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UNIT-3, CLASSIFICATION OF, ELEMENTS AND, PERIODICITY IN, PROPERTIES
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Periodic Table, An arrangement of all the known, elements according to their properties, so that similar elements fall within the, same vertical column and dissimilar, elements are separated.
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EARLIER CLASSIFICATION, OF ELEMENTS
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1. DOBEREINER’S LAW OF TRIADS, 2. NEWLANDS’ LAW OF OCTAVES, 3. MENDELEEV’S PERIODIC TABLE, 4. MODERN PERIODIC TABLE
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In 1829, Dobereiner arranged the known, elements of at that time in the ascending, order of atomic masses., He found out three elements group called, triad., In a triad, the properties of the middle, elements are the average of the other two., This law is known as Dobereiner’s law of, triads.
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EXAMPLES OF TRIADS
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In 1865, Newlands arranged all the known, elements of at that time in the ascending, order of atomic masses., He observed that the properties of the, eighth elements are the simple repetitions, of the first one like eighth note in an octave, in music., This law is known as Newlands law of, octaves.
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In 1869, a Russian chemist Mendeleev, arranged the known elements of at, that time in the ascending order of, atomic masses., He observed that same properties are, repeated, , in, , regular, , intervals, , and, , proposed a law known as Mendeleev’s, periodic law.
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MENDELEEVE'S PERIODIC LAW, The law states that “the physical and, chemical properties of elements are, ., periodic functions of their atomic, masses”, , Dimitri Mendeleev
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NOTE, Gallium and Germanium were unknown at, the time Mendeleev published his periodic, table., He left a gap under aluminium and a gap, under silicon., He called these elements Eka-Aluminium, and Eka-Silicon.
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DEMERITS OF MENDELEEVE'S PERIODIC TABLE, , Elements with dissimilar properties are, found in same group., He could not give an exact position for, hydrogen., He could not give exact position for, Lanthanoids and Actinoids and also for, isotopes., Did not strictly obey the increasing order, of atomic weights.
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MODERN PERIODIC TABLE, , Henry Moseley
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Moseley’s work on the x-ray spectra of the, elements reveals that atomic number is a, more fundamental property than atomic, mass., On the basis of this, he put forward the, modern periodic law.
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The law states that “the physical and, chemical properties of elements are, periodic functions of their atomic, numbers”., , Modern Periodic Law
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PERIODS AND GROUPS
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PERIODS, ✔The horizontal rows present in the modern periodic, table are called periods., ✔There are seven periods., ✔The first period consists of 2 elements., ✔Second and third period consists of 8 elements, each., ✔Fourth and fifth period consists of 18 elements., ✔Sixth period consists of 32 elements., ✔The last seventh period is an incomplete period.
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GROUPS, ✔The vertical columns present in the modern, periodic table are called groups., ✔There are 18 vertical columns., ✔Therefore 18 groups are present in the, modern periodic table.
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MODERN CLASSIFICATION, OF ELEMENTS
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, , In the modern periodic table, elements, are classified into four blocks., , , , They are s, p d and f block elements., , , , Classification is based on the orbital in, which the last electron of the atom of, the element enters.
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s-BLOCK ELEMENTS, ✔ The elements in which the last electron, enters the s orbital of their valence shell are, called s block elements., ✔ It consists of elements of group 1 and group, 2., ✔ The ground state configuration of the, valence shell is ns1 or ns2 .
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p-BLOCK ELEMENTS, ✔ The elements in which the last electron, enters the p orbitals of their valence shell, are called p block elements., ✔ It consists of group 13―18 except He., ✔ The ground state configuration of the, valence shell is ns 2 np1 to ns2 np6 .
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d-BLOCK ELEMENTS, ✔, , The elements in which the last electron, enters the d orbitals are called d block, elements., , ✔, , It consists of groups 3―12., , ✔, , The, , general, , electronic, , (n―1)d 1―10 ns 1―2 ., , configuration, , is
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d-BLOCK ELEMENTS
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f-BLOCK ELEMENTS, , , The elements in which the last electron, enters the f orbitals are called f block, elements., , , , Their general electronic configuration is, (n―2)f 1―14 (n―1)d 0―1 ns 2 .
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f-BLOCK ELEMENTS
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TYPES OF ELEMENTS
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REPRESENTATIVE ELEMENTS, All the elements of the s and p block, elements, , together, , constitute, , representative elements., , the
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NOBLE GASES, The elements of the 18th group, are called noble gases or inert, gases or rare gases.
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TRANSITION ELEMENTS, ✔ The d block elements i.e., elements of, group 3―12 are called transition, elements., ✔ They are placed in between s and p, block elements.
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INNER TRANSITION ELEMENTS, The f block elements are called inner, transition elements., It consists of Lanthanides and actinides., The elements coming after Lanthanum are, called lanthanides., The elements coming after actinium are called, actinides.
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CLASSIFICATION OF ELEMENTS INTO, METALS, NON METALS AND METALLOIDS
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METALS, More than 75% of all known elements are, metals., Appear on the left side of the periodic table., Usually solids at room temperature., Have high melting and boiling points., Good conductors of heat and electricity., Malleable and ductile.
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NON METALS, Non-metals are located at the top right, hand side of the periodic table., Usually exists as solids or gases at room, temperature., Low melting and boiling points., Bad conductors of heat and electricity.
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METALLOIDS, Metalloids or semi metals are elements, which show both the properties of metals, and non metals., Eg: Boron, Silicon, Germanium, Arsenic,, Antimony, Selinium, Tellurium and, Polonium.
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NOTE, ✔The metallic character increases from, top to bottom of a group., ✔Non metallic character increases from, left to right across a period.
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NOMENCLATURE OF ELEMENTS, WITH ATOMIC NUMBER GREATER THAN 100, The elements are named using the numerical roots, for 0 and numbers 1-9., The roots are put together in the order of digits, which make up the atomic number., ‘ium’ is added at the end.
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The IUPAC names for the elements with Z, above 100 are shown below.
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PERIODIC, PROPERTIES
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Properties, indirectly, , which, related, , are, to, , directly, , the, , or, , electronic, , configuration of the elements and show, a regular gradation when we move from, left to right across a period or from top, to bottom in a group are called periodic, properties.
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1. IONISATION ENERGY, Ionisation energy is, Ionisation Potential., , also, , known, , as, , The minimum amount of energy required, to, , remove, , the, , most, , loosely, , bound, , electron from an isolated gaseous atom.
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✔The energy required to remove the first, electron is called first Ionisation energy, (IE1 )., ✔The energy required to remove the, second electron is called second, ionisation energy (IE2 )., ✔In general, IE2 > IE1.
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ATOMIC SIZE, The larger the atomic size, smaller the, ionisation energy. Smaller the atomic size,, larger the ionisation energy., , NUCLEAR CHARGE, Ionisation, , energy, , increases, , increase in nuclear charge., , with
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SHIELDING EFFECT, The inner electrons repel the outer electrons, and cut down the attractive force between the, nucleus and the valence shell., This effect is known as shielding effect or, screening effect., As the shielding increases the ionisation, energy decreases.
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EFFECT OF HALF FILLED AND, COMPLETELY FILLED SUB LEVELS, If an atom has half filled or completely, filled sub levels, its ionisation energy is, higher, , than, , that, , expected, , position in the periodic table., , from, , its
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2. ELECTRON AFFINITY, The energy released when an isolated, gaseous atom changed into an anion by, accepting an electron.
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FACTORS INFLUENCING, ELECTRON AFFINITY
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ATOMIC SIZE, Larger the size of the atom, the smaller, will be the electron affinity and vice versa., , ATOMIC SIZE, Greater the nuclear charge, greater the, electron affinity.
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ELECTRONIC CONFIGURATION, When the electronic configuration of the atom is, stable, the less will be the tendency of the atom to, accept an additional electron and hence lower will be, the electron affinity., The electron affinity values of halogens are very high, because of their strong tendency to accept an electron, to attain the stable noble gas configuration.
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3. ELECTRONEGATIVITY, The tendency of an atom to attract the, shared pair of electrons towards itself.
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Small atoms are more electronegative because, they attract electrons more strongly than the, larger ones., Atoms with nearly filled shells will have higher, electronegativities than those with less densely, filled ones., NOTE: The least electronegative element is, cesium and the most electronegative element, is fluorine.
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PAULING’S SCALE, This scale is based on an empirical relation, between the energy of a bond and the, electronegativities of bonded atoms., MULLIKEN’S SCALE, According to this scale, electronegativity, could be regarded as the average of the, ionization energy and electron affinity of an, atom.
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ACROSS A PERIOD, The, , Ionisation, , energy,, , Electron, , Affinity, , and, , Electronegativity increases from left to right along a, period. This is because, i) The decrease in atomic size of the elements along a, period., ii) The increase in nuclear charge on moving along a, period., iii) Decrease in shielding effect.
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WITHIN A GROUP, The ionisation Energy, Electron Affinity and, Electronegativity, , decreases, , down, , group., This is because along a group, i) The size of the atom increases., ii) The nuclear charge decreases., iii) Increase in shielding effect., , the
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4. ATOMIC RADIUS, A) COVALENT RADIUS, It is one half of the distance between the centres of, the nuclei of two bonded atoms of the same element., Eg: The inter nuclear distance between the covalently, bonded Hydrogen atoms is 74 pm., The covalent radius of Hydrogen is 37 pm.
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B) VANDER WAALS RADIUS, It is one half of the distance between, the centres of the nuclei of two non, bonded, , atoms, , of, , the, , adjacent, , molecules of the element in the solid, state.
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C) METALLIC RADIUS, It, , is, , half, , the, , inter-nuclear, , distance, , separating the metal atoms in the metallic, crystal., Eg: The distance between two adjacent, copper atoms in solid copper is 256 pm., The metallic radius of copper is 128pm.
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D) IONIC RADIUS, The effective distance from the centre, of the nucleus of an ion up to which it, has an influence on the electron cloud.
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1. A cation is smaller than its parent atom but an anion is, larger than its parent atom. Give reason., A cation is smaller than its parent atom., It has fewer electrons while its nuclear charge remains, the same., An anion is larger than the corresponding parent atom, The addition of one or more electrons would result in, increased repulsion among the electrons and decrease, in effective nuclear charge.
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2. The electron affinity of chlorine is higher than that of, fluorine. Why?, Fluorine atom is much smaller than chlorine atom., Due to this, there is much crowding of electrons in small, space around the fluorine nucleus., Due to this crowding, fluorine atom has less attraction for, the outside electron in comparison to chlorine in which the, crowding of electrons is less due to the bigger size of, chlorine atom., As a result of this, electron affinity of fluorine is less than, that of chlorine.
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The Ionisation Energy of Nitrogen is greater than, that of Oxygen. Why?, ✔ The electronic configuration of Nitrogen, is 1s2 , 2s2 , 2p3, ✔The electronic configuration of Oxygen is, 1s2 , 2s2 , 2p4 ., ✔ In the case of Nitrogen atom, the p orbitals are, half filled., ✔Atoms with half-filled electronic configurations, have extra stability., ✔Therefore, the ionization energy of Nitrogen is, greater than that of Oxygen.
