1. Which sequence of Group 18 elements demonstrates a gradual decrease in the strength of the Van der Waals forces?

A)    Ar, Kr, Ne, Xe

B)    Xe, Kr, Ar, Ne

C)    Kr, Xe, Ar, Ne

D)    Ne, Ar, Kr, Xe

Correct Answer: Option B - Xe, Kr, Ar, Ne

The van der Waals force (or van der Waals interaction) is the attractive or repulsive force between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. These forces decrease as the molecule gets smaller and increase as the molecule increases. Therefore the Van der Waals forces would also be decreasing.

Xe, Kr, Ar, Ne are in the order of decreasing molecule size. Hence, only option B is the correct answer and the other 3 options are incorrect.

2. Which combination of atoms can form a polar covalent bond?

A)    N and N

B)    H and H

C)    Na and Br

D)    H and Br

Correct Answer: Option D - H and Br

Polarity refers to a separation of electric charge leading to a molecule having an electric dipole. Polar molecules can bond together due to dipole-dipole intermolecular forces between one molecule (or part of a large molecule) with asymmetrical charge distribution and another molecule also with asymmetrical charge distribution. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, a molecule of water is polar because of the unequal sharing of its electrons in a "bent" structure, whereas methane is considered non-polar because the carbon shares the electrons with the hydrogen atoms uniformly. Polarity underlies a number of physical properties including surface tension, solubility, and melting- and boiling-points.

The electronegativity difference between H (2.1) and Br (3.0) is 0.9. If the difference is less than 1.7 (but not zero, usually more than 0.4), a polar covalent bond is formed. A nonpolar covalent bond is formed when the difference is close to zero (usually up to 0.4). If the difference is more than 1.7, an ionic bond is formed.

Hence, options A, B and C are incorrect.

3. As a bond between a hydrogen atom and a sulfur atom is formed, electrons are:

A)    Shared to form a covalent bond

B)    Shared to form an ionic bond

C)    Transferred to form an ionic bond

D)    Transferred to form a covalent bond

Correct Answer: Option A - Shared to form a covalent bond

Covalent bonds result from the sharing of valence electrons, thus the term "covalent". Since the octet rule is still observed for most covalent compounds, elements obtain a noble gas configuration by sharing, rather than by transferring, electrons. In the Periodic Table, sulfur is directly underneath oxygen. According to trends in the Periodic Table, one knows that the size of the atom of sulfur is larger than that of oxygen because there are more electrons in a higher energy level surrounding the nucleus. The bond between both hydrogen and oxygen and hydrogen and sulfur are covalent. That means that the lone electron around the hydrogen is equally shared between the hydrogen and the sulfur. Simply because sulfur is a larger atom, it will have a larger bond length between anything it bonds with than that if oxygen were to bond with that same atom.

Hence, options B, C and D are incorrect.

4. Atoms of which element can bond to each other to form chains, rings, and networks?

A)    Hydrogen

B)    Fluorine

C)    Oxygen

D)    Carbon

Correct Answer: Option D - Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent-making four electrons available to form covalent chemical bonds. There are several allotropes of carbon of which the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form.

Carbon has the ability to form very long chains of interconnecting C-C bonds. This property is called catenation. Carbon-carbon bonds are strong, and stable. This property allows carbon to form an almost infinite number of compounds; in fact, there are more known carbon-containing compounds than all the compounds of the other chemical elements combined except those of hydrogen

At normal pressures carbon takes the form of graphite, in which each atom is bonded trigonally to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The resulting network is 2-dimensional, and the resulting flat sheets are stacked and loosely bonded through weak Van der Waals forces. This gives graphite its softness and its cleaving properties (the sheets slip easily past one another). Because of the delocalization of one of the outer electrons of each atom to form a p-cloud, graphite conducts electricity, but only in the plane of each covalently bonded sheet. This results in a lower bulk electrical conductivity for carbon than for most metals. The delocalization also accounts for the energetic stability of graphite over diamond at room temperature. At very high pressures carbon forms the more compact allotrope diamond, having nearly twice the density of graphite. Here, each atom is bonded tetrahedral to four others, thus making a 3-dimensional network of puckered six-member rings of atoms.

Hence, options A, B and C are incorrect.

5. Which element has an atom with the greatest attraction for electrons in a chemical bond?

A)    Arsenic

B)    Nitrogen

C)    Bismuth

D)    Phosphorus

Correct Answer: Option B - Nitrogen

Electronegativity, denoted by symbol χ, is a chemical property that describes the ability of an atom (or, more rarely, a functional group) to attract electrons (or electron density) towards itself in a covalent bond. An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it. The electronegativity difference between two covalently bonded atoms is used to assess the degree of polarity.

The most commonly used method of calculation of electronegativity is that originally proposed by Pauling. This gives a dimensionless quantity, commonly referred to as the Pauling scale, on a relative scale running from 0.7 to 4.0 (hydrogen = 2.2). When other methods of calculation are used, it is conventional to quote the results on a scale that covers the same range of numerical values: this is known as electronegativity in Pauling units

Nitrogen has an electronegativity of 3.04 Pauling Units

Incorrect Options

• Arsenic has an electronegativity of 2.18 Pauling Units
• Bismuth has an electronegativity of 2.02 Pauling Units

6. Which formula represents a polar molecule?

A)    Br₂

B)    CH₄

C)    CO₂

D)    NH₃

Correct Answer: Option D - NH₃

A molecule in which there is some separation of charge in the chemical bonds, so that one part of the molecule has a slight positive charge and the other a slight negative charge, i.e., the molecule has a permanent electric dipole (a pair of equal and opposite charges a short distance apart). Water is a well-known example of a polar molecule.

A polar molecule forms when an atom of high electronegativity (one that attracts electrons), such as chlorine, bonds with a less electronegative atom such as hydrogen. Because the more electronegative atom pulls the electron(s) away from the other atom, the molecule formed has one end which is negatively charged and another which is positively charged. Polar molecules tend to align themselves because the negative end of each molecule is attracted to the positive end of other molecules, and vice versa. Polar molecules containing hydrogen are so strongly polar (and therefore attracted to each other) that they form a type of chemical bond called a hydrogen bond.

The ammonia molecule has a triangular pyramidal shape, as predicted by Valence shell electron pair repulsion theory (VSEPR). The nitrogen atom in the molecule has a lone electron pair, and ammonia acts as a base, a proton acceptor. This shape gives the molecule a dipole moment and makes it polar so that ammonia readily dissolves in water.

Hence, options A, B and C are incorrect.

7. What can be concluded if an ion of an element is smaller than an atom of the same element?

A)    The ion is negatively charged because it has fewer electrons than the atom.

B)    The ion is negatively charged because it has more electrons than the atom.

C)    The ion is positively charged because it has fewer electrons than the atom.

D)    The ion is positively charged because it has more electrons than the atom.

Correct Answer: Option C - The ion is positively charged because it has fewer electrons than the atom.

Atomic radius trend

• In groups, atomic radii increase with atomic number because the number of energy level increases.
• In periods, atomic radii decrease when atomic number increases because the nuclear charges increase attracting electrons closer to the nucleus.

Ionic radius trend

• Cations (positive ions) are always smaller than their parent atoms and anions (negative ions) are always larger.

Hence, options A, B and D are incorrect.

8. Which compound contains both ionic and covalent bonds?

A)    Sodium nitrate

B)    Ammonia

C)    Methane

D)    Potassium chloride

Correct Answer: Option A - Sodium nitrate

Ionic bond, also known as electrovalent bond is a type of bond formed from the electrostatic attraction between oppositely charged ions in a chemical compound. These kinds of bonds occur mainly between a metallic and a non metallic atom. An ionic bond is formed between a metal and a non-metal. Non-metals(-ve ion) are "stronger" than the metal(+ve ion) and can get electrons very easily from the metal. These two opposite ions attract each other and form the ionic bond. Pure ionic bonding is not known to exist. All ionic compounds have a degree of covalent bonding. The larger the difference in electronegativities between two atoms, the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.

Covalent bonding is a form of chemical bonding between two non metallic atoms which is characterized by the sharing of pairs of electrons between atoms and other covalent bonds. A covalent bond is formed between two non-metals that have similar electronegativities. Neither atom is strong enough to attract electrons from the other. For stabilization, they share their electrons from outer molecular orbit with others.

Sodium nitrate is the chemical compound with the formula NaNO₃. This salt is a white solid which is very soluble in water. It is made up of sodium being a metal and nitrogen being a nonmetal. Hence, this compound has both ionic and covalent bonds.

Incorrect options

Ammonia has a molecular formula of NH₃ with nitrogen and hydrogen forming covalent bond. Hence, option B is incorrect.

Methane has a molecular formula of CH₄ with carbon and hydrogen forming double covalent bonds. Hence, option C is incorrect.

Potassium chloride has a molecular formula of KCl with potassium and chlorine forming ionic bond. Hence, option D is incorrect.

9. Which bond has the greatest ionic character?

A)    H---N

B)    H---F

C)    H---Cl

D)    H---O

Correct Answer: Option B - H---F

Electronegativity, denoted by symbol χ, is a chemical property that describes the ability of an atom (or, more rarely, a functional group) to attract electrons (or electron density) towards itself in a covalent bond. An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it. The electronegativity difference between two covalently bonded atoms is used to assess the degree of polarity.

The most commonly used method of calculation of electronegativity is that originally proposed by Pauling. This gives a dimensionless quantity, commonly referred to as the Pauling scale, on a relative scale running from 0.7 to 4.0 (hydrogen = 2.2). When other methods of calculation are used, it is conventional to quote the results on a scale that covers the same range of numerical values: this is known as electronegativity in Pauling units. The term means which bond has the greatest polarity and is thus most similar to an ionic bond, which involves the transfer of an electron (in opposition to covalent bonds, which share electrons).

H has an electronegativity of 2.1 and F has an electronegativity of 4.0. Subtract the two values and the difference is 1.9. Ionic bonds have differences of 1.7 or greater.

Hence, options A, C and D are incorrect.

10. A barium atom attains a stable electron configuration when it bonds with:

A)    1 chlorine atom

B)    1 sodium atom

C)    2 chlorine atoms

D)    2 sodium atoms

Correct Answer: Option C - 2 chlorine atoms

1 atom of Barium atom has 2 valence electrons and 1 atom of chlorine has 7 valence electrons. Hence, for Barium to stabilize it gives 2 electrons to 2 atoms of chlorine forming an ionic bond with, 1 barium cation and 2 chlorine anions. Hence, option A is incorrect and option C is correct.

Sodium also has 1 valence electron and the energies of both sodium and barium are insufficient to form a bond. Hence, options B and D are incorrect.