodic Table, which are the metals among the first 10th elements?(1 mark)
Answer: Beryllium and Lithium are the metals among the first ten elements in the Modern Periodic Table.
Read More:
| Related Topics: | |
|---|---|
| Electrochemical Cells | Batteries |
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Comments