Overview
Periodic Classification of Elements is the story of how chemists brought order to a growing crowd of elements, and of how each attempt failed in a way that pointed to the next. It begins with Dobereiner's triads of 1817, in which the atomic mass of the middle element was very nearly the average of the other two - an idea that worked beautifully for lithium, sodium and potassium but could identify only three triads among the elements then known. It moves to Newlands' Law of Octaves of 1866, which arranged elements in increasing order of atomic mass and found that every eighth element had properties similar to the first, like the notes of a musical scale; it worked only up to calcium, forced unlike elements into the same column, and assumed that only 56 elements existed in nature. Then comes Mendeleev, who in 1869 arranged 63 known elements by atomic mass and by chemical properties, using the formulae of their hydrides and oxides, and whose Periodic Law stated that the properties of elements are the periodic function of their atomic mass. His table's greatness lay in what he left out: he boldly left gaps for elements not yet discovered, named them eka-boron, eka-aluminium and eka-silicon, predicted their properties, and was vindicated when scandium, gallium and germanium were found. The chapter then sets out his three limitations - the position of hydrogen, the problem of isotopes, and pairs like cobalt and nickel that had to be placed out of mass order - and resolves them with Moseley's discovery that atomic number, not atomic mass, is the fundamental property, giving the Modern Periodic Table of 18 groups and 7 periods and its trends in valency, atomic size and metallic character.
Learning Objectives
- Explain why a classification of elements was needed.
- State Dobereiner's law of triads and give an example.
- State Newlands' Law of Octaves and explain its limitations.
- Describe how Mendeleev arranged the elements and state Mendeleev's Periodic Law.
- Explain the achievements of Mendeleev's Periodic Table, including the prediction of undiscovered elements.
- State the limitations of Mendeleev's Periodic Table.
- State the Modern Periodic Law and explain why atomic number replaced atomic mass.
- Describe the structure of the Modern Periodic Table in terms of groups and periods.
- Explain how the number of elements in each period is determined.
- Explain the trend in valency across a period and down a group.
- Explain how atomic size varies across a period and down a group, with reasons.
- Explain how metallic and non-metallic character vary in the periodic table.
Topics in this chapter
6 topics · tap a topic title to jump straight to it.
Early Attempts: Dobereiner's Triads
Why classify at all
We know that Dmitri Ivanovich Mendeleev was the most important contributor to the early development of a periodic table of elements. But before we take up his work, let us look at some of the earlier attempts at the classification of elements.
The classification of elements was necessary because the number of known elements kept growing, and studying each one separately would have been impossible. A pattern would let chemists predict properties instead of memorising them.
Dobereiner's triads
In the year 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each. So he called these groups 'triads'.
Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
Take the triad lithium (Li), sodium (Na) and potassium (K) with atomic masses 6.9, 23.0 and 39.0. The average of the first and third is (6.9 + 39.0) / 2 = 22.95, which is very close to the atomic mass of sodium.
The other two triads he found were calcium, strontium and barium, and chlorine, bromine and iodine.
The limitation
Dobereiner could identify only three triads from the elements known at that time. Hence, this system of classification into triads was not found to be useful.
Notice the shape of the failure: the idea was correct as far as it went, but it covered too few elements to be a system. Every later attempt in this chapter is an attempt to widen the coverage.
- Li (6.9), Na (23.0), K (39.0): the average of 6.9 and 39.0 is 22.95, very close to sodium's 23.0.
- The three triads Dobereiner found: Li-Na-K, Ca-Sr-Ba, and Cl-Br-I.
- Only three triads could be identified among the elements then known, so the system was not useful.
- Dobereiner's triad: atomic mass of the middle element is approximately (mass of first + mass of third) / 2
Newlands' Law of Octaves
The idea
The English scientist John Newlands in 1866 arranged the then known elements in the order of increasing atomic masses. He started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element.
He found that every eighth element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it the 'Law of Octaves'. It is known as 'Newlands' Law of Octaves'.
In Newlands' Octaves, the properties of lithium and sodium were found to be the same. Sodium is the eighth element after lithium. Similarly, beryllium and magnesium resemble each other.
The four limitations
- It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
- It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.
- In order to fit elements into his table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note. For example, cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties from these elements. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.
- Thus, Newlands' Law of Octaves worked well with lighter elements only.
Newlands' law is the second stage of the same problem: it covered far more elements than triads did, but broke down beyond calcium and forced unlike elements together. The next step needed a principle that could accommodate elements not yet discovered.
- Newlands began with hydrogen and ended at thorium, the 56th element then known.
- Cobalt and nickel were put in one slot, in the same column as fluorine, chlorine and bromine, which are quite unlike them.
- The Law of Octaves worked only up to calcium and only for lighter elements.
Mendeleev's Periodic Table
The basis of his arrangement
When Mendeleev started his work, 63 elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties.
Among chemical properties, Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.
Mendeleev then took 63 cards and on each card he wrote down the properties of one element. He sorted out the elements with similar properties and pinned the cards together on a wall. He observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses.
Mendeleev's Periodic Law
It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On this basis, Mendeleev formulated a Periodic Law, which states that 'the properties of elements are the periodic function of their atomic mass'.
The structure
Mendeleev's Periodic Table contains vertical columns called 'groups' and horizontal rows called 'periods'. There were eight groups and six periods.
What made it great: the gaps
- Mendeleev's Periodic Table could predict the existence of some elements that had not been discovered at that time. Mendeleev left some gaps in his Periodic Table.
- Instead of taking this as a limitation of his Table, Mendeleev boldly predicted the existence of some elements that had not been discovered at that time.
- Mendeleev named them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka-boron, Eka-aluminium and Eka-silicon, respectively.
- The noble gases like helium (He), neon (Ne) and argon (Ar) have been mentioned in the last group of the Periodic Table. These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. One of the strengths of Mendeleev's Periodic Table was that, when these gases were discovered, they could be placed in a new group without disturbing the existing order.
Anomalies he accepted
Some elements were placed in a way that did not follow the order of increasing atomic mass. Mendeleev did this so that elements with similar properties could be grouped together. For example, cobalt (58.9) appears before nickel (58.7), and tellurium (127.6) before iodine (126.9). He gave priority to chemical properties over atomic mass.
- Mendeleev used the formulae of hydrides and oxides as a basic classifying property, writing each of 63 elements on a card.
- Eka-boron, eka-aluminium and eka-silicon turned out to be scandium, gallium and germanium.
- Noble gases were discovered late but fitted into a new group without disturbing the existing order.
Limitations of Mendeleev's Classification
The position of hydrogen
No fixed position could be given to hydrogen in Mendeleev's Periodic Table.
Hydrogen resembles alkali metals: like them it combines with halogens, oxygen and sulphur to form compounds with similar formulae. But hydrogen also resembles halogens: it exists as a diatomic molecule H2, just as fluorine and chlorine exist as F2 and Cl2. So hydrogen could be placed either with the alkali metals or with the halogens, and this was the first limitation of Mendeleev's Periodic Table.
Isotopes
Isotopes were discovered long after Mendeleev had proposed his periodic classification of elements. Isotopes of all elements posed a challenge to Mendeleev's Periodic Law.
Isotopes have the same chemical properties but different atomic masses. If elements are arranged by atomic mass, the isotopes of one element would have to be placed in different positions - which is clearly wrong.
The anomalous pairs
Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements - especially when we consider the heavier elements.
Cobalt (atomic mass 58.9) had to be placed before nickel (58.7), and tellurium (127.6) before iodine (126.9), in violation of the very law the table was built on.
The resolution
Every one of these three problems has the same root: atomic mass is not the fundamental property of an element. Once the correct fundamental property was identified, all three difficulties disappeared at once.
- Hydrogen resembles alkali metals in its compounds and halogens in existing as a diatomic molecule H2.
- Isotopes have identical chemical properties but different atomic masses, so mass cannot be the ordering principle.
- Cobalt (58.9) before nickel (58.7), and tellurium (127.6) before iodine (126.9) - anomalies Mendeleev had to accept.
The Modern Periodic Table
Moseley's discovery
In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
Accordingly, Mendeleev's Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table.
The Modern Periodic Law can be stated as follows: 'Properties of elements are a periodic function of their atomic number.'
How it resolves the three problems
- The position of cobalt and nickel is resolved: cobalt has atomic number 27 and nickel 28, so cobalt correctly comes first. The same holds for tellurium (52) and iodine (53).
- Isotopes are all placed in one and the same position, because they have the same atomic number.
- The position of hydrogen can still be regarded as anomalous, but atomic number settles where it sits in the sequence.
Groups and periods
The Modern Periodic Table has 18 vertical columns known as 'groups' and 7 horizontal rows known as 'periods'.
Elements in a group have the same number of valence electrons, and hence similar chemical properties. Look at group 1: hydrogen, lithium, sodium, potassium, rubidium, caesium and francium - all have one valence electron.
Elements in a period have the same number of shells. Each period marks a new electronic shell getting filled. The number of elements in a period is determined by the number of electrons that can be accommodated in the shells.
- First period: 2 elements (K shell, maximum 2 electrons).
- Second period: 8 elements.
- Third period: 8 elements.
- Fourth period: 18 elements.
- Fifth period: 18 elements.
- Sixth period: 32 elements.
- Seventh period: incomplete.
The position of an element in the Periodic Table tells us about its reactivity.
- Moseley (1913) showed atomic number is more fundamental than atomic mass.
- Cobalt (27) before nickel (28) and tellurium (52) before iodine (53) - the anomalies disappear.
- Elements in a group share the number of valence electrons; elements in a period share the number of shells.
- Modern Periodic Law: properties of elements are a periodic function of their atomic number
- Maximum electrons in a shell = 2n^2, where n is the shell number
- Number of elements per period: 2, 8, 8, 18, 18, 32, and an incomplete seventh
Trends in the Modern Periodic Table
Valency
The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
- Down a group, the number of valence electrons remains the same, so all elements of a group have the same valency.
- Across a period, the valency first increases from 1 to 4 and then decreases to zero. Lithium has valency 1, beryllium 2, boron 3, carbon 4, nitrogen 3, oxygen 2, fluorine 1 and neon 0.
Atomic size
The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.
- Across a period, atomic radius decreases as we move from left to right. This is because the nuclear charge increases, which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
- Down a group, atomic radius increases. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases, in spite of the increase in nuclear charge.
Metallic and non-metallic properties
Metals tend to lose electrons while forming bonds, that is, they are electropositive in nature. As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease.
- Down a group, metallic character increases. This is because the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus.
- Across a period, metallic character decreases and non-metallic character increases.
In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The borderline elements - boron, silicon, germanium, arsenic, antimony, tellurium and polonium - are intermediate in properties and are called metalloids or semi-metals.
The oxides
Metal oxides are basic in nature, while non-metal oxides are acidic in nature. Since metallic character decreases across a period, the nature of the oxides changes from basic through amphoteric to acidic as we move from left to right.
- Valency across period 2: Li 1, Be 2, B 3, C 4, N 3, O 2, F 1, Ne 0.
- Atomic radius decreases across a period because nuclear charge pulls electrons in; it increases down a group because new shells are added.
- Metalloids - boron, silicon, germanium, arsenic, antimony, tellurium, polonium - lie along the zig-zag line separating metals from non-metals.
Key Concepts
- Dobereiner's triads
- Groups of three elements in which, when arranged by increasing atomic mass, the mass of the middle element is roughly the average of the other two.
- Newlands' Law of Octaves
- The 1866 arrangement in which every eighth element, ordered by increasing atomic mass, had properties similar to the first, like notes on a musical scale.
- Mendeleev's Periodic Law
- The law that the properties of elements are the periodic function of their atomic mass.
- Eka-elements
- Mendeleev's names for undiscovered elements, using the Sanskrit prefix eka (one) - eka-boron, eka-aluminium and eka-silicon became scandium, gallium and germanium.
- Group
- A vertical column of the periodic table; elements in a group have the same number of valence electrons and therefore similar chemical properties.
- Period
- A horizontal row of the periodic table; elements in a period have the same number of shells.
- Moseley's discovery
- The 1913 finding that atomic number, not atomic mass, is the more fundamental property of an element.
- Modern Periodic Law
- The law that the properties of elements are a periodic function of their atomic number.
- Atomic size
- The radius of an atom - the distance between the centre of the nucleus and the outermost shell of an isolated atom.
- Valence electrons
- The electrons in the outermost shell, which determine the valency of an element.
- Electropositive
- The tendency of metals to lose electrons while forming bonds.
- Metalloids
- Borderline elements - boron, silicon, germanium, arsenic, antimony, tellurium, polonium - intermediate in properties between metals and non-metals.
- Isotopes
- Atoms of the same element with the same atomic number but different atomic masses; their existence undermined a classification based on atomic mass.
- Noble gases
- The very inert gases of the last group, discovered late because of their low atmospheric concentration, which fitted into Mendeleev's table without disturbing it.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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State the modern periodic law and explain how it differs from Mendeleev's periodic law. / आधुनिक आवर्त नियम बताइए तथा समझाइए कि यह मेंडलीव के आवर्त नियम से कैसे भिन्न है।
Show answer
The modern periodic law states that the properties of elements are a periodic function of their atomic number. / आधुनिक आवर्त नियम के अनुसार तत्वों के गुण उनके परमाणु क्रमांक के आवर्ती फलन होते हैं। Mendeleev's law was based on atomic mass, whereas the modern law is based on atomic number, which removed the anomalies of mass-based ordering. / मेंडलीव का नियम परमाणु द्रव्यमान पर आधारित था, जबकि आधुनिक नियम परमाणु क्रमांक पर आधारित है, जिसने द्रव्यमान आधारित क्रम की विसंगतियों को दूर किया।
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What were the limitations of Mendeleev's periodic table? State any two. / मेंडलीव की आवर्त सारणी की सीमाएं क्या थीं? कोई दो बताइए।
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1) The position of hydrogen was not clearly fixed, as it resembled both alkali metals and halogens. / 1) हाइड्रोजन का स्थान स्पष्ट रूप से निश्चित नहीं था, क्योंकि यह क्षार धातुओं व हैलोजन दोनों से समानता रखता है। 2) Isotopes had no separate place, and in some pairs an element of higher atomic mass was placed before one of lower mass. / 2) समस्थानिकों के लिए अलग स्थान नहीं था, तथा कुछ युग्मों में अधिक परमाणु द्रव्यमान वाले तत्व को कम द्रव्यमान वाले से पहले रखा गया।
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How does atomic size (radius) vary across a period and down a group? Give reasons. / आवर्त में तथा समूह में नीचे जाने पर परमाणु आकार (त्रिज्या) कैसे बदलता है? कारण दीजिए।
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Across a period atomic size decreases because nuclear charge increases while electrons are added to the same shell, pulling them closer. / आवर्त में परमाणु आकार घटता है क्योंकि नाभिकीय आवेश बढ़ता है जबकि इलेक्ट्रॉन उसी कोश में जुड़ते हैं, जिससे वे पास खिंच जाते हैं। Down a group atomic size increases because new shells are added, increasing the distance of outer electrons from the nucleus. / समूह में नीचे जाने पर आकार बढ़ता है क्योंकि नए कोश जुड़ते हैं, जिससे बाहरी इलेक्ट्रॉनों की नाभिक से दूरी बढ़ती है।
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Why are elements in the same group of the periodic table chemically similar? / आवर्त सारणी के एक ही समूह के तत्व रासायनिक रूप से समान क्यों होते हैं?
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Elements in the same group have the same number of valence electrons in their outermost shell. / एक ही समूह के तत्वों के बाह्यतम कोश में संयोजी इलेक्ट्रॉनों की संख्या समान होती है। Since chemical properties depend mainly on valence electrons, these elements show similar chemical behaviour. / चूँकि रासायनिक गुण मुख्यतः संयोजी इलेक्ट्रॉनों पर निर्भर करते हैं, ये तत्व समान रासायनिक व्यवहार दिखाते हैं।
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How does metallic character vary across a period and down a group? / आवर्त में तथा समूह में नीचे जाने पर धात्विक गुण कैसे बदलता है?
Show answer
Metallic character decreases across a period (left to right) because the tendency to lose electrons decreases as nuclear attraction increases. / आवर्त में (बाएं से दाएं) धात्विक गुण घटता है क्योंकि नाभिकीय आकर्षण बढ़ने से इलेक्ट्रॉन त्यागने की प्रवृत्ति घटती है। It increases down a group because atomic size increases and electrons are lost more easily. / समूह में नीचे जाने पर यह बढ़ता है क्योंकि परमाणु आकार बढ़ता है और इलेक्ट्रॉन अधिक सरलता से त्यागे जाते हैं।
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Mendeleev left gaps in his periodic table. Why is this considered an achievement? / मेंडलीव ने अपनी आवर्त सारणी में रिक्त स्थान छोड़े। इसे एक उपलब्धि क्यों माना जाता है?
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He left gaps for elements that were not yet discovered and predicted their properties (e.g., eka-aluminium, eka-silicon). / उसने अनदेखे तत्वों के लिए रिक्त स्थान छोड़े और उनके गुणों की भविष्यवाणी की (जैसे एका-एल्युमिनियम, एका-सिलिकॉन)। When elements like gallium and germanium were later discovered with matching properties, it confirmed the validity of his table. / जब गैलियम व जर्मेनियम जैसे तत्व बाद में समान गुणों के साथ खोजे गए, तो इसने उसकी सारणी की वैधता की पुष्टि की।
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An element X is in group 1 and another element Y is in group 17 of the periodic table. Predict the formula and nature of the bond in the compound formed between them. / तत्व X आवर्त सारणी के समूह 1 में तथा तत्व Y समूह 17 में है। इनके बीच बने यौगिक का सूत्र व बंध की प्रकृति की भविष्यवाणी कीजिए।
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X has 1 valence electron and Y has 7; X loses one electron and Y gains one, forming the compound XY. / X में 1 संयोजी इलेक्ट्रॉन तथा Y में 7 हैं; X एक इलेक्ट्रॉन त्यागता है व Y एक ग्रहण करता है, जिससे यौगिक XY बनता है। The bond is ionic, formed by transfer of an electron from the metal X to the non-metal Y. / बंध आयनिक होता है, जो धातु X से अधातु Y को इलेक्ट्रॉन के स्थानांतरण से बनता है।
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Why is the modern periodic table arranged into 18 groups and 7 periods, and what does the period number indicate? / आधुनिक आवर्त सारणी 18 समूहों व 7 आवर्तों में क्यों व्यवस्थित है, तथा आवर्त संख्या क्या दर्शाती है?
Show answer
Elements are arranged in increasing order of atomic number into 18 vertical groups and 7 horizontal periods based on electronic configuration. / तत्वों को परमाणु क्रमांक के बढ़ते क्रम में इलेक्ट्रॉनिक विन्यास के आधार पर 18 ऊर्ध्वाधर समूहों व 7 क्षैतिज आवर्तों में व्यवस्थित किया जाता है। The period number indicates the number of electron shells (energy levels) present in the atoms of that period. / आवर्त संख्या उस आवर्त के परमाणुओं में उपस्थित इलेक्ट्रॉन कोशों (ऊर्जा स्तरों) की संख्या दर्शाती है।
Related Laws & Principles
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