Sindh Class 9 Notes Chemistry Chapter 5 Chemical Bonding
Sindh and Karachi board class 9 chemistry notes chapter 5 chemical bonding short, long question, multiple question, and MCQs.
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Chemical Bonding Chemistry Notes Chapter 5
Sindh Class 9 Notes Chemistry Chapter 5 Chemical Bonding. Contains solved review questions, MCQs, and important questions.
Q.1) Define chemical bond? Discuss how atoms unite and change into molecules?
Answer: Chemical bond: A chemical bond is a force of attraction that holds two or more atoms together. Atoms combine to form various types of substances. The forces of attraction that lead to chemical bonding between atoms are electrical in nature. Atoms unite and form molecules that tend to gain a table electronic configuration of a noble gas. When bonding occurs, atoms acquire eight electrons configuration in their valence shell. In the formation of chemical compounds, the valence electrons of one atom are transferred or shared to the valence electrons of the combining atom. A stable molecule is formed when the total energy of the combined atoms is less than that of the separate atoms. For example, there is a combination of two atoms of hydrogen and one atom of oxygen to form a water molecule. When two atoms combine they release energy and form a water molecule. The energy of a water molecule is less than the constituent atoms. Both hydrogen and oxygen get stable electronic configurations by sharing the electrons.
Q.2) What are the valence electrons of an atom? How many valence electron does a nitrogen atom possess?
Answer: The electrons found in the outermost shell of an atom are called valence electrons. Nitrogen has five valence electrons. 2s2, 2p3. Electronic configuration = 1s2, 2s2, 2p3 Valence electrons = 2 + 3 = 5
Q.3) What happens to electrons, when elements combine?
Answer: When elements combine the valence electrons are either transferred from one atom to the other or shared between two atoms. When atoms chemically combine they usually attain the stable eight or two electronic configurations of their nearest stable gas. Only hydrogen and helium attain the duplet configuration during a chemical reaction. All other elements attain the octet configuration. Noble gases have completely filled their outer most electrons. Thus elements to attain stability like noble gases react and combine through their valence electrons.
Q.4) What part of the atom is involved in the formation of chemical bond?
Answer: The electrons present in the outer most shell of atoms are involved in the formation of the chemical bond. They are also called valence electrons. They are either transferred from one atom to another atom or shared between two atoms.
Q.5) Explain with examples? How elements are united by electro-valent bond?
Answer: Electro-valent Bond: Electro-valent or ionic bonding is the complete transfer of valence electrons between atoms. It is a type of chemical bond that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion. Ionic bonds require an electron donor, often a metal, and an electron acceptor, a nonmetal. Formation of Na+ ion: For example, the sodium atom donates the electron from its valence orbital in order to achieve octet configuration. This creates a positively charged cation due to the loss of electron. Na 1s2 2s2 2p63s1 → 1s2 2s2 2p6 + 1e–
Formation of Cl- ion: The chlorine atom receives one electron to achieve its octet configuration, which creates a negatively charged anion. Cl 1s2 2s2 2p6 3s2 3p5 + 1e– → 1s2 2s2 2p6 3s2 3p6
What common properties are shown by ionic compounds?
Properties of ionic compounds: i) In ionic bond, it is impossible to say that any two ions are bonded to each other to produce molecule, but in the crystals of ionic compounds, the oppositively charged ions are `mutually surrounded by each other in orderly. arrangement. Thus ionic compounds are solids at room temperature. ii) Ionic compounds have high melting and boiling points because of the strong electrovalent bonds existing between the ions. iii) Ionic solids, do not conduct electricity as the ions are not free to move. Once an ionic compound is melted (fused) the ions are free to move and conducts electricity. Similarly, solutions of ionic compounds conduct electricity. iv) Ionic compounds are usually soluble in polar solvents, i.e. solvents of high di-electric constant such as water. But ionic compounds are insoluble in non polar (organic) solvents. These solvents have low di-electric constant such as benzene, carbon tetrachloride, etc. They are mostly inorganic compounds.
What is meant by covalent bond? Write electronic formulas of any two covalent molecules? What is single, double and triple covalent bond?
Answer: Covalent bond: Covalent bond is formed by mutual sharing of electrons between two atoms. Atoms will covalently bond with other atoms in order to gain more stability. By sharing their valence electrons, atoms can fill up their outer electron shell and gain stability. For example, consider the formation of covalent bond in hydrogen molecule. A hydrogen atom has a single valence electron. Two hydrogen atoms share their valence electrons to form a hydrogen molecule. In this bond formation each hydrogen atom achieves the stable electronic configuration and electron pair is in the region between two attached atoms.
Another example is the formation of the HCl molecule.
i) Single covalent bond When one pair of electrons is shared between the two bonding atoms, it is called a single covalent bond. It is indicated by a single horizontal line between the two atoms. Example:
ii) Double covalent bond When two bonding atoms share two pairs of electrons with each other, it is called a double covalent bond. This bond is indicated by a double line between the two atoms in the structure of such molecules. Example:
iii) Triple covalent bond When two bonding atoms share three pairs of electrons with each other, it is called a triple covalent bond. This bond is indicated by three lines between those atoms in the molecules of such compounds. Example:
Q.9) Classify the following bonds as ionic or covalent. For those bonds that are covalent indicate whether they are polar or non-polar.
a) H2 b) H-Cl c) NaCl d) CaCO3 e) HC≡CH f) O=O
Answer: a) H2 Covalent bond (non-polar) b) HCl Covalent bond (polar) c) NaCl Ionic bond d) CaCO3 Ionic bond e) HC≡CH Covalent bond (polar) f) O=O Covalent bond (non-polar)
Q.11) Account for the fact that some covalent bonds are polar while others are non-polar.
Answer: There are two types of covalent bonds that are polar and non-polar. i) Non-polar covalent bond: A covalent bond formed between two similar atoms is called non-polar covalent bond. The electrons are shared equally between the two atoms in such molecules. For example, consider the formation of covalent bond in hydrogen molecule. A hydrogen atom has a single valence electron. Two hydrogen atoms share their valence electrons to form a hydrogen molecule. In this bond formation, each hydrogen atom achieves the stable electronic configuration and electron pair is in the region between two attached atoms.
ii) Polar Covalent bond: The covalent bond formed between two unlike atoms which differ in their electronegativity is said to be a polar bond. The bonding pair of electrons does not lie exactly midway between the two atoms. It lies more towards the atom which has more electronegativity. The atom with higher electronegativity gets slight negative charge. For example, consider the formation of covalent bond in HCl. A hydrogen atom has single valence electron. Chlorine needs only one electron to fill the outer most shell. Thus two atoms hydrogen and chlorine share their valence electrons to form HCl molecule. In this bond formation, both hydrogen atom and chlorine atom achieve the stable electronic configuration and electron pair is located near the chlorine atom because it is more electronegative.
Q.12) What is co-ordinate covalent bond? Explain with examples?
Answer: Co-ordinate Covalent Bond A coordinate covalent bond or dative covalent bond is formed when shared pair of electrons comes from only one atom from the two atoms involved in the bonding. The atom which donates the electron pair is called the donor atom, while the atom which accepts the electron pair is called the acceptor atom. A coordinate covalent bond is represented by an arrow. This arrow indicates the atom and pair of electrons being donated. It starts from the donor atom and ends in the acceptor atom. Examples: i) Formation of ammonium ion Ammonium ion is formed by the combination of ammonia molecule (NH3) and a hydrogen ion (H+). Nitrogen has a lone pair of electrons. It donates this lone pair to the hydrogen ion and ammonium ion (NH4+) is formed. Thus the nitrogen atom acts as a donor and hydrogen ion acts as an acceptor.
ii) Formation of hydronium ion Hydronium ion is formed by the combination of water molecule (H2O) and a hydrogen ion (H+). Oxygen atom has two lone pairs of electrons. It donates one pair to the hydrogen ion and hydronium ion (H3O+) is formed. Thus oxygen atom acts a donor and hydrogen ion acts as an acceptor.
Define the term covalent bond? How does a covalent bond differ from a co-ordinate covalent bond?
Covalent bond: When two or more atoms of the same element or atoms of different elements having similar electronegativities react, the transfer of electrons does not occur. In these instances, the atoms achieve inert gas (noble gas) structure by sharing electrons. Thus, the atoms complete their outermost shell by means of sharing of unpaired electrons, and a covalent bond is formed. In a covalent bond, each atom has to contribute an equal number of unpaired electrons. The shared pair of electrons which links the atoms in a molecule is known as a covalent bond. A coordinate bond is a bond where both electrons come from the same atom. While in a covalent bond, both the atoms share a pair of electrons.
Answer: Electronegativity: If the covalent bond is formed between two like atoms, that molecule is called non-polar because the electron pair is shared equally between the two atoms, as in case of (H-H), (Cl-Cl), (O=O) and (N≡N) molecules. However, if the covalent bond is formed between the two dissimilar atoms as in hydrogen chloride (H-Cl) molecule, the attraction for electron pair, would not be equal, one atom will attract more than the other. Hence the electron pair will be displaced from the central position and reaches near to the chlorine atom. This power of an atom to attract the shared pair of electrons towards, itself, is known as electronegativity. Consequently, the chlorine atom being more electronegative tends to be partially negative and hydrogen atom would be partially positive.
Q.15) Explain in your own words Pauling (E.N.) table? Explain its usefulness in predicting the relative ionic and covalent character of a given compound?
Answer: Electronegativity is the relative tendency of an atom in a molecule to attract shared pair of electrons to itself. For example, in case of HCl, Cl is more electronegative than hydrogen then it will attract shared pair electrons more to itself. It has no unit therefore it is denoted by a number. Linus Pauling, an American chemist, calculated the numerical values of electronegativities. He has given a scale by taking fluorine as standard with its electronegativity as four. It is very useful. It helps to determine the relative ionic and covalent character of a given compound. If the electronegativity difference of bonded atoms is zero the bond is pure covalent and non polar. If the electronegativity difference between the bonded atoms is up to 1.7 then the bond is polar covalent. If the electronegativity difference is more than 1.7 then the bond is ionic in nature.
Q.16) Give the characteristics of covalent compounds?
Answer: Characteristics of covalent compounds: i) Covalent compounds usually consist of discrete molecules. The force of attraction between adjacent covalent molecules is weak. Due to these weak forces, most of the covalent compounds exist as gases or liquids. ii) They have low melting and boiling points as compared to ionic bonding. iii) Covalent solids exist in crystal structures. iv) Giant molecules of covalent solids do not show electrical conductivity. They are insulators. v) Majority of covalent solids are insoluble in polar solvents like H2O, but readily soluble in non-polar solvents like benzene (C6H6).
Q.17) What do you understand by ionic character of covalent bond? Under what conditions are the following formed
a) Polar covalent bond. b) Non-polar covalent bond. c) Ionic bond.
Answer: If covalent bond is formed between two like atoms e.g. in case (H-H), the hydrogen atoms are identical, hence the shared pair of electrons is not disturbed from the centre. This molecule is called non-polar because it is electrically neutral as well as symmetrical. If covalent bond is formed between dissimilar atoms e.g. in case of H-Cl molecule, the shared pair of electrons move closer to one of the both atoms. In H-Cl molecule chlorine is more electronegative, hence the shared pair of electrons, is drawn nearer to chlorine than hydrogen atom. This results in partial positive charge on H atom and partial negative charge on Cl atom. The covalent bond between H and Cl in H-Cl is partially ionic or polar covalent bond, because of positive and negative charges or poles. Covalent bonds are partially ionic, if they exist between two dissimilar atoms and their ionic character depends upon the difference in electronegativities of bonded atom. i) Polar covalent bond If the difference in the electronegativities of bonded atoms is upto 1.7 that bond is called polar covalent bond. The elements of different electronegativities always form polar covalent bond.
ii) Non-Polar Covalent Bond According to the scale of Linus Pauling, if the difference in the electronegativities of bonded atoms is zero, then the bond is pure covalent bond or non-polar bond. The molecule containing like atoms or the atoms of same electronegativities form pure covalent bonds or non-polar bonds.
iii) Ionic Bond: If the difference in the electronegativities of bonded atoms is more than 1.7 then that bond is purely ionic.
Define the term metal? And describe metallic bond?
Metal: A substance that consists of positively charged ions, fixed in a crystal lattice with negatively charged electrons moving freely in the crystal is called a metal. Metals are good conductors of heat and electricity. They are lustrous, malleable, and ductile. Metallic Bond The metallic bond is a sharing of mobile or free electrons between the atoms of a metal element. Metals have large-sized atoms. The electrostatic force of attraction between the nucleus and the valence electrons is very weak. And because of low ionization energies, they tend to lose their valence electrons easily and form positively charged ions. The valence electrons are thus not attached to any particular atom. The loose or free electrons of all the metals are free to move in spaces between the atoms of metal. All of the atoms in a metal share a big pool of valence electrons known as a sea of electrons. These electrons hold the nuclei of the metals together forming a metallic bond. A schematic diagram of metallic bond showing its positive nuclei embedded in a sea of free electrons is as below:
Explain the following properties of metals?
a) Lustre. b) Conductivity. c) Malleability. d) Ductility.
a) Lustre: It is an important property of metals. Free electrons in the crystal lattice of metals move from one atom to another. These free-moving electrons readily absorb light falling upon them and move to higher energy levels. When they come down to the ground state they emit radiations. This causes the lustre in metals. b) Conductivity: As free electrons in the crystal lattice of metals move from one atom to another, therefore, they are good conductors of heat and electricity. When metals are heated the mobile electrons absorb heat and transfer to neighboring electrons. This means that metals are good conductor of heat. c) Malleability: It means metals can be hammered and converted into sheets. This is due to an external force which is when applied the atoms slip down over one another. d) Ductility: Metals can also be converted into thin wires. This is due to when an external force has applied the atoms of metal slip down over one another. That is why metals are ductile in nature.
Why are some metals, such as sodium is soft, while others are hard?
Answer: The softness and hardness of metals can be compared on the basis of the strength of metallic bonding in metals. Greater the strength of the metallic bond, the harder the metal is. The strength of a metallic bond depends upon the number of valence electrons and the size of the metal atom. All valence electrons of a given metal combine to form a sea of electrons. Atoms having a greater number of valence electrons for delocalization result in stronger metallic bonds because the sea has greater electron density. In smaller-sized atoms, the delocalized electrons are closer to the nuclei.
Both these factors increase the strength of the metallic bond and therefore, making the metal hard. Similarly, the metal having fewer valence electrons and bigger atomic size will have weak metallic bonding making them soft. In sodium metal, each sodium atom has only one free electron. The metallic bond in sodium is weak that is why it is soft. Other metals are hard because the number of positive ion and free electrons is more and they possess strong metallic bonding.
Explain the following properties of metals?
a) Lustre. b) Conductivity. c) Malleability. d) Ductility.
Answer: a) Lustre: It is an important property of metals. Free electrons in the crystal lattice of metals move from one atom to another. These free-moving electrons readily absorb light falling upon them and move to higher energy levels. When they come down to the ground state they emit radiations. This causes the lustre in metals. b) Conductivity: As free electrons in the crystal lattice of metals move from one atom to another, therefore, they are good conductor of heat and electricity. When metals are heated the mobile electrons absorb heat and transfer to neighboring electrons. This means that metals are good conductor of heat. c) Malleability: It means metals can be hammered and converted into sheets. This is due to an external force which is when applied the atoms slip down over one another. d) Ductility: Metals can also be converted into thin wires. This is due to when external force is applied the atoms of metal slip down over one another. That is why metals are ductile in nature.
Why are some metals, such as sodium is soft, while other are hard?
Answer: Softness and hardness of metals can be compared on the basis of strength of metallic bonding in metals. Greater the strength of metallic bond, harder the metal is. The strength of metallic bond depends upon the number of valence electrons and size of the metal atom. All valence electrons of a given metal combine to form a sea of electrons. Atoms having greater number of valence electrons for delocalization result in stronger metallic bonds because the sea has greater electron density. In smaller sized atoms, the delocalized electrons are closer to the nuclei. Both these factors increase the strength of the metallic bond and therefore, making the metal hard. Similarly, metal having less valence electrons and bigger atomic size will have weak metallic bonding making them soft. In sodium metal, each sodium atom has only one free electron. The metallic bond in sodium is weak that is why it is soft. Other metals are hard because the number of positive ion and free electrons are more and they possess strong metallic bonding.
Explain the origin of dipole-dipole forces between the molecules? Give an example?
A dipole-dipole force is an attractive intermolecular force resulting from the interaction of the positive end of one molecule with the negative end of other. For example, hydrochloric acid (HCl) molecule, the electronegativity of chlorine is more than hydrogen, a partial negative charge on chlorine and partial positive charge on hydrogen is developed. It has dipole moment due to these charges. Due to dipole- dipole molecule possess the strong force of attraction between the positive and negative charges. Thus HCl boils at (-85°C) as compared to (F2) non polar molecule (-188°C), although both have same molecular weight.
What do you mean by dispersion forces? Why they are also called London forces?
Dispersion Forces: The dispersion forces are the weak attractive forces between temporarily polarized atoms caused by the varying positions of the electrons during their motion about the nuclei. For example, let us consider the dispersion forces in Neon. Ten electrons are distributed around the nucleus of neon spherically symmetrical. When two Neon atoms are come close together the symmetry of the separate neon atoms become distorted because electronic clouds of both atoms will repel each other. The attraction is strong when they are close but rapidly weaken as they move apart. This polarized each molecule and give rise to an induced or temporary dipoles. These dipoles are called weak attractive forces or Lodon dispersion forces. These forces are called London dispersion forces after the identification of them by Fritz London in 1930.
What is hydrogen bonding? What type of forces, either intra-molecular or inter-molecular forces are present in hydrogen bonding?
Hydrogen Bonding: The interaction of a highly electron deficient hydrogen and lone pair on a nearby highly electronegative atom such as N, O or F is called hydrogen bond. For example, consider the molecules in which hydrogen atom is bonded to a highly electronegative element such as fluorine, oxygen or nitrogen. In these molecules, the shared pair of electrons lie more towards the electronegative element. Water is a typical polar compound which exhibits hydrogen bonding. The covalent bond between hydrogen atom and oxygen atom becomes polar enough to create partial positive charge on hydrogen atom and partial negative charge on oxygen atom.
The small size and high partial positive charge on the hydrogen atom enables it to attract the highly electronegative atom oxygen of the other molecule. Hydrogen bonding also affects the physical properties of the molecules. The hydrogen bond is week and lost only in a tiny fraction of seconds but it takes lot of energy to overcome combined attraction of many hydrogen bonds. For example, water (100°C) is higher than that of alcohol (78°C) because of more and stronger hydrogen bonding in water.
Hydrogen bond is an intermolecular force, which exist between the positive pole of hydrogen of one molecule and negative pole of (F,O, or N) of another molecule. The intramolecular bonding is the covalent bonding which exists between the highly electronegative atom (O, N, or F) and partial positive hydrogen atom in a molecule.