Revision: The Periodic Table Chemistry (English Medium) ICSE Class 9 CISCE

The horizontal rows are called periods. The table consists of 7 periods, numbered from 1 to 7 from top to bottom.

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.

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.

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.

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.

Elements are pure substances made up of one type of atom.

The physical and chemical properties of elements are the periodic functions of their atomic number.

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.

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.

It is the distance between the centre of the nucleus of an atom and its outermost shell.

"Electropositivity of an element is the tendency to form a cation by losing its valence electron."

"Electronegativity of an atom is the attractive force with which the valence electrons are held."

The Group 1 elements (Li, Na, K, Rb, Cs, Fr) are collectively called alkali metals. All elements except Hydrogen in this group are called alkali metals.

The Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra) are called alkaline earth metals. They are less reactive than alkali metals. Radium (Ra) is the first radioactive element in this group, discovered by Madame Curie.

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.

• 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.

• 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.

• The modern periodic table has 7 periods, 18 groups, and 118 elements.
• A zig-zag line separates metals, metalloids, and nonmetals.
• Periods are horizontal rows.
• Groups are vertical columns.
• Period number represents the number of shells.
• The group number generally represents the valence electrons of s- and p-block elements.
• Elements are divided into s, p, d and f blocks.
• Lanthanoids and actinoids are placed separately at the bottom.

• Atomic radius is the distance from the nucleus to the outermost shell of an atom.
• Across a period, atomic size decreases due to increase in effective nuclear charge.
• Down a group, atomic size increases due to addition of new shells.
• Atomic size depends on number of shells and nuclear charge.
• Cations are smaller than their parent atoms due to loss of electrons.
• Anions are larger than their parent atoms due to gain of electrons.
• In isoelectronic species, greater nuclear charge leads to smaller size.
• Noble gases show larger atomic size due to consideration of van der Waals radius.

Electronic Configurations:

Biological Importance

• Na⁺ ions participate in the transmission of nerve signals
• High concentration of Na⁺ and low concentration of K⁺ is found in blood plasma
• High concentration of K⁺ and low concentration of Na⁺ is found inside human cells
• K⁺ ions are essential for nerve impulse transmission and maintaining osmotic balance

Uses of Alkali Metals

• Lithium is used in lithium-ion batteries
• Potassium is used in manufacturing potassium superoxide (KO₂) for oxygen generation in submarines, mountaineering, and space equipment
• Potassium chloride is used as a fertilizer
• Be is used as a moderator in nuclear reactors

Electronic Configurations:

Periodic Trends in Group 2:

Importance of Alkaline Earth Metals

• Mg²⁺ ions are a constituent of chlorophyll — the green pigment in plants responsible for photosynthesis
• Ca²⁺ ions play a vital role in blood clotting (coagulation cascade)
• Ca²⁺ is essential for the formation and strength of bones and teeth (as calcium phosphate)
• Mg²⁺ is required for the activity of many enzymes in the body
• Ca²⁺ ions are involved in muscle contraction and nerve signal transmission
• Sr and Ba compounds are generally toxic to biological systems