Chapter 3: Chemical BondingShort Questions with Answers

Metals generally have a tendency to lose electrons to form positive ions called cations; this is their electropositive character. Usually metals have low values of ionisation energy, so their atoms lose their outer electrons easily. Alkali metals (Na, K) are highly electropositive; alkaline earth metals (Mg, Ca) lose their outer electrons less easily.

Non-metals have an affinity towards electrons. They tend to gain electrons and become negatively charged ions called anions; they are electronegative elements. Fluorine is the most electronegative element, followed by oxygen.

Covalent compounds mostly exist as discrete neutral molecules. The forces between these molecules (intermolecular forces) are very weak compared with the bonding forces inside them. Lower molecular mass covalent compounds are gases or low boiling liquids because little energy is needed to separate their molecules. Examples: CH₄, NH₃, H₂S, HCl, CO₂ and SO₂ are gases at room temperature.

Diamond (carbon). Carbon, phosphorus and sulphur exist as covalent solids, in both amorphous and crystalline forms. In diamond, each carbon atom is surrounded by four other carbon atoms linked together by strong covalent bonds. Graphite and rhombic sulphur are other examples.

In metals the atoms are strongly held and arranged in the form of rows one above the other. When pressure is applied on the metals, the upper rows of atoms slip past the lower rows. As a result, their shapes are changed. Metals can therefore be easily drawn into wires and hammered into sheets. The mobile electrons keep holding the rows together, so the metal bends instead of breaking.

Yes. A coordinate covalent bond is a type of covalent bond, and once formed it is as strong as an ordinary covalent bond. The difference between a covalent bond and a coordinate covalent bond lies in the way they are formed. Once such bonds are formed, there does not remain any difference. All four N–H bonds in the ammonium ion behave exactly alike.

H shares one pair with O; that O shares one pair with N; N shares two pairs with a second O (double bond) and gives its lone pair to the third O (coordinate bond). Every O and N then has an octet, and H has a duplet.

Ionic bonds are formed between elements that lose electrons easily (metals) and elements that gain electrons easily (non-metals). Alkali and alkaline earth metals are electropositive metals which form bonds with electronegative elements like oxygen and chlorine. Examples: NaCl, KCl, MgF₂, CaCl₂.

Non-metals, which combine with other non-metals to form a wide variety of molecular substances: H, C, N, O, S, P and the halogens. They lower their energy by mutually sharing their electrons. Examples: H₂, Cl₂, H₂O, CH₄, NH₃.

A coordinate bond is formed when a species has an electron pair to donate to another species.

(a) Neither BF₃ nor AlCl₃ has a lone pair on its central atom; B and Al are both acceptors, so no ordinary coordinate bond forms between them. AlCl₃ forms one with a donor such as NH₃ (H₃N→AlCl₃), or with a Cl lone pair of another AlCl₃ molecule (Al₂Cl₆).

(b) A lone pair on the oxygen of dimethyl ether bonds the proton: (CH₃)₂O + H⁺ → [(CH₃)₂OH]⁺.

A coordinate bond needs a species that has an electron pair to donate and one that can accept it. A compound with no lone pair and no empty space cannot form one, such as methane, CH₄: carbon has four bond pairs and no lone pair, and hydrogen already has its duplet.

Metal atoms, which usually have low values of ionisation energy, so they lose their outer electrons easily (Na, Mg, Al, Cu, Fe). The nuclei of metallic atoms cannot hold their outer electrons firmly, and these electrons become a sea of mobile electrons.

Atoms have a tendency to decrease their energy. They can do this by combining with other atoms. It is a natural phenomenon because it increases the stability of atoms. Atoms form bonds because they would like to lower their energy by completing their duplet or octet.

Helium has two electrons in its outermost shell, while all other noble gases have eight, and these gases are unreactive. Having two electrons (for hydrogen and helium, which have only the first shell) or eight electrons in the outermost shell means stability and hence unreactivity. This principle was named the duplet or octet rule.

Neither their atoms combine with themselves nor with other atoms. The probable reason for this lack of reactivity was their stability, due to the presence of two electrons in helium and eight electrons in the outermost shells of the rest of the gases.

For a sodium atom it is easier to lose one electron and stabilise itself than to gain seven electrons to complete its octet. Sodium therefore adopts the energetically easier path and loses its electron to form a bond.

It is energetically favourable for a hydrogen atom to lose one electron to become a proton (H⁺) or gain one electron to become a hydride ion (H⁻). In the latter case, it completes its duplet. It can also complete its duplet by sharing (H₂, HCl).

The arrangement of electrons around the nucleus of an atom in shells and sub-shells is called electronic configuration. Example: Na 2, 8, 1; Cl 2, 8, 7.

A chemical bond is a force of attraction between atoms which holds them together in the form of a molecule or a compound.

When atoms approach each other there are two possibilities: they may attract or repel each other. If the forces of attraction dominate the forces of repulsion, the energy of the system gets lowered and the two atoms react to form a new molecule. Conversely, the two atoms simply move away from each other.

(1) Ionic bond, (2) covalent bond, (3) coordinate covalent bond, and the metallic bond found in metals.

An ionic bond is a bond which is formed by the complete transference of electron or electrons from one atom to another atom. The oppositely charged ions formed are held together by the electrostatic force of attraction. It is also called an electrovalent bond, and compounds having such a bond are called ionic compounds.

An electron from the outermost shell of the sodium atom is transferred to the outermost shell of the chlorine atom, and in doing so both atoms acquire the electronic configurations of their nearest noble gases. The Na⁺ and Cl⁻ ions are held together by electrostatic attraction.

Calcium, an alkaline earth metal, loses two electrons, one to each of two chlorine atoms, to form calcium chloride:

The electron or electrons which take part in a chemical reaction come only from the outermost shells of the atoms.

The ions of an ionic compound surround each other three-dimensionally to form a crystal lattice. Since ions are spherical and oppositely charged, they surround each other from all sides. In NaCl each Na⁺ is surrounded by six Cl⁻ ions.

Since ions are spherical and oppositely charged, they can surround each other from all the sides; therefore ionic bonds are non-directional.

KCl, MgF₂, NaF, KBr, CaF₂, NaCl and CaCl₂.

A covalent bond is a bond formed by the mutual sharing of an electron pair provided by the bonded atoms. This is called a single covalent bond. It is the strong electrostatic attraction between the shared electrons and the two nuclei.

In some compounds the atoms share two electrons each to form a double covalent bond, and in the same way atoms can share three electrons each to form a triple covalent bond. Double and triple bonds have two and three electron pairs mutually shared between the two atoms. A single bond is shown by a single line (−), a double bond by two lines (=) and a triple bond by three lines (≡).

When two atoms approach, the electrons of one atom come under the attractive influence of the nucleus of the other atom; this lowers the energy. The electrons and nucleus of one atom also repel those of the other, which increases the energy. The two atoms bring themselves to such a distance that the attractive forces dominate the repulsive forces. The total energy at this distance will be minimum and thus a stable molecule is formed.

The force of attraction of an atom for the shared pair of electrons is called its electronegativity. Fluorine is the most electronegative element in the periodic table, followed by oxygen.

When a covalent bond is formed between two identical atoms, the shared pair of electrons lies at the middle of the two bonded atoms. This gives rise to a pure covalent bond which is essentially non-polar (H₂, Cl₂).

When a covalent bond is formed between two non-identical atoms, the shared pair of electrons bends towards the atom which has more electronegativity. A partial negative charge is created on the more electronegative atom and an equal partial positive charge on the less electronegative atom: a polar covalent bond (HCl, H₂O).

In the HCl molecule the shared pair of electrons bends towards the more electronegative chlorine atom, creating a partial negative charge on it. H gets an equal partial positive charge, so the H–Cl bond is a polar covalent bond: H(δ+)–Cl(δ−).

H₂O has 2 lone pairs on oxygen, NH₃ has 1 lone pair on nitrogen, and CH₄ has none.

Coordinate covalent bond is a type of covalent bond in which the shared electron pair is donated by one atom only. This bond is formed when a species has an electron pair to donate to another species. The species which donates the electron pair is called a donor, while that which accepts it is called an acceptor. An arrowhead (→) pointing towards the acceptor represents this bond.

Acids provide protons (H⁺) when dissolved in water. This proton has an empty outer shell and can accept one of the two pairs of electrons present on the oxygen atom of the water molecule. As a result, a hydroxonium ion (H₃O⁺) is formed. The positive charge covers the whole ion, and all three bonds of oxygen behave exactly alike.

During the reaction, an electron pair from nitrogen of ammonia fills the partially empty outer shell of boron present in boron trifluoride, forming a coordinate covalent bond: H₃N→BF₃.

When a coordinate covalent bond breaks, both the bonded electrons go to the donor species.

In ammonium chloride a coordinate covalent bond links the nitrogen of ammonia and the proton. The positive charge is spread all over the ammonium ion, and all the four bonds between nitrogen and hydrogen behave exactly alike. This proves that the difference lies only in the way they are formed; once formed, there does not remain any difference.

A metallic bond is a type of chemical bond which has positively charged ions bound together by the mobile electrons. A metal appears to have a sea of electrons in which all the nuclei of the atoms are submerged.

The strength of a metallic bond depends upon two factors: the number of positive charges present on the positive ions and the number of mobile electrons set free by each atom.

In sodium metal each sodium atom sets free only one electron, so the metallic bond is not very strong. In magnesium, each atom releases two electrons to acquire two positive charges, so the metallic bond is stronger. This explains why magnesium melts at a higher temperature than sodium (650 °C against 98 °C).

The presence of freely moving electrons in metals makes them good conductors of heat and electricity.

In metals, the atoms are strongly held and arranged in the form of rows one above the other. This arrangement makes them hard and heavy.

Metals are extensively used in machinery, automobiles, railways, aircraft, rockets, the construction industry, the electronics industry, jewellery, electric wires and many more.

Metals generally have a tendency to lose electrons to form positive ions called cations. This property is called the electropositive character of metals. It is related to reactivity: metals which lose electrons easily are more reactive.

Alkali metals (Na, K) are highly electropositive elements and undergo reactions very easily; sodium and potassium react vigorously with water and halogens to give hydroxides and halides, and with acids to give salts and hydrogen. Alkaline earth metals (Mg, Ca) lose their outer electrons less easily, so they are less electropositive and their reactions are less vigorous.

Non-metals have an affinity towards electrons. They tend to gain electrons and become negatively charged ions called anions; they are therefore named electronegative elements. Non-metals react with metals to form ionic bonds, and with other non-metals to form a wide variety of molecular substances.

Fluorine is the most electronegative element in the periodic table, followed by oxygen, then chlorine and nitrogen.

The forces of attraction which are present between the molecules of elements and compounds are named intermolecular forces of attraction. They are generally very weak compared with the bonding forces between atoms; among the three states they are weakest in gases and strongest in solids.

The melting and boiling points of substances depend on the strength of these forces. The stronger the forces among the molecules of a liquid, the higher is its boiling point and vice versa; similarly, the stronger the forces, the higher the melting point of a solid.

These attractive forces are present between the molecules of a polar compound like HCl. Chlorine attracts the shared pair more strongly, so the bond is polar (H(δ+)–Cl(δ−)). Due to these partial charges the molecules of HCl start attracting each other. These forces of attraction are called dipole-dipole forces. Such compounds show relatively higher melting and boiling points.

Hydrogen bonding is a special case of dipole-dipole attractive forces. When hydrogen is covalently bonded to highly electronegative elements like F, O or N, the large difference of electronegativity makes the bond highly polar, and strong dipole-dipole attractions are observed among the molecules. The attractive force between the molecules of water is called hydrogen bonding.

The strength of the hydrogen bonds causes water to have relatively higher melting and boiling points as compared to compounds like H₂S and NH₃. Water boils at 100 °C, NH₃ at −33 °C and H₂S at −60 °C.

In ionic compounds the oppositely charged ions are held together by the strong electrostatic force of attraction in the form of a crystal lattice. Since the ions are rigid, such compounds exist as very stable solids with significantly high melting points. The melting point of NaCl is 801 °C.

If an external force is applied on the crystal lattice, it breaks easily. It shows that ionic solids are highly brittle. The force pushes like charges next to each other and they repel.

In the solid form, ionic compounds do not conduct electricity because the ions are tightly held and cannot move. In the molten state the ions get free and start conducting electricity. In aqueous solution the free ions also move towards their electrodes.

In the process of hydration, water molecules surround and interact with ions or molecules. Water not only breaks the electrostatic attraction in an ionic solid but also hydrates the free ions.

When we mix two solutions of ionic compounds, the positive ions of one compound may react with the negative ions of the other to form a new compound. A white precipitate of silver chloride comes out of the aqueous solution.

Electrolysis of molten sodium chloride gives sodium metal and chlorine gas. Electrolysis of aqueous sodium chloride gives sodium hydroxide and chlorine gas.

Elements on the right side of the periodic table (except inert gases) exist as covalently bonded diatomic molecules. Due to very weak forces of attraction between their molecules, their densities and boiling points are very low. Bromine (Br₂) exists as a volatile fuming liquid.

In diamond, each carbon atom is surrounded by four other carbon atoms linked together by strong covalent bonds. Due to this rigid structure, diamond is the hardest thing on this planet. It is used as a cutting, polishing and drilling tool.

Graphite consists of a layered structure made of hexagonal rings of carbon. Since the layers are not bonded strongly, they can slip past each other, so graphite is used as a lubricant in industry. The layers have mobile electrons between them, so graphite is a good conductor of electricity and is used as an electrode.

Coal is the amorphous form of carbon, whereas diamond and graphite are its crystalline forms. Coal is used as a fuel in electricity generating plants.

Binary covalent compounds generally exist as low boiling gases, except water. Methane (CH₄), ammonia (NH₃), hydrogen sulphide (H₂S), hydrogen chloride (HCl), nitrogen dioxide (NO₂), carbon dioxide (CO₂) and sulphur dioxide are all gases at room temperature, because the forces between their molecules are weak.

Covalent molecules like hydrogen chloride, sulphuric acid and nitric acid ionise completely in water, behaving as very strong acids.