Atomic Structure & Periodicity — Part 2: 40 MCQs with Explanations

Alkenes contain a carbon-carbon double bond, which is electron-rich. They typically undergo electrophilic addition reactions to become saturated.

Benzene has a delocalized π-electron system (resonance) that makes it unusually stable. Addition reactions would destroy this stability, so it prefers substitution where the ring is preserved.

The azimuthal or angular momentum quantum number (l) defines the subshell and determines the shape of the orbital (s is spherical, p is dumbbell, etc.).

Water (H2O) has strong intermolecular hydrogen bonding due to high electronegativity of oxygen and small size. H2S, despite higher molar mass, has weaker dipole-dipole interactions, hence a lower boiling point.

Electron affinity is the energy released when an atom gains an electron. Although Fluorine is the most electronegative, Chlorine has the highest electron affinity due to Fluorine's small size causing inter-electronic repulsion.

An ionic bond (or electrovalent bond) is formed when one atom completely transfers one or more electrons to another atom, resulting in the formation of ions.

l = 2 corresponds to the d-subshell. A d-subshell has 5 orbitals, and each orbital can hold a maximum of 2 electrons with opposite spins. Therefore, maximum electrons = 5 * 2 = 10.

According to Molecular Orbital Theory, the electronic configuration of N2 gives a bond order of (Nb - Na)/2 = (8-2)/2 = 3. It has a triple bond (one sigma and two pi bonds).

Hybridization is the concept of mixing atomic orbitals to form new, equivalent hybrid orbitals that are suitable for bonding, as in the formation of CH4, C2H4, etc..

Alkenes (CH2=CH2) are electron-rich due to the pi bond and readily undergo electrophilic addition with HBr. Alcohols react slowly, alkynes are less reactive than alkenes, and benzene is stabilized by resonance.

VSEPR theory states that electron pairs repel each other. Four bonding pairs arrange themselves as far apart as possible, pointing to the corners of a tetrahedron, giving a tetrahedral geometry (bond angle 109.5°).

Chlorine has the electronic configuration [Ne] 3s2 3p5. Since the last electron enters the p-orbital, it belongs to the p-block of the periodic table.

Melting points of transition metals are high due to strong metallic bonding. In the 3d series, Chromium has an exceptionally high melting point (1907 K) due to its half-filled d-orbital configuration leading to stronger bonding.

Leading zeros (to the left of the first non-zero digit) are not significant. Trailing zeros after the decimal point are significant. So, the significant digits are '500' (5,0,0), which gives 3 significant figures.

Zinc (Zn) has the electronic configuration [Ar] 3d10 4s2. It has a completely filled d-orbital, so it typically exhibits only a +2 oxidation state by losing the two 4s electrons. Transition metals usually show variable states due to involvement of (n-1)d electrons.

sp hybridization involves the mixing of one s and one p orbital, resulting in two hybrid orbitals oriented at 180°. This gives a linear geometry.

In the ammonium ion (NH4+), the bond between NH3 and H+ is formed when the nitrogen atom donates its lone pair to the proton. This shared pair of electrons comes from only one of the bonding atoms, defining a coordinate covalent bond.

Aromatic compounds are cyclic, planar, and contain a conjugated system with (4n+2) π electrons (Hückel's rule). Benzene (C6H6) is the classic example with 6 π electrons. The others are alicyclic compounds.

Mass of one electron = 9.1 × 10⁻³¹ kg. Mass of one mole electrons = (9.1 × 10⁻³¹ kg) × (6.02 × 10²³) = 5.48 × 10⁻⁷ kg = 5.48 × 10⁻⁴ g = 0.548 mg ≈ 0.55 mg.

Atomic number (Z) is equal to the number of protons in the nucleus of an atom. Therefore, sodium (Z=11) has 11 protons.

A d-subshell has 5 orbitals (magnetic quantum numbers -2, -1, 0, +1, +2). Each orbital holds 2 electrons, so total = 5 × 2 = 10 electrons.

sp³ hybridization involves the mixing of one s and three p orbitals to form four equivalent hybrid orbitals. These orbitals are directed towards the four corners of a regular tetrahedron (bond angle 109.5°).

By definition, the cathode is the electrode where reduction (gain of electrons) takes place. The anode is where oxidation occurs.

Alkali metals are the elements of Group 1 (I-A) of the periodic table. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

Electronegativity is the tendency of an atom to attract a shared pair of electrons. Fluorine is the most electronegative element on the Pauling scale (value = 4.0).

Carbon-14 is a radioactive isotope with a half-life of about 5730 years. It is used in radiocarbon dating to determine the age of carbonaceous materials (up to ~60,000 years).

Benzene (C₆H₆) is the parent compound of aromatic hydrocarbons. It has a cyclic, planar structure with delocalized π-electrons, which gives it special stability (aromaticity).

According to Brønsted-Lowry theory, a conjugate base is formed when an acid donates a proton (H⁺). NH₃ can act as an acid by donating a proton to form NH₂⁻ (amide ion).

The azimuthal or angular momentum quantum number (l) defines the subshell and the shape of the orbital. For example, l=0 is s (spherical), l=1 is p (dumbbell), l=2 is d (cloverleaf).

In molten NaCl (an ionic compound), the ions (Na⁺ and Cl⁻) are free to move and conduct electricity. Copper and graphite conduct via electrons. Silicon is a semiconductor.

Alkaline earth metals are the elements of Group 2 (II-A). Their general electronic configuration is [noble gas] ns², meaning they have 2 electrons in their outermost (valence) shell.

The nucleus is the small, dense, positively charged center of an atom. It contains protons (positively charged) and neutrons (neutral), collectively called nucleons.

The Balmer series corresponds to electronic transitions from higher energy levels (n ≥ 3) to n=2. These transitions produce wavelengths of light that fall in the visible region of the electromagnetic spectrum.

Atomic radius generally decreases across a period from left to right due to an increase in nuclear charge which pulls the electron cloud closer. In the third period (Na to Ar), Si is to the right of Al, so it has a smaller radius than Na, Mg, Al.

Plasma is often called the fourth state of matter. It consists of a gas of ions and free electrons and is the most common form of matter in the universe (stars, including the sun).

Down a group, atomic radius increases due to the addition of new shells. This increases the distance between the nucleus and the valence electrons, and also increases the shielding effect. This makes it easier to remove an electron, decreasing ionization energy.

An alpha particle (α-particle) is identical to the nucleus of a helium-4 atom. It consists of two protons and two neutrons, so it has a +2 charge and a mass number of 4.

Ferromagnetic materials can form permanent magnets and are strongly attracted to a magnet. They have unpaired electrons and can have their magnetic domains aligned. Examples: iron (Fe), cobalt (Co), nickel (Ni).

A covalent bond is formed by the mutual sharing of electrons between two atoms. This typically occurs between non-metal atoms that have similar electronegativities.

sp hybridization involves the mixing of one s and one p orbital to form two equivalent linear hybrid orbitals oriented at 180° to each other. Example: BeCl₂, C₂H₂ (acetylene).