Periodic Classification Of Elements: The study of the Periodic Classification of elements is one of the most important and basic aspects of chemistry. It is essentially used to classify all the elements in groups. It helps us to approach a systematic study of various elements found in nature without which it would have been impossible to study the Periodic Table.
With the help of the Periodic Classification of Elements, a comparative study of the elements and their compounds can be done. It also helps us to analyze the regular variations in various properties such as ionization potential, electron affinity, electronegativity, etc. This article will give you a detailed understanding of CBSE Class 10 Science Periodic Classification of Elements.
Theories On Periodic Classification of Elements
Doberiner’s Triads, 1829
The first scientist to classify the elements was Johann Wolfgang Döbereiner. He classified the elements showing similar properties in groups of three elements which he called ‘Triads’. In the triads, the atomic weight of the element in the center was the arithmetic mean of the atomic weights of the other two elements. For example:
Dobereneir could only arrange nine elements (listed below) in the triad form. Thus, his idea of periodic classification of elements into triads was not widely embraced.
- Li, Na, K
- Ca, Sr, Ba
- Cl, Br, I
Newlands’ Law of Octaves, 1865
In 1865, John Newlands proposed new law on the periodic classification of elements. The law states, “If elements are arranged by the increasing order of their atomic masses, property of every eighth element (starting from the first element) repeats”.
Newlands compared this to the octaves found in music and called it the ‘Law of Octaves’.
Inert gases were not discovered at the time Newlands proposed this law. Thus, the properties of new elements that were not discovered could not be classified. Only 56 elements incorporated in this law were known at that time. The elements yet to be discovered were not considered. At many places, two elements were placed in a single slot (like Cobalt and Nickel). Placing of iron far away from cobalt and nickel, which have similar properties as iron, were also not explained.
Mendeleev’s Periodic Table
In 1869, a Russian chemist called Dmitri Mendeleev published a periodic table. He arranged all 63 elements known at the time in order of relative atomic mass. He put those elements in a group that had similar properties. Mendeleev’s periodic law states that when elements are arranged in the order of increasing atomic masses, the elements with similar properties occur at regular intervals. This periodic table is divided into nine columns which are called groups and seven rows which are called periods.
Because of the way he arranged the elements, there were gaps in the periods. Mendeleev thought the gaps meant that the elements which would fit in the gaps had not been discovered yet. He was also able to work out the atomic mass of the missing elements and predict their properties. He predicted the properties of an element undiscovered at that time that should fit below aluminum in his table. This element’s (gallium) properties were later found to be close to Mendeleev’s predictions.
- The study of the properties of elements became easier as all known elements were classified in a group according to their similar properties for the first time.
- It encouraged the discovery of new elements as some gaps were left in it. Mendeleev gave details on the position and properties of the elements like Sc, Ge, and Tc before they were discovered and even left blank spaces for them in his table.
- Properties of elements in a particular period of the table show regular gradation (i.e. increase or decrease) from left to right.
- IN this periodic table, the elements in a particular group share similar properties. They show a regular gradation in their physical properties and chemical reactivities.
- Position of hydrogen – Hydrogen resembles both, the alkali metals and halogens in properties so Mendeleev could not decide where to place it.
- Position of isotopes – Isotopes are atoms of the same element having different atomic masses but have similar chemical properties. As the atomic weight of isotopes differs, they should have been placed in different positions in Mendeleev’s periodic table. However, no such positions were there in Mendeleev’s periodic table.
- There were some pair of elements (anomalous pairs) which did not follow the increasing order of atomic weights. For example, Ar and Co have higher atomic weights but they were placed before K and Ni, respectively.
- Elements with similar properties were placed in different groups.
Modern Periodic Table Of Elements
In 1913, Moseley proved that the atomic number is a very important property of an element. After that, Neils Bohr made the modern periodic table using the atomic number.
According to the modern periodic classification, the properties of elements are the periodic function of their atomic numbers. The periodicity in properties is due to the repetition of similar outer shell electronic configuration at certain regular intervals. The modern periodic table is based on modern periodic law in which elements are arranged in increasing order of their atomic numbers.
In this table, the elements are arranged in rows and columns. These rows are called ‘periods’ and the columns are known as ‘groups’. The table is made up of 7 periods and 18 groups. Period denotes the value of ‘n’ (principal quantum number) for the outermost or valence shell. The same number of electrons are present in the outer orbitals.
Characteristics of Periods:
- The first period is called the shortest period and contains only two elements. The second and third periods are called short periods containing eight elements each. The fourth and fifth periods are long periods containing eighteen elements each. The sixth period contains 32 elements. It is the longest period. The seventh period is an incomplete period containing nineteen elements.
- Lanthanide and actinide series (with 14 elements each) are placed separately under the main periodic table. These are related to the sixth and seventh periods of Group 3 respectively.
- Elements of the third period from sodium (Na) to Chlorine (Cl) are called representative or typical elements.
- In a period, the valency of an element increases from 1 to 7 with respect to oxygen.
- From left to right in a period generally atomic weight, effective nuclear charge, ionization potential, electronegativity and electron affinity of an element increases. From left to right in a period, atomic radius, electropositive character, and metallic character of an element decrease.
- Elements like Li, Be, and B of the second period are very similar to the elements Mg, Al, and Si of the third period in the next higher group.
- Elements of the second period are called bridge elements.
Characteristics of Groups:
- There are nine groups in the modern periodic table and they are represented by Roman numerals as I, II, III, IV, V, Vi, VII, VIII, and zero.
- Groups I to VII are divided into two subgroups (A and B), Group VIII is made up of three sets, each one containing three elements.
- Zero groups contain inert gases or noble gases.
- The valency of an element in a group is the same as the group number.
- Normal elements are the elements of the groups which resemble the typical elements. These are elements are of group IA, IIA, IIIA, IVA, VA, VIA, VIIA.
- Transition elements are those elements of the groups which do not resemble the typical elements. These are elements are of group IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII. Both IA and VIIA groups contain Hydrogen.
- Atomic weight, atomic size, electropositive character, and metallic character of elements increases as we go down the group.
- Ionization potential, electron affinity, and electronegativity of elements decrease down the group.
Characteristics of Elements:
The elements are divided into four groups on the basis of electronic configuration –
S – block elements
- These are the elements in which the electron enters s subshell.
- These are located on the left side of the periodic table.
- These include 1 and 2 group elements.
- All the s- block elements are metals.
- The general electronic configuration of the valence shell for s block elements is ns1-2 ( n = 1 to 7).
P – block elements
- These are the elements in which the last electron enters p subshell.
- These are located on the right side of the periodic table.
- Group 13 to 18 of the periodic table are included in P-block elements.
- Most of the elements in P-block are metalloids and non-metals. A few of them are metals.
- The general electronic configuration of the valence shell is ns2 np1-6 ( n = 2 to 7).
- ns2 np6 is a stable noble gas configuration. The electronic configuration of He is Is2.
D – block elements
- These are the elements in which the electron enters d-subshell.
- These are present between S and P-block elements.
- Group 3 to 12 groups of the periodic table are included in D-block elements.
- All are D-block elements are metals.
- The last electrons fill in (n – 1)d orbital.
- The outermost electronic configuration of D-block elements is (n-1)d1-10 ns1-2 (n = 4 to 7).
- There are three series of D-block elements:
- 3d series – Sc(21) to Zn (30)
- 4d series – Y (39) to Cd (48)
- 5d series – La (57), Hf (72) to Hg (80)
F – block elements
- These are placed in a separate section below the main periodic table.
- These are mainly related to IIIB, that is, group 3 of the periodic table.
- There are two series of F-block elements:
- 4f series – Lanthanides – 14 Elements, that is, Ce (58) to Lu (71)
- 5f series – Actinides – 14 Elements, that is, Th (90) to Lw (103)
- The last electron fills in (n – 2)¦-orbital
- Their general outermost electronic configuration is (n-2)¦1-14 (n-1)s2 (n-1)p6 (n-1)d0-1ns2 (n = 6 and 7).
Tips To Learn Periodic Classification Of Elements
Since the modern periodic table is extensive, students find it difficult to learn. Many students learn different groups and periods in the periodic table in the form of mnemonics. We have compiled some general tips that can help you remember the content of the periodic table easily:
- Get hard copies of the table – Get yourself multiple hard copies of the table. Read the table or its selective parts like a story when you are sitting idle. If you have a habit of writing while reading, you can copy the table yourself. This will make remembering the table easier as you are writing it yourself.
- Divide the table into segments – The periodic table has different segments. This segregation into different parts is based on parameters like the increasing order of the atomic number. You can choose, read, and remember the parts of the table with respect to these parameters. You can revise and reexamine the segments occasionally for better learning.
- Make abbreviations, acronyms, or mnemonics – Abbreviations, acronyms, or mnemonics are very useful while learning something. These are especially helpful when dealing with monotonous sets of data like the periodic table. You can link the elements to a sentence, a place, or something familiar in order to remember the elements. You can also form a sentence or a rhyme that can be used as a mnemonic.
- Correspond each element with an image – Corresponding each element with an image will help you remember the table. It will also act as a memory exercise when you are trying to remember which image you associated with which element. You can also create a list or chart based on this formula.
- Take online quizzes/tests – You can find numerous free tests/quizzes online on the topic. You can take these exclusive tests/quizzes to check how much you remember. These are excellent sources to jog your memory from time to time.
- Use flashcards – You can make your own flashcards or buy a readymade set to memorize the periodic table. The flashcards should have details of each element like symbol, atomic number, atomic weight, etc. These flashcards are a fun way of learning.
Use of Periodic Table in Chemistry
From the periodic table, one can get the following information about an element –
- Atomic size: Atomic size of the element increases from top to bottom in a group.
- The number of shells: from top to bottom number of shell increases by one for each element.
- The number of Valence electrons: Number of valence electrons is constant.
- Ionization potential: It decreases from top to bottom.
- Electronegativity: Electronegativity decreases from top to bottom.
- Metallic Character: Metallic character increases down the groups.
The Periodic Classification of Elements is an important topic for students not only from the examination point of view but also because it is one of the fundamental concepts in chemistry. It is important to have a sound knowledge of the Periodic Table of Elements in order to understand the subject better and ease the learning process.
You can take free Class 10 Foundation Test on Embibe to understand the paper pattern and test your preparation level for both Science and Maths. Also, you can take a free JEE Main Mock Test to solve higher level and in detailed questions of the Periodic Table of Elements.2857 Views