- Newlands’ Law of Octaves says every 8th element has similar properties.
- He arranged elements by increasing atomic mass in 1866.
- He compared the pattern to musical octaves, Sa, Re, Ga, Ma, Pa, Dha, Ni, Sa.
- The law worked only up to calcium and failed for heavier elements.
- It had no space for new elements and placed unrelated elements together.
Definitions [12]
Definition: Dobereiner’s Triads
Triads are groups of three elements with similar chemical properties, arranged in increasing order of atomic masses, where the atomic mass of the middle element is approximately the mean of the other two.
Definition: Modern Periodic Law
The physical and chemical properties of elements are the periodic functions of their atomic number.
Define groups of modern periodic table.
In the modern periodic table, groups (also known as families) are the 18 vertical columns that organize chemical elements by shared properties. Unlike periods, which are horizontal rows representing electron shells, groups are vertical columns of elements with similar chemical properties.
Definition: Elements
Elements are pure substances made up of one type of atom.
Definition: Modern Periodic Table
A tabular arrangement of elements in groups (vertical columns) and periods (horizontal rows), highlighting the regular trends in properties of elements, is called a Periodic Table.
OR
The classification of elements resulting from an arrangement of the elements in an increasing order of their atomic numbers is the modern periodic table.
Define periods of modern periodic table.
The horizontal rows are called periods. The table consists of 7 periods, numbered from 1 to 7 from top to bottom.
Define:
Periodic table
The periodic table is a tabular arrangement of elements in horizontal rows, called periods, and vertical columns, called groups, to classify elements and facilitate their systematic study.
Definition: Periodic Trends
When the properties of elements in a period or a group of the modern periodic table are compared, certain regularity is observed in their variations. It is called the periodic trends in the modern periodic table.
Definition: Valency
The valency of an element is determined by the number of electrons present in the outermost shell of its atoms, that is, the valence electrons.
Definition: Atomic Size (Atomic Radius)
It is the distance between the centre of the nucleus of an atom and its outermost shell.
Definition: Electropositivity
"Electropositivity of an element is the tendency to form a cation by losing its valence electron."
Definition: Electronegativity
"Electronegativity of an atom is the attractive force with which the valence electrons are held."
Theorems and Laws [1]
Law: Modern Periodic Law
Statement:
“The properties of elements are a periodic function of their atomic numbers.”
Explanation / Proof:
- When Mendeleev proposed his periodic table, the structure of the atom was not yet known.
- After the discovery of the electron, scientists began to link atomic number with the number of electrons and protons in an atom.
- In 1913, Henry Moseley used X-ray experiments to show that the atomic number (Z) equals the number of protons in an atom's nucleus.
- This showed that atomic number determines the chemical properties of elements more accurately than atomic mass.
Conclusion:
- Atomic number is the true basis for the classification of elements.
- Thus, the modern periodic table is arranged by increasing atomic number, correcting the issues in Mendeleev’s table.
Key Points
Key Points: Classification of Elements
- Around 1800, only about 30 elements were known, whereas today the number has increased to 118.
- To manage the growing volume of information, scientists began identifying patterns in element properties to facilitate systematic classification.
- Elements were initially grouped as metals and nonmetals, and later, a third category called metalloids was recognised.
Key Points: Newland's Law of Octaves
Key Points: Mendeleev’s Periodic Table
- Mendeleev arranged 63 elements by increasing atomic mass.
- He stated: Properties of elements repeat with atomic mass.
- The periodic table had groups (columns) and periods (rows).
- Classification was based on chemical and physical properties.
- He left gaps for elements yet to be discovered.
Key Points: Insights into Mendeleev’s Periodic Table
- Mendeleev corrected atomic masses to correctly place elements.
- He predicted new elements like Sc, Ga, and Ge with correct properties.
- He later added a zero group for noble gases.
- Co and Ni have similar masses, which can cause confusion about their order.
- Isotopes couldn’t be placed due to different masses.
- Atomic mass gaps are uneven, so new elements couldn’t be predicted.
- Hydrogen's position is unclear—it’s like both alkali metals and halogens.
Key Points: Modern Periodic Table
- Dobereiner grouped elements in threes (triads) with similar properties and a pattern in atomic masses.
- Newlands found that every 8th element shared similar properties (the Law of Octaves).
- Mendeleev arranged elements by atomic mass and predicted new elements, but couldn’t explain isotopes and rare earths.
- Moseley fixed the flaws by arranging elements by atomic number, forming the modern periodic table.
- Bohr proposed the long-form periodic table based on electron arrangement.
Key Points: Structure of the Modern Periodic Table
- The modern periodic table has 7 periods, 18 groups, and 118 elements.
- It has 4 blocks: s, p, d, and f (with d-block as transition elements).
- A zig-zag line separates metals, metalloids, and nonmetals.
Key Points: Modern Periodic Table and Electronic Configuration of Elements
- Electronic configuration determines an element’s position in a group and period of the modern periodic table.
- Groups show similarity and gradation, while periods show gradual changes in element properties from left to right.
Key Points: Groups and Electronic Configuration
- Elements in the same group have the same number of valence electrons, so they show similar chemical properties.
- Moving down a group, the number of shells increases, but the number of valence electrons remains the same.
- In the modern periodic table, groups are vertical (18), showing similarities, and periods are horizontal (7), showing gradual changes in properties.
Key Points: Periods and Electronic Configuration
- Elements in the same period have the same number of electron shells, while valence electrons increase by one from left to right.
- Periods are determined by the number of shells; e.g., second-period elements have electrons in K and L shells.
- Valency and reactivity depend on the number of valence electrons and the shell number, both identifiable from the element’s position in the periodic table.
Key Points: Atomic Size
- Atomic radius is the distance from the nucleus to the outermost shell.
- Over time, atomic size decreases due to a stronger nuclear pull.
- Down a group, atomic size increases as new shells are added.
- Atomic size depends on the number of shells and the nuclear charge.
- Cations are smaller, anions are larger, and in isoelectronic ions, size decreases with more protons.
Key Points: Metallic and Non-metallic Characters
- Metals are on the left, nonmetals on the right, and metalloids along the zig‑zag line in the periodic table.
- Metallic character is the tendency to lose electrons (electropositivity).
- Nonmetallic character is the tendency to gain electrons (electronegativity).
- Down a group: metallic character increases, nonmetallic character decreases.
- Across a period (left to right): metallic character decreases, nonmetallic character increases.
Key Points: Gradation in Halogen Family
- In Group 17, state changes from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂) down the group.
- Group 2 metals react more with water as you go down the group.
- Reactivity increases from Be to Ba in Group 2; Be doesn’t react, Ba reacts easily.
Important Questions [30]
- Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in ______.
- The Element Eka-silicon in Mendeleev’S Periodic Table is Known as in the Modern Periodic Table.
- Fill in the Blank: the Modern Periodic Table Consists of ……………. Periods.
- Write the Merits of the Modern Periodic Table Over Mendeleev’S Periodic Table
- Define periods of modern periodic table.
- Define groups of modern periodic table.
- Select the appropriate options and complete the following paragraph: (metals, non-metals, metalloids, four, seven, s-block, p-block, d-block, f-block). On the basis of electronic configuration,
- An Element Has Its Electron Configuration as 2, 8, 2. Now Answer the Following Question. to Which Period Does this Element Belong?
- An element has its electron configuration as 2, 8, 2. Now answer the following question. What is the group of this element?
- An Element Has Its Electron Configuration as 2, 8, 2. Now Answer the Following Question. What is the Atomic Number of this Element?
- An element has its electron configuration as 2, 8, 1. Now answer the following questions: What is the atomic number of this element? What is the group of this element?
- Classify the following elements into metals, non-metals and metalliods As, C, Hg, Mg, S, Si
- Differentiate Between Normal Elements and Transition Elements.
- Fill in the Blank: the Formulae of Chloride of Metal M is Mcl2. the Metal M Belongs to ……………. Group.
- Taking into Consideration the Period of the Elements Given Below, Answer the Following Questions : (Pm)
- Fill in the Blank and Rewrite the Completed Statement: ________ Group in the Periodic Table Contains Elements that Are All Gases at Room Temperature.
- Give Scientific Reason: Atomic Size Increase Down the Group.
- Answer the Following Question: State the Four Blocks of the Modern Periodic Table Based on the Electronic Configuration of Elements.
- Elements in the Same Group Show the Same Valency. Give Scientific Reason.
- Write the Electronic Configuration of K and Ne.
- The atomic masses of three elements A, B and C having similar chemical properties are 7, 23 and 39, respectively
- Write scientific reason. Elements belonging to the same group have the same valency.
- Write the name and symbol of the element from the description. The atom having the smallest size.
- Give scientific reasons for the following: Atomic radius goes on decreasing while going from left to right in a period.
- Write Scientific Reason. Atomic Radius Goes on Increasing Down a Group.
- Write the name and symbol of the element from the description. The atom having the smallest atomic mass.
- Write the name and symbol of the element from the description. The noble gas with the smallest atomic radius.
- Write a scientific reason. Metallic character goes on decreasing while going from left to right in a period.
- Complete the correlation: Group 1 : Alkali metals : : ______ : Halogens.
- Write the name and symbol of the element from the description. The most electronegative atom.
Concepts [15]
- Classification of Elements
- Dobereiner’s Triads
- Newland's Law of Octaves
- Mendeleev’s Periodic Table
- Insights into Mendeleev’s Periodic Table
- Modern Periodic Law
- The Modern Periodic Table
- Structure of the Modern Periodic Table
- Modern Periodic Table and Electronic Configuration of Elements
- Groups and Electronic Configuration
- Periods and Electronic Configuration
- Periodic Trends in the Modern Periodic Table
- Atomic Size
- Metallic and Non-metallic Characters
- Gradation in Halogen Family
