Physical Science Classes VI-X for School Assistants and TET Paper 2A · Chapter 8

Periodic Classification and Chemical Bonding

What to remember

The modern periodic law says: properties of elements are a periodic function of their atomic number (Moseley). The table has 18 groups and 7 periods.
Across a period atomic size falls and ionisation enthalpy, electronegativity and non-metallic character rise; down a group the trends reverse.
VSEPR gives the shape from the number of electron pairs (bond pairs + lone pairs); the central atom's hybridisation (sp, sp2, sp3, sp3d, sp3d2) matches the electron-pair geometry.

Early attempts at classification

Dobereiner (triads): three elements with similar properties, in which the atomic mass of the middle element is about the mean of the other two. Examples: Li-Na-K, Ca-Sr-Ba, Cl-Br-I. Check: (7 + 39)/2 = 23, which is the mass of Na.
Newlands (law of octaves): when elements are arranged by increasing atomic mass, every eighth element repeats the properties of the first, like musical notes. It failed after calcium and could not place new elements.
Mendeleev (periodic law, mass based): properties are a periodic function of atomic mass. He left gaps for undiscovered elements and predicted them: eka-aluminium (later gallium), eka-silicon (later germanium) and eka-boron (later scandium). He also moved some elements against mass order to keep similar elements together. Noble gases had not been discovered and were added later without disturbing the table.
Defects of Mendeleev's table: no fixed place for hydrogen, isotopes had no place, and some pairs (for example Co-Ni, Ar-K) were in wrong mass order.
Moseley (1913): atomic number, not atomic mass, is the basic property. This removed the anomalies and gave the modern periodic law.

The modern periodic table

18 vertical columns are groups; 7 horizontal rows are periods. Elements in one group have the same number of valence electrons. Elements in one period have the same number of shells.
Period lengths: 1st = 2 elements, 2nd and 3rd = 8 each, 4th and 5th = 18 each, 6th = 32, 7th = 32 (incomplete or recently completed).
Number of valence electrons: groups 1 and 2 = 1 and 2; groups 13 to 18 = group number minus 10 (so group 17 has 7). Valency of a main-group element is the number of valence electrons (if 4 or fewer) or 8 minus that number.
Blocks: s-block (groups 1, 2), p-block (groups 13 to 18), d-block (groups 3 to 12, transition elements), f-block (lanthanoids and actinoids, each 14 elements, shown below the table).
Special groups: group 1 alkali metals, group 2 alkaline earth metals, group 17 halogens, group 18 noble gases (zero valency; helium has 2 valence electrons, others 8).
To find position from atomic number: write the electronic configuration. Number of shells = period; valence electrons give the group. Example: Z = 17 gives 2, 8, 7, so period 3, group 17 (chlorine). Z = 12 gives 2, 8, 2, so period 3, group 2 (magnesium).

Property	Along a period (left to right)	Down a group
Atomic radius	Decreases (nuclear charge rises, shells same)	Increases (new shells)
Ionisation enthalpy	Increases	Decreases
Electronegativity	Increases	Decreases
Metallic character	Decreases	Increases
Non-metallic character	Increases	Decreases
Electron gain enthalpy (magnitude)	Generally increases	Generally decreases
Oxides	Basic to amphoteric to acidic	More basic

Atomic radius: a cation is smaller than its atom; an anion is larger than its atom. In isoelectronic species (same electrons) the one with more protons is smaller.
Ionisation enthalpy: energy needed to remove the outermost electron from a gaseous atom. Exceptions: Be is higher than B, and N is higher than O (stable filled or half-filled subshells).
Electronegativity: tendency of a bonded atom to attract the shared pair (Pauling scale). Fluorine is the most electronegative element (4.0). Cs and Fr are the least.
Electron gain enthalpy: chlorine releases more energy than fluorine because fluorine is very small and its electron cloud is crowded.
Noble gases have very high ionisation enthalpy and are very stable.

Chemical bonding: why atoms combine

Atoms combine to reach a stable noble-gas configuration (octet; duplet for H). This is the octet rule. It has exceptions: BF3 (6 electrons around B), PCl5 and SF6 (expanded octet), NO (odd electrons).

Ionic bond: transfer of electrons between a metal and a non-metal. Example: Na + Cl gives Na⁺ Cl⁻. It forms when the difference in electronegativity is large. Properties: high melting and boiling point, hard but brittle crystals, soluble in water, conduct electricity in molten state or solution, not in solid state.
Covalent bond: sharing of electron pairs between non-metals. Single bond (one pair, H2, CH4), double bond (two pairs, O2, CO2), triple bond (three pairs, N2). Properties: low melting and boiling points, poor conductors, often insoluble in water.
Coordinate (dative) bond: both shared electrons come from one atom. Examples: NH4⁺ (N gives the pair to H⁺), H3O⁺, ozone.
Metallic bond: positive ions in a sea of free electrons. It explains electrical conductivity, malleability and lustre of metals.
Lewis structure: valence electrons drawn as dots; shared pairs show bonds. Formal charge = valence electrons - non-bonding electrons - half of bonding electrons.
Hydrogen bond: weak attraction between H bonded to F, O or N and a lone pair on another F, O or N. It explains the high boiling point of water and ice floating on water.
Polarity: a bond is polar when the two atoms differ in electronegativity (HCl). A molecule is non-polar if bond dipoles cancel (CO2, CH4, BF3) even though bonds are polar.

VSEPR theory

Valence Shell Electron Pair Repulsion: electron pairs around the central atom arrange themselves as far apart as possible. Repulsion order: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair. So lone pairs squeeze bond angles.

Bond pairs	Lone pairs	Shape	Ideal/real angle	Example
2	0	Linear	180°	BeCl2, CO2
3	0	Trigonal planar	120°	BF3
2	1	Bent (V-shaped)	less than 120°	SO2
4	0	Tetrahedral	109.5°	CH4
3	1	Trigonal pyramidal	about 107°	NH3
2	2	Bent	about 104.5°	H2O
5	0	Trigonal bipyramidal	90° and 120°	PCl5
6	0	Octahedral	90°	SF6
4	1	See-saw	-	SF4
3	2	T-shaped	-	ClF3
4	2	Square planar	90°	XeF4

Method: central atom's valence electrons plus electrons from other atoms (adjust for charge), divide by 2 to get electron pairs, subtract bond pairs to get lone pairs. Example: H2O has 4 pairs around O, 2 bonded and 2 lone, so bent.

Valence bond theory and hybridisation

Valence bond theory (VBT): a covalent bond forms when half-filled orbitals of two atoms overlap; the shared electrons have opposite spins. Greater overlap gives a stronger bond.
Sigma (σ) bond: head-on (axial) overlap; strong; free rotation. Pi (π) bond: sideways overlap of p orbitals; weaker; restricts rotation. A single bond is one σ; a double bond is one σ + one π; a triple bond is one σ + two π.
Hybridisation: mixing of atomic orbitals of nearly equal energy to form the same number of equivalent hybrid orbitals (Pauling). It explains shapes such as the tetrahedral carbon in methane.

Hybridisation	Orbitals mixed	Shape	Angle	Example
sp	one s + one p	Linear	180°	BeCl2, ethyne (each C)
sp2	one s + two p	Trigonal planar	120°	BF3, ethene (each C)
sp3	one s + three p	Tetrahedral	109.5°	CH4, ethane, NH3, H2O
sp3d	one s + three p + one d	Trigonal bipyramidal	90°, 120°	PCl5
sp3d2	one s + three p + two d	Octahedral	90°	SF6

To find hybridisation, count electron pairs (steric number): 2 gives sp, 3 gives sp2, 4 gives sp3, 5 gives sp3d, 6 gives sp3d2.
NH3 and H2O are sp3 even though lone pairs change the shape; ammonia angle is reduced to about 107° and water to about 104.5°.
Counting σ and π bonds (worked examples): ethene C2H4 has 5 σ and 1 π. Ethyne C2H2 has 3 σ and 2 π. Benzene C6H6 has 12 σ and 3 π. Methane has 4 σ, no π. In CO2 (O=C=O) there are 2 σ and 2 π.
Carbon in diamond is sp3, in graphite sp2, in ethyne sp.

Classroom angle

Use ball-and-stick models and balloons tied together (two balloons give a line, four give a tetrahedron) to show VSEPR. Use the "musical octave" idea for Newlands and a simple Mendeleev card game to show how gaps predict elements.

Exam traps

Dobereiner used atomic mass triads; Moseley used atomic number. Do not mix them.
Newlands' octave means every eighth element, not every seventh.
Atomic size increases down a group but decreases across a period.
Electron gain enthalpy of Cl is more than that of F, though F is more electronegative.
Ionisation enthalpy of N is greater than O, and of Be greater than B.
A cation is smaller than its parent atom; an anion is larger.
NH3 and H2O are both sp3 but not tetrahedral in shape (pyramidal and bent).
CO2 has polar bonds yet is a non-polar molecule.
A σ bond is stronger than a π bond; π bonds do not allow free rotation.
Hydrogen bond is an attraction between molecules, not a covalent bond.

One-liners

1. Dobereiner's triads: Li-Na-K, Ca-Sr-Ba, Cl-Br-I.
2. Newlands' law of octaves works up to calcium.
3. Mendeleev predicted gallium, germanium and scandium.
4. Moseley showed atomic number is the basis of classification.
5. The modern table has 18 groups and 7 periods.
6. Fluorine is the most electronegative element.
7. Noble gases are in group 18 and have zero valency.
8. Lanthanoids and actinoids have 14 elements each.
9. Bond angle in methane is 109.5°; in water about 104.5°.
10. BF3 is sp2 and planar; BeCl2 is sp and linear.
11. SF6 is sp3d2 and octahedral; PCl5 is sp3d and trigonal bipyramidal.
12. A triple bond has one σ and two π bonds.

Practice questions

Triads of elements such as Li-Na-K were proposed by
1. Mendeleev
2. Newlands
3. Moseley
4. Dobereiner
Answer
D. Dobereiner
Dobereiner grouped similar elements in triads where the middle mass is about the mean of the other two.
The modern periodic law is based on
1. number of neutrons
2. atomic number
3. atomic mass
4. atomic volume
Answer
B. atomic number
Moseley showed atomic number is the fundamental property.
The number of groups in the modern periodic table is
1. 7
2. 16
3. 8
4. 18
Answer
D. 18
The long form table has 18 groups and 7 periods.
The most electronegative element is
1. fluorine
2. chlorine
3. nitrogen
4. oxygen
Answer
A. fluorine
Fluorine has electronegativity 4.0 on the Pauling scale.
Mendeleev's eka-aluminium was later discovered as
1. scandium
2. gallium
3. indium
4. germanium
Answer
B. gallium
Eka-aluminium = gallium, eka-silicon = germanium, eka-boron = scandium.
The bond angle in methane is
1. 120°
2. 104.5°
3. 107°
4. 109.5°
Answer
D. 109.5°
Four bond pairs, no lone pair, give a regular tetrahedron.
The hybridisation of each carbon atom in ethyne is
1. sp3d
2. sp3
3. sp
4. sp2
Answer
C. sp
Each C has two electron pairs (a σ to C and a σ to H), so it is sp and linear.
The shape of the ammonia molecule is
1. tetrahedral
2. trigonal planar
3. trigonal pyramidal
4. bent
Answer
C. trigonal pyramidal
NH3 has three bond pairs and one lone pair.
The shape of SF6 is
1. trigonal bipyramidal
2. tetrahedral
3. square planar
4. octahedral
Answer
D. octahedral
Six bond pairs and no lone pair around S give an octahedron (sp3d2).
The bond in sodium chloride is
1. ionic
2. metallic
3. covalent
4. coordinate
Answer
A. ionic
Na transfers an electron to Cl to form Na⁺ and Cl⁻.
The bond formed when NH3 donates its lone pair to H⁺ to form NH4⁺ is
1. metallic
2. coordinate
3. ionic
4. hydrogen
Answer
B. coordinate
Both shared electrons come from nitrogen, so it is a coordinate bond.
The sixth period of the modern periodic table contains how many elements?
1. 18
2. 24
3. 32
4. 8
Answer
C. 32
Periods 6 and 7 have 32 elements each, including the f-block.
The hybridisation of phosphorus in PCl5 is
1. sp3d2
2. sp3d
3. sp3
4. sp2
Answer
B. sp3d
Five bond pairs give a trigonal bipyramidal shape with sp3d hybridisation.
On moving from left to right across a period, the atomic radius generally
1. decreases
2. first increases then decreases
3. increases
4. stays the same
Answer
A. decreases
Nuclear charge rises while the number of shells stays the same.
Newlands' law of octaves says that the properties repeat after every
1. eighteenth element
2. seventh element
3. third element
4. eighth element
Answer
D. eighth element
Newlands noticed the eighth element resembled the first.
Fluorine has a lower electron gain enthalpy magnitude than chlorine mainly because
1. it has more shells than chlorine
2. it is a metal
3. it has lower nuclear charge than chlorine
4. its small 2p subshell has strong electron-electron repulsion
Answer
D. its small 2p subshell has strong electron-electron repulsion
Crowding of electrons in the compact fluorine atom reduces the energy released.
Water has a bond angle (about 104.5°) smaller than methane (109.5°) because
1. lone pairs repel bond pairs more strongly
2. water has no lone pair
3. hydrogen atoms are bigger
4. oxygen is less electronegative
Answer
A. lone pairs repel bond pairs more strongly
Two lone pairs on O squeeze the H-O-H angle.
CO2 is a non-polar molecule even though C=O bonds are polar because
1. oxygen is not electronegative
2. the bond dipoles cancel in the linear shape
3. the molecule is bent
4. carbon is a metal
Answer
B. the bond dipoles cancel in the linear shape
Equal and opposite dipoles in a linear molecule give zero net dipole.
A cation is smaller than its parent atom because
1. it has an extra shell
2. it has more protons
3. it has fewer electrons while the nuclear charge is unchanged
4. its electrons repel more
Answer
C. it has fewer electrons while the nuclear charge is unchanged
Loss of electrons (often a whole shell) lets the nucleus pull the rest closer.
Ice floats on water because
1. ice has covalent network bonds
2. hydrogen bonding gives an open, less dense structure
3. ice is a metal
4. water is an ionic compound
Answer
B. hydrogen bonding gives an open, less dense structure
Hydrogen-bonded cages in ice make it less dense than liquid water.
The first ionisation enthalpy of nitrogen is higher than that of oxygen because
1. nitrogen is larger
2. nitrogen has more protons than oxygen
3. nitrogen has a stable half-filled 2p subshell
4. oxygen has no 2p electrons
Answer
C. nitrogen has a stable half-filled 2p subshell
Removing an electron from a half-filled subshell is harder.
An element with atomic number 17 is placed in
1. period 3, group 17
2. period 17, group 3
3. period 2, group 17
4. period 3, group 7
Answer
A. period 3, group 17
Configuration 2, 8, 7 gives three shells and seven valence electrons.
Using Dobereiner's idea, the atomic mass of bromine from chlorine (35.5) and iodine (127) is about
1. 63.5
2. 91.25
3. 162.5
4. 81.25
Answer
D. 81.25
(35.5 + 127)/2 = 81.25.
The number of valence electrons in an atom of atomic number 15 is
1. 3
2. 15
3. 5
4. 8
Answer
C. 5
Configuration 2, 8, 5 gives five valence electrons.
The number of σ bonds in ethene (C2H4) is
1. 4
2. 6
3. 3
4. 5
Answer
D. 5
One C-C σ and four C-H σ bonds make 5; the π bond is extra.
The number of π bonds in ethyne (C2H2) is
1. 1
2. 2
3. 3
4. 0
Answer
B. 2
A triple bond has one σ and two π bonds.
The total number of σ bonds in benzene (C6H6) is
1. 12
2. 6
3. 15
4. 9
Answer
A. 12
Six C-C σ and six C-H σ bonds make 12; three π bonds are extra.
A central atom with four electron pairs shows hybridisation
1. sp2
2. sp3d
3. sp
4. sp3
Answer
D. sp3
Four electron pairs correspond to one s and three p orbitals mixing.
The number of lone pairs on the central oxygen of water is
1. 2
2. 1
3. 0
4. 3
Answer
A. 2
Oxygen has six valence electrons; two are used in bonds, leaving two lone pairs.
The number of valence electrons in a calcium atom (Z = 20) is
1. 1
2. 8
3. 2
4. 10
Answer
C. 2
Configuration 2, 8, 8, 2 puts calcium in group 2.
Which of the statements is/are correct? 1. Atomic radius decreases across a period. 2. Ionisation enthalpy decreases across a period.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Ionisation enthalpy generally increases across a period.
Which of the statements is/are correct? 1. Mendeleev's table was based on atomic mass. 2. Moseley's work gave the modern law based on atomic number.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Which of the statements is/are correct? 1. BF3 is sp2 hybridised and planar. 2. NH3 has a tetrahedral molecular shape.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
NH3 is sp3 but its shape is trigonal pyramidal.
Which of the statements is/are correct? 1. A σ bond is formed by sideways overlap of p orbitals. 2. A π bond restricts free rotation about the bond.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
σ bonds form by head-on overlap; π bonds form by sideways overlap and restrict rotation.
Which of the statements is/are correct? 1. Noble gases have zero valency. 2. Helium has eight valence electrons.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Helium has only two valence electrons.
Which of the statements is/are correct? 1. Ionic compounds conduct electricity in the molten state. 2. Covalent compounds generally have high melting points.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Covalent compounds usually have low melting points.
Which of the statements is/are correct? 1. BeCl2 is a bent molecule. 2. XeF4 has a tetrahedral shape.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
BeCl2 is linear and XeF4 is square planar.
Which of the statements is/are correct? 1. Electronegativity increases down a group. 2. Metallic character increases down a group.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Electronegativity decreases down a group; metallic character increases.
Which of the statements is/are correct? 1. A hydrogen bond is weaker than a covalent bond. 2. Hydrogen bonding explains the high boiling point of water.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Which of the statements is/are correct? 1. In ethene each carbon is sp hybridised. 2. In ethyne each carbon is sp2 hybridised.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
Ethene carbons are sp2 and ethyne carbons are sp.
Which of the statements is/are correct? 1. An anion is larger than its parent atom. 2. A cation is larger than its parent atom.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Cations are smaller; anions are larger.
Which of the statements is/are correct? 1. The s-block contains groups 1 and 2. 2. The d-block elements are called transition elements.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Match: (a) CH4 (b) BF3 (c) BeCl2 (d) SF6 with (i) sp (ii) sp2 (iii) sp3 (iv) sp3d2
1. a-ii, b-iii, c-i, d-iv
2. a-iii, b-ii, c-i, d-iv
3. a-iii, b-i, c-ii, d-iv
4. a-iv, b-ii, c-i, d-iii
Answer
B. a-iii, b-ii, c-i, d-iv
CH4 sp3, BF3 sp2, BeCl2 sp, SF6 sp3d2.
Match the scientist with the contribution: (a) Dobereiner (b) Newlands (c) Mendeleev (d) Moseley; (i) atomic number (ii) triads (iii) octaves (iv) gaps for undiscovered elements
1. a-iii, b-ii, c-iv, d-i
2. a-i, b-iii, c-iv, d-ii
3. a-ii, b-iii, c-i, d-iv
4. a-ii, b-iii, c-iv, d-i
Answer
D. a-ii, b-iii, c-iv, d-i
Dobereiner triads, Newlands octaves, Mendeleev gaps, Moseley atomic number.
Among Na, Mg, Al and Cl in period 3, the atom with the largest radius is
1. Mg
2. Na
3. Al
4. Cl
Answer
B. Na
Radius falls across a period, so sodium on the left is the largest.

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1.The chemical symbol P stands for-
2.The atomic size _______ moving from left to right and _______ moving from up to down in the periodic table.
3.The electron distribution in an aluminum atom is-
4.The first attempt to classify elements was made by-
5.The first scientific experiments on genetics and inheritance were conducted in the mid nineteenth century. Who performed these hybridisation experiments on garden peas?
6.The filament of a light bulb is made up of-
7.The elements with atomic numbers 35, 53 and 85 are _________.
8.The elements that have intermediate properties between those of metals and non-metals are called metalloids. Which among the following is known as a metalloid?
9.The element with atomic number 21 is ______.
10.The element, which has zero electron affinity in the 3rd period is -

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