CBSE Class 11 Chemistry Notes For Chapter 3 Long Form Of Periodic Table

CBSE Class 11 Chemistry Notes For Chapter 3 Long Form Of Periodic Table

Description of periods:

• Like Mendeleev’s modified table, it also consists of numbers from as1 to 7 from top to bottom.
• The period number is equal to the value of i.e., the principal quantum number corresponding to the outermost shell of the atoms of the elements belonging to that period.
• Each period begins with the filling of electrons in a new energy level. Several elements in each period are twice the total number of atomic orbitals available in the energy level that are being filled.

First period:

This period begins with the filling of the first energy level (n = 1). Since the first shell has only one orbital [i.e., Is), which can accommodate a maximum of two electrons, there can be only two elements in the first period. These are hydrogen (Is¹) and helium (Is²).

Second period:

• It starts with the filling of the second energy level (n = 2). Since the second shell contains four orbitals (one 2s and three 2p), it can accommodate a maximum of (2 × 4) = 8 electrons. So, there are eight elements in the second period.
• It begins with lithium (Li) in which 1 electron enters the 2s -orbital (₃Li: 2s¹) and ends up with neon (Ne) in which the second shell gets filled (₁₀Ne: 2s²2p⁶).

Third period:

• The third period begins with the filling of the third energy level (n = 3). This energy level contains nine orbitals (one 3s, three 3p and five 3d).
• According to the Aufbau principle, 3d -orbitals will be filled up only after filling the 4s -orbital.
• Consequently, the third period involves filling only four orbitals (one 3s and 3p ) which can accommodate a maximum of (2 × 4) = 8 electrons. So, there are 8 elements in the third period from ₁₁Na (3s¹) to ₁₈Ar (3s²3p⁶).

Fourth period:

• This period corresponds to the filling of the fourth energy level (n = 4). Out of 4s,4p,4d and 4f-orbitals belonging to this shell, filling of 4d -and 4f-orbitals does not occur in this period since their energies are higher than that of even 5s -orbital.
• It must however be remembered that after filling the 4s -orbital, the filling of five 3d -orbitals begins since the energy of the -orbital is greater than that of the 4s orbital but less than that of the 4p -orbital.
• So the fourth period involves filling of or 9 Orbitals (one 4s, five 3d and three 4p), which can accommodate (2 × 9) = 18 electrons.
• Therefore, the fourth period contains 18 elements from potassium (₁₉K: 4s¹) to krypton (₃₆Kr: 4s²3d¹⁰ 4p⁶).
• This period contains 10 elements more than the third period corresponding to filling off 3d -orbitals. These 10 elements [₂₁Sc(3d¹4s²) to ₃₀Zn(3d¹⁰4s²)] are called the first series of transition elements.

Fifth period:

• The fifth period corresponds to the filling of electrons in the fifth energy level (n = 5). Like the fourth period, it also accommodates 18 elements since only nine orbitals (one 5s, five 4d and three 5p) are available for filling with electrons.
• It starts with rubidium in which one electron enters 5s -orbital (₃₇Rb: 5s¹) and ends up with xenon in which the filling of 5p -orbital is complete (₅₄Xe: 5s²4d¹⁰5p⁶). 10 elements from ₃₉Y(5s²4d¹) to ₄₈Cd(5s²4d¹⁰) corresponding to filling of five 4d orbitals are called second series oftransition elements.

Sixth period:

• The sixth period corresponds to the filling of electrons in the sixth energy level (n = 6).
• This period involves the filling of sixteen orbitals (one 6s, seven4f, five 5d and three 6p) which can accommodate a maximum of (2 × 16) = 32 electrons. So there are 32 elements in the sixth period.
• It begins with caesium (Cs) in which one electron enters 6s -orbital (₅₅Cs: 6s1) and ends up with radon in which filling of 6p -orbital is complete (₈₆Rn: 4f¹⁴5d¹⁰6s²6p⁶).
• Filling up of 4f-orbitals begins with cerium(₅₈Ce) and ends with lutetium (₇₁Lu). These 14 elements constitute the first inner-transition series, also called lanthanoids or rare earth elements.
• These are separated from the main frame periodic table and are placed in a horizontal row at the bottom of the table Again, 10 elements lanthanum (₅₇La), hafnium (₇₂Hf) to mercury (₈₀Hg), corresponding to successive filling of (10) 5d -orbitals, constitute the third transition series.

Seventh period:

• This period corresponds to the filling of electrons in the seventh energy level (n = 7). Like the sixth period, it is expected to accommodate 32 elements corresponding to the filling of 16 orbitals (one 7s, seven5f, five 6d and three 7p ).
• However, at present this period is incomplete consisting of 28 elements. The last element of this period will have an atomic number of of118 and will position the inert gas family.
• In this period, after filling of 7s -orbital [₈₇Fr: 7s¹ and ₈₈Ra: 7s²], the next two electrons enter the 6rf-orbital (this is against the Aufbau principle) corresponding to the elements 8gAc and goTh.
• Thereafter, the filling up of 57- orbital begins with ₉₁Pa and ends from Th to Lr are commonly called actinoids, which constitute the second Inner-transition series, Although Th does not contain any electron.

‘if orbital, it is considered to be a member of the actinoid series. Like lanthanoids, 14 members of the actinoid series are placed separately in a horizontal row at the hotter of the periodic table.

Number of elements in different periods and types of orbitals being filled up:

Description of groups:

Each of the 18 groups in the long form of the periodic table consists of many elements whose atoms have similar electronic configurations ofthe outermost shell (valence shell). The members of each group exhibit similar properties. Successive members in a group are separated by magic numbers of either 8 18 or 32. According to the recommendation of IUPAC (1988), the groups are numbered from 1 to 18.

Designations of these groups in different systems are presented in the following table

Designations of different groups [including outer electronic configuration:

Specific names of the elements of certain groups:

Long form of the periodic table

The superiority of the long form of the periodic table over Mendeleev’s periodic table

• In the long form of the periodic table, it is easy to remember and reproduce all the elements more easily in a sequence of atomic numbers.
• It relates the positions of the elements in the table to their electronic configurations more dearly.
• Gradual change In properties along the periods or similarity in properties along the groups can be interpreted by considering electronic configurations of the elements.

For example:

Elements of the same group exhibit marked similarities due to similar outer electronic configurations.

• Splitting the periodic table into s-,p-, cl- and f-blocks has made the study of the elements easier.
• The maximum capacity of each period to accommodate a specific number of elements is related to the capacity of different electronic shells to accommodate the maximum number of electrons.
• Due to the elimination of sub-groups, dissimilar elements do not fall In the same group. Each vertical column (group) accommodates only those elements which have similar outer electronic configurations, thereby, showing similar properties.
• Group-8 elements (involving triads) of Mendeleev’s table, have been provided separate positions in groups 8, 9 and 10.
• Elements belonging to 1, 2, and 13-17 groups are classified as representative elements, while those belonging to 3-12 groups are classified as transition elements.

Elements are further classified as active metals (belonging to groups 1 and 2), heavy metals (belonging to groups 3-12) and non-metals (belonging to groups 13-18).

• Transition elements of the 4th, 5th, 6th and 7th periods are assigned appropriate positions in this periodic table.
• The completion of each period with an inert gas element is more logical. In a period as the atomic number increases, the quantum shells are gradually filled up until an inert gas configuration is achieved at group 18.
• It thus eliminates the even and odd series belonging to the periods 4, 5 and 6 of Defects of the long form of the periodic table.

If the Position of hydrogen:

The position of hydrogen is not settled. It can be placed along with alkali metals in group 1 or with halogens in group 17, as it resembles the alkali metals as well as the halogens.

Position of helium:

Based on electronic configuration, He (Is²) should be placed in group 2. But, it is placed in group 18 along with the p -p-block elements. No other p -p-block element has the electronic configuration ofthe type ns².

Position of lanthanoids and actinoids:

Lanthanoids and actinoids have not been accommodated in the main frame of the periodic table.

Position of isotopes:

Isotopeshavenotgot separate places. Properties of isotopes of heavier elements are more or less the same, but isotopes of lighter elements differ drastically in their physical, kinetic and thermodynamic properties. So it is not desirable to place the isotopes in the same position. Despite these limitations, the long form of the periodic table, based on electronic configurations, is much more scientific and thus finds extensive use.

Classification Of Elements Into Different Blocks

Amount In tin long form of the periodic table it has been divided Into four blocks viz, s -block, p -block, d -block and f- block. It Is done based on the nature of atomic orbitals into which the Inst electron (the differentiating electron) gets accommodated. Elements of s and p -blocks except Inert gases, are called representative elements, and d -block elements, on the other hand, are called transition elements.

S -block elements

Elements In which the last electron enters the -subshell of their outermost energy level (n) are called s – s-block elements.

Since s- s-subshell can accommodate a maximum of 2 electrons, only two groups are included in this block.

Elements of group-1 (alkali metals) and group-2 (alkaline earth metals) which have outermost electronic configurations rui1 and ns2 respectively constitute the s -block. This block is situated at the extreme left portion of the periodic table.

Outermost electronic configuration of s-block elements:

ns¹‾² Inert gas element, helium (He, Is²) Is also considered as an s -s-block element.

Characteristics of s-block elements:

In the case of these elements, all shells except the outermost one, are filled with electrons.

• Except for H, all other elements of this block are metals. Because of their low ionisation potential, these metals are very reactive and do not occur freely in nature.
• All the metals of this block are good reducing agents because of the value of ionisation potential.
• They are good conductors of heat and electricity.

They are soft metals. They have low melting points, boiling points and low densities as compared to the adjacent transition elements.

• Cations of group-IA and group-DA elements are diamagnetic and colourless since their orbitals do not contain odd electrons.
• Except for Be and Mg, -block elements impart specific colour to the flame (flame test).
• Salts of these elements except dichromate, permanganate arid chromate, are colourless.
• Compounds of these elements are mainly ionic (only Li and Be can form covalent compounds in many cases).

They form stable oxides with oxygen (Na₂O, CaO), produce chlorides with chlorine (NaCl, CaC₂) and also form salt-like hydrides (NaH, KH, CaH₂) with hydrogen.

• Hydroxides of these elements [except Ca(OH)₂, Mg(OH)₂ and Be(OH)₂] are soluble in water at ordinary temperature.
• The non-luminous flame of the Bunsen burner is rich in electrons. During the flame test, metal ions are converted into short-lived neutral atoms by accepting electrons from the flame.
• Valence electrons of these neutral atoms absorb energy from the flame and get promoted to higher energy levels.

When the electrons return to lower energy levels, the absorbed energy is emitted in the form of radiation of different wavelengths in the visible range and as a consequence, different colours, depending.

• The wavelengths of emitted light radiations are Imparted to the flame.
• For instance, the generation of golden-yellow flame during the flame test with sodium salt is due to the transition of one electron of Na -atom from 3s -orbital to 3p -orbital and its return to 3sorbltal after a very short interval.
• The ionisation potentials of Be and Mg are sufficiently high because of their smaller size.
• So, their electrons cannot be excited to higher energy levels by absorbing energy from the flame. As a result, they fail to respond to the flame test.

P -block elements

Elements in which the last electron enters p -subshell of their outermost energy level (n) are called p-block elements.

Since p -p-subshell can accommodate a maximum of six electrons, 6 groups are included in this block.

Elements of group-13, 14, 15, 16, 17 and 18 (excluding helium) having the outermost electronic configurations:

ns²np1, ns²np², ns²np³, ns⁶np⁴, ns²np⁵ and ns²np⁶ respectively, constitute the p -block. This block is situated at the extreme right portion of the periodic table

The elements of group 18 are balled noble gases or inert gases. They have the shell electronic configuration ns²np⁶ in the outermost shell. Group-17 elements are called halogens (salt producers)’ and group-16 elements are called chalcogens (ore-forming).

These two groups of elements have high electron-gain enthalpies (high negative values of ΔH) and hence readily accept one or two electrons respectively to attain the stable noble gas configuration thereby forming negative and negative anions respectively.

The elements of s- and p -blocks taken together are called representative normal or main group elements.

Outermost electronic config. of-flock elements: ns²np⁶

Based on electronic configuration, helium (Is2) should not be considered as a p -p-block element, but from the standpoint of its chemical inertness (owing to the presence of a filled valence shell) it is justified to place group-18 along with other noble gas elements.

Characteristics of block elements:

Ionisation enthalpies of p -p-block elements are higher as compared to those of —-block elements. Most of the p -p-block elements are non-metals, some are metals and a few others are metalloids and inert gases.

• The metallic character increases from top to bottom within a group non-metallic character increases from left to right along a period.
• Hence, metals exist at the bottom ofthe left side ofthe p -block whereas non-metals lie at the top of the right ofthe p -block. Metalloids (B, Si, Ge, As, Sb ) stand midway between them.
• The oxidising character of -p-block elements increases from left to right in a period and the reducing character increases from top to bottom in a group.
• Most of them form covalent compounds, although ionic character increases continually down the group.
• Elements of this block are non-conductors of heat and electricity, except metals and graphite.

Elements of this block are mostly electronegative.

• Some of them exhibit variable oxidation states or valence states. Oxidation states may be both positive and negative.
• Non-metallic elements of this block form acidic oxides. They can form both coloured and colourless compounds.
• 4th, 5th and 6th-period elements can form complex compounds by coordinate covalency due to the presence of vacant d-orbitals.
• Some of the p -block elements Fe.g.-Q Si, P, S, B, Ge, Sn, As etc.) show the phenomenon of allotropy.

Carnation property is shown by some- block elements (Example: C, Si, Ge, N, S etc.)

d-block elements (Transition elements) Elements in which the last electron enters d -the subshell of their penultimate shell (i.e., the second from the outermost) are called d -block elements, d -subshell can accommodate a maximum of 10 electrons.

Therefore, ten groups are included in d -the block. Elements of group-3 [(n-1)d¹ns²], 4, 5, 6, 7, 8, 9, 10, 11 and 12 [(n- 1)d¹⁰ns²] constitute the d -block.

Atoms of the elements belonging to these groups usually contain or 2 (sometimes zero) electrons in the s -s-orbital of their outermost shell (i.e., n -th shell), while the differentiating electrons are being progressively filled in, one at a time, in the d -subshell of their penultimate shell [i.e., (n- 1) -th shell].

Electronic configuration of outer shell: (n-1) d^1-10ns^1-2

This block is situated in between s -and p -blocks. D -block elements form a bridge between the chemically active metals of groups 2 on one side and the less reactive elements of groups 13 and 14 on the other side.

Hence, d -block elements are called transition elements These elements have been divided into four series called the first, second, third and fourth transition series.

First transition series or 3d-series:

The first transition series consists of 10 elements, belonging to the 4th period, from scandium ₂₁Sc) to zinc (₃₀Zn) in which 3d -orbitals are being progressively filled in. Zn is not a transition element.

Second transition series or 4d-series:

The second transition series also consists of 10 elements, belonging to the 5th period, from yttrium (₃₉Y) to cadmium (₄₈Cd) in which 4d -orbitals are being progressively filled in. Cd is not a transition element.

Third transition series or 5d-series:

Third transition series also consists of 10 elements, belonging to the 6th period. These are lanthanum (₅₇La) and elements from hafnium (₇₂Hf) to mercury (₈₀Hg). In all these elements, 5d orbitals are being successively filled in. Hg is not a transition element.

Fourth transition series (6d-series):

The fourth transition series is formed from a part of the seventh period and it contains 10 elements. These, are actinium (₈₉Ac) and elements from rutherfordium (104Rf) to ununbium (₁₁₂Uub), in which 6d -orbitals are being progressively filled in.

All d -block elements are not transition elements. Only those d -d-block elements in which atoms in their ground state or any stable oxidation state contain incompletely filled subshells are considered transition elements.

Characteristics of d-block elements:

• All d -block elements are metal. Their ionisation potential lies mid-way between those of s and p-block elements.
• Elements of the 5d series (especially Pt. Au and Hg) are inert under ordinary conditions. Thus, they are known as noble metals.
• Elements of this block exhibit variable oxidation states and valencies because ofthe presence of partially filled d orbitals in their atoms, ns -electrons and different numbers of(n-1)d electrons participate in bonding at the time of reaction with atoms of other elements.
• They are solids (except Hg), hard and have high melting and boiling points. They can form both ionic and covalent compounds.
• They exhibit paramagnetic character due to the presence of one or more unpaired electrons in their atoms or ions (exception-Sc³⁺, Ti⁴⁺, Zn²⁺, and Cu⁺ which do not contain odd electrons and are diamagnetic). Fe and Co can be converted into magnets and hence, they are ferromagnetic.
• They frequently form coloured ions in solids or solutions. With the change in their oxidation numbers, there also occurs a change in the colour ofthe formed ions.
• d -block elements exhibit a very distinctive property of forming coloured coordination complexes.
• This tendency may be ascribed to the small size of the atom or ion, a high nuclear charge of the ion and the presence of an incomplete d -d-orbital, capable of accepting electrons from the ligands.
• They are less electropositive than s -s-block elements but more electropositive than p -p-block elements.
• Several transition metals such as Cr, Mn, Fe, Co, Ni, Cu etc., and their compounds are used as catalysts. Many transition metals form alloys.

F-block elements (Inner-transition elements)

Elements in which the differentiating electron (i.e., the last electron) enters the f-subshell of their antepenultimate shell (i.e., the 3rd from the outermost) are called f-block elements.

All the F-block elements belong to group 3 (3B) of the periodic table. In these elements, s -orbital last shell (n) is filled, d -subshell of the penultimate shell [i.e., (n- 1) th shell] contains 0 or 1 electron, while f-subshell of the antepenultimate shell [i.e., (n-2)th shell] gets progressively filled in.

General electronic config.: (n-2)f^1-14(n-1)d^0-1ns²

Lanthanide series or 4f-Series:

• The first series follows lanthanum (La) in the 6th period and consists of 14 elements from cerium (₅₈Ce) to lutetium (₇₁Lu).
• These 14 elements are collectively called lanthanoids because they closely resemble lanthanum in their properties.
• These are also called rare-earth elements since most of these elements occur in very small amounts in the earth’s crust.

Actinoid series or 5f-series:

The second series follows actinium (sgAc) in the 7th period and consists of 14 elements from thorium (goTh) to lawrencium (103Lr).

These 14 elements are collectively called actinoids because they closely resemble actinium in their properties.

All the actinoids are radioactive elements. 4fand 5f-series of elements are also called inner-transition elements because they form transition series within the transition elements of d -block.

Characteristics of f-block elements:

• They are all heavy metals.
• They exhibit variable valency. +3 oxidation state is most common. Few elements are found to occur in +2 and +4 oxidation states.
• Some members exhibit paramagnetism due to the presence of odd electrons.
• They form complex compounds, most of which are coloured.
• They have high densities.
• They generally have high melting and boiling points.
• Within each series, the properties of the elements are quite similar. It is very difficult to separate them from a mixture.
• Actinoids are radioactive. The first three members (Th, Pa, U ) occur in nature, while the others are man-made.
• The elements after uranium are called transuranic elements.

Stair-step diagonal

• The right side of the long form of the periodic table is composed of p -block elements belonging to groups 13 (3A), 14(4A), 15(5A), 16(6A), 17(7A) and 18 (WlA or 0).
• This segment includes four types of elements viz., metals, nonmetals, metalloids and inert gases.
• There is no sharp line of demarcation to classify the metals and non-metals, but the zig¬ zag diagonal line (looking like stair-steps) running across the periodic table from boron (B) to astatine (At) is considered as a separation between the metals and non-metals.

This line is called the stair-step diagonal. The elements B, Si, Ge, As, Sb and Te bordering this line and- running diagonally across the periodic table are 8 known as metalloids (which exhibit properties that are characteristics of both metals and non-metals).

The elements (except Al ) lying between the stair-step diagonal line and the d -block elements are referred to as post-transition elements.

Determination Of The Position Of An Element In Long Form Of Periodic Table

Since there is a close relationship between the long form of the periodic table and the electronic configuration of elements, the serial numbers of periods and groups and the type of block to which an element belongs can be predicted by following the guidelines given below:

1. Period:

Serial number of the period = principal quantum number (n) ofthe valence shell.

Example:

Mg (ls²2s²2p⁶3s²) belongs to the third period because the principal quantum number of its valence shell is 3.

2. Block:

The publicly into which the differentiating electron [i.e., the last electron) enters, represents the block to which the given element belongs (except He ).

Example:

Sc (ls²2s²2p⁶3s²3p⁶4s²3d¹) belongs to d -block because the last electron [i.e., the 21st electron) enters the 3d -subshell.

3. Group:

The group to which an element belongs can be predicted based on the number of electrons present in the outermost [i.e., fth) and the penultimate [i.e., [n —1) th] shell.

For s -s-block elements: Group-number = Number of valence electrons i.e., no. of electrons in the ns -orbital.

For p -p-block elements: Group-number = 10 + no. of valence electrons = 10 + no. of ns -electrons + no. of np electrons.

For d -d-block elements: Group-number = no. of ns- electrons + no. of (n- l)d -electrons.

For f- f-block elements: Group number= 3 (fixed).

Examples:

Determination of the position of the elements with the following electronic configurations in the long form of the periodic table—

• ls²2s²2p⁶3s¹
• ls²2s²2p⁴
• ls²2s²2p⁶3s²3p⁶3d²4s²
• ls²2s²2p⁶3s²3p⁶4s²
• ls²2s²2p⁶3s²3p⁶3d¹⁰4s¹

1.

• The given element belongs to s -the block because the differentiating electron (i.e., the 11th electron) enters the 3s orbital.
• For this -block element, group-number = no. of electrons in the 3s -orbital =1.
• Serial no. of the period = principal quantum number ofthe valence shell =

2.

• The differentiating electron [i.e., the 8th election) enters the p-subshell.
• So, tile given element belongs to p block, [b] Serial no. of the period = principal quantum number of the valence shell
• = For this p -block element, group number= 10+ no. ofvalence electrons = 10 + number of ns electrons + no. of np -electrons =10 + 2 + 4 =16.

3.

• The differentiating electron [i.e., the 22nd electron) enters the 3d -subshell.
• So, the given element belongs to d -the block,
• Serial no. of the period = principal quantum number of the valence shell = 4.
• For die d block element, group-number = no. of ns -electrons + no. of (n- 1)d -electrons = 2 + 2 = 4.

4.

• The differentiating electron {l.e., the 20th electron) enters the 4s -subshell. So, the given element belongs to s -block,
• Serial no. of the period = principal quantum number of the valence shell =4.
• For this -block element, group-number = no. of electrons in outermost shell = 2.

5.

• The differentiating electron [i.e., the 29th electron) enters the 3d -subshell.
• So, the given element belongs to d -block, [b] Serial no. of the period = principal quantum number of the valence shell = 4.
• For this d -d-block element, group no. = no. of ns electrons + no. of[n- 1)d -electrons = 1 + 10 = 11.