- 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.
Definitions [15]
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:
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.
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.
Define the atomic size.
Atomic size is the distance between the center of an atom i.e., from the nucleus to the outermost shell (valence shell) of that atom.
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.
Definition: Elements
Elements are pure substances made up of one type of atom.
Definition: Modern Periodic Law
The physical and chemical properties of elements are the periodic functions of their atomic number.
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."
Definition: Fusible Alloy
Alloys melting in the range of about 51 °C to 260°C, usually contain bismuth, lead, tin, etc. These alloys are called fusible alloys.
Definition: Dental Amalgam
Dental amalgam is a mixture of mercury and a silver-tin alloy.
Define:
Alloy
A homogeneous mixture of two or more metals or metals with non-metals is known as an alloy.
For example - Duralumin is a homogeneous mixture of aluminium, copper, magnesium and manganese.
Definition: Alloy Steel
A steel containing up to 10% of elements such as chromium, molybdenum, nickel, etc., usually with a low percentage of carbon is known as alloy steel.
Definition: Amalgam
A mixture or an alloy of mercury with a number of metals or alloys such as sodium, zinc, gold and silver as well as with some non metals is known as amalgam.
Definition: Alloy
An alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements.
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: 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: Insights into Mendeleev’s Periodic Table
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: 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: 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.
