Revision: Periodic Classification of Elements Science SSLC (English Medium) Class 10 Tamil Nadu Board of Secondary Education

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

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.

Elements are pure substances made up of one type of 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.

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

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.

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.

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

The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion is called Ionization energy or ionization potential.

The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion is called ionisation potential (l.P) or ionisation energy (I.E.) or first ionisation energy (IE₁).

The amount of energy released while converting a neutral gaseous isolated atom into a negatively charged gaseous ion (anion) by the addition of electron is called Electron Affinity (E.A.)

Electron affinity is the energy released when an isolated gaseous atom acquires an electron to form an anion.

\[\ce{X (g) + e- -> \underset{(anion)}{X-(g)}+ E.A.}\]

X is any element taken in its gaseous state. 

The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is called its electronegativity.

The science and technology regarding the extraction of metals from ores and their purification for the use is called metallurgy.

Ore: Those minerals from which a metal can be extracted profitably are called ores.

Charge: The mixture of materials fed into a furnace to extract a metal is called charge.

Mineral: The naturally occurring compounds of metals which are generally mixed with earthy such as soil, sand, limestone and rocks are known as minerals.

The product obtained by the combination of gangue with flux is called slag.

Most of the elements occur in nature as in combined state as minerals. The chemical composition of minerals is fixed.

Minerals from which metals can be extracted profitably are known as ores.

If minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. These minerals are called ores.

If an ore is a carbonate or a hydrated oxide, it is heated in the absence of air to a temperature that is high but insufficient to melt the ore. The process is known as calcination.

The impurities (sand, silt, soil, gravel, etc.) present in the ore are called gangue.

Roasting is the process of heating concentrated ore to a high temperature in the presence of air. The process is usually carried out for sulphide ores.

The substance added to get rid of gangue in the extraction of metal is called flux.

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.

An alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements.

Alloys melting in the range of about 51 °C to 260°C, usually contain bismuth, lead, tin, etc. These alloys are called fusible alloys. 

Dental amalgam is a mixture of mercury and a silver-tin alloy.

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.

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.

Corrosion is the gradual damage of metals caused by their reaction with components of the atmosphere, such as oxygen and moisture.

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.

• Ionisation energy is the energy needed to remove an electron from a neutral gaseous atom to form a positive ion.
• Larger atomic size → lower ionisation energy because outer electrons are farther from the nucleus.
• Higher nuclear charge → higher ionisation energy due to stronger attraction of electrons.
• Across a period, ionisation energy increases from left to right as atomic size decreases.
• Down a group, ionisation energy decreases with increasing atomic size; metals have low I.E., non‑metals have high I.E.

• Smaller atoms and higher nuclear charge → higher (more negative) electron affinity due to stronger attraction for electrons.
• Over time, E.A. becomes more negative (increases), with the largest increase for halogens (Group 17).
• Down a group, E.A. becomes less negative (decreases) due to increased atomic size and electron repulsion.
• Exceptions: Fluorine < Chlorine and Oxygen < Sulphur in E.A. due to strong electron repulsions in very small atoms.

• Electronegativity is the ability of an atom to pull shared electrons; it is highest for fluorine (4.0).
• It increases across a period (left to right) and decreases down a group (top to bottom).
• Non-metals have high electronegativity (gain electrons), while metals have low electronegativity (lose electrons).

• Reactive metals occur in combined form (as oxides, sulphides, etc.), while unreactive metals like gold and silver occur in the free state.
• Minerals are natural metal compounds; those from which metals can be extracted economically are called ores, and impurities present in ores are called gangue.
• The complete process of extracting and purifying metals from ores is called metallurgy.