NED Admission Test Chemistry Syllabus

How does atomic radius vary across a period and down a group? Explain why.

Across a period (left to right) it decreases because increasing nuclear charge pulls electrons in. Down a group it increases because each new period adds a shell, increasing distance and shielding.

Define ionization energy and describe its periodic trend.

Ionization energy is the minimum energy needed to remove the most loosely bound electron from a gaseous atom. It increases across a period (left to right) and decreases down a group.

Define electron affinity and describe its general periodic trend.

Electron affinity is the energy change when a gaseous atom gains an electron to form a negative ion. It generally becomes more negative (more energy released) across a period and less negative down a group.

Define electronegativity and identify the most electronegative element.

Electronegativity is the tendency of an atom to attract a shared (bonding) pair of electrons toward itself. It increases across a period and decreases down a group; fluorine is the most electronegative element.

How does an ionic (cation) radius compare to its parent atom, and how does an anion radius compare?

A cation is smaller than its parent atom (it loses electrons and often a whole shell, increasing effective nuclear charge per electron). An anion is larger than its parent atom (added electrons increase repulsion).

What is an ionic bond, and between what types of elements does it typically form?

An ionic bond is the electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons. It typically forms between a metal (low ionization energy) and a non-metal (high electronegativity).

What is a covalent bond, and what is the difference between a polar and a nonpolar covalent bond?

A covalent bond is formed by the sharing of electron pairs between atoms. It is nonpolar when shared equally (between identical or similar-electronegativity atoms) and polar when shared unequally, creating partial charges.

What is a coordinate (dative) covalent bond? Give an example.

A covalent bond in which both shared electrons are donated by the same atom. Example: the bond between NH3 and H+ to form the ammonium ion (NH4+), where nitrogen donates its lone pair.

State the octet rule and give one common exception.

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell (a noble-gas configuration). An exception is boron in BF3, which is stable with only six valence electrons.

What is the main postulate of VSEPR theory?

Valence Shell Electron Pair Repulsion theory states that electron pairs (bonding and lone pairs) around a central atom arrange themselves to be as far apart as possible to minimize repulsion, which determines the molecule's shape.

In VSEPR theory, rank the strength of repulsions between lone pairs (lp) and bonding pairs (bp).

Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. Lone pairs repel more strongly because they are held by only one nucleus and occupy more space.

State the molecular geometry and bond angle for 2, 3, and 4 bonding pairs with no lone pairs on the central atom.

2 pairs: linear, 180 degrees. 3 pairs: trigonal planar, 120 degrees. 4 pairs: tetrahedral, 109.5 degrees.

What are the shapes and bond angles of water (H2O) and ammonia (NH3), and why do they differ from the ideal tetrahedral angle?

H2O is bent/angular (about 104.5 degrees) with 2 lone pairs; NH3 is trigonal pyramidal (about 107 degrees) with 1 lone pair. Lone pair repulsion compresses the bond angle below the ideal 109.5 degrees.

What is the molecular geometry of a molecule with 5 bonding pairs and one with 6 bonding pairs (no lone pairs)?

5 bonding pairs: trigonal bipyramidal (angles 120 and 90 degrees), e.g. PCl5. 6 bonding pairs: octahedral (90 degrees), e.g. SF6.

What is hybridization?

Hybridization is the mixing of atomic orbitals of similar energy (e.g. s and p) on an atom to form an equal number of new, equivalent hybrid orbitals that have suitable geometry for bonding.

State the hybridization, geometry, and bond angle for sp, sp2, and sp3 hybridization.

sp: linear, 180 degrees (2 orbitals). sp2: trigonal planar, 120 degrees (3 orbitals). sp3: tetrahedral, 109.5 degrees (4 orbitals).

What is the hybridization of carbon in methane (CH4), ethene (C2H4), and ethyne (C2H2)?

Methane (CH4): sp3. Ethene (C2H4): sp2 (contains a C=C double bond). Ethyne (C2H2): sp (contains a C-triple-bond-C).

What is the difference between a sigma and a pi bond?

A sigma bond forms by head-on (axial) overlap of orbitals and is stronger. A pi bond forms by sidewise (lateral) overlap of unhybridized p orbitals and is weaker. A double bond = 1 sigma + 1 pi; a triple bond = 1 sigma + 2 pi.

List the three main types of intermolecular forces in order of increasing strength.

London dispersion forces (weakest) < dipole-dipole forces < hydrogen bonding (strongest). All are weaker than covalent or ionic bonds.

What is hydrogen bonding, and which atoms must be involved?

Hydrogen bonding is a strong dipole-dipole attraction between a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and a lone pair on another electronegative atom (F, O, or N).

What are London dispersion forces, and how does their strength depend on molecular size?

London (dispersion) forces are weak attractions arising from temporary instantaneous dipoles in electron clouds. They are present in all molecules and increase with molecular size, mass, and number of electrons (greater polarizability).

Why does water have an unusually high boiling point compared to H2S?

Water molecules form extensive hydrogen bonds (O is highly electronegative), which require much more energy to break, whereas H2S has only weak dipole-dipole and dispersion forces, giving it a much lower boiling point.

State Boyle's Law (equation and conditions).

At constant temperature and fixed amount of gas, the volume of a gas is inversely proportional to its pressure: P x V = constant, or P1V1 = P2V2.

State Charles's Law (equation and conditions).

At constant pressure and fixed amount of gas, the volume of a gas is directly proportional to its absolute (Kelvin) temperature: V/T = constant, or V1/T1 = V2/T2.

Write the ideal gas equation and define each term with SI/standard units.

PV = nRT, where P = pressure, V = volume, n = number of moles, T = absolute temperature (K), and R = the universal gas constant (0.0821 L atm K^-1 mol^-1, or 8.314 J K^-1 mol^-1).

State Avogadro's Law and the molar volume of an ideal gas at STP.

Avogadro's Law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. At STP (0 degrees C, 1 atm), one mole of any ideal gas occupies 22.4 litres.

What is vapour pressure, and how does it change with temperature?

Vapour pressure is the pressure exerted by a vapour in equilibrium with its liquid in a closed container at a given temperature. It increases as temperature increases because more molecules have enough energy to escape the liquid.

Define the boiling point of a liquid in terms of vapour pressure.

The boiling point is the temperature at which the vapour pressure of the liquid becomes equal to the external (atmospheric) pressure. At higher altitude, lower atmospheric pressure lowers the boiling point.

What is the difference between a crystalline solid and an amorphous solid? Give one example of each.

A crystalline solid has a regular, repeating long-range ordered arrangement of particles and a sharp melting point (e.g. NaCl, diamond). An amorphous solid has a disordered arrangement, no sharp melting point (e.g. glass, plastic).

Name the four main types of crystalline solids classified by bonding, with an example of each.

Ionic (e.g. NaCl), covalent/network (e.g. diamond), metallic (e.g. copper), and molecular (e.g. solid CO2 / dry ice). They differ in the particles and forces holding the lattice together.