Depending on the tendency of an atom to lose, gain, or share electrons, there are two types of bonds:
lonic bonds are formed between two atoms, when one atom loses electron to form cation and the other atom gains this electron to form anion.
Describe the formation of Na+ and Mg2+ cations.
Problem Solving Strategy:
(a) Formation of Na⁺ ion:
Na (1s² 2s² 2p⁶ 3s¹) - e⁻ → Na⁺ (1s² 2s² 2p⁶)
You can also represent this by the following electron dot structure.
(b) Formation of Mg²⁺ ion:
Mg (1s² 2s² 2p⁶ 3s²) - 2e⁻ → Mg²⁺ (1s² 2s² 2p⁶)
You can also represent this by an electron dot structure.
Describe the formation of anions for the following non-metal atoms:
Problem Solving Strategy:
(a) Formation of anion by oxygen atom:
Oxygen belongs to Group VIA on the periodic table. It has six electrons in its valence shell. It needs two electrons to achieve noble gas configuration.
O (1s² 2s² 2p⁴) + 2e⁻ → O²⁻ (1s²
(b) Formation of anion by fluorine atom:
Fluorine belongs to Group VIIA on the periodic table. It has seven electrons in the valence shell. A fluorine atom therefore, requires only one electron to complete octet.
F (1s² 2s² 2p⁵) + e⁻ → F⁻ (1s²
You can also represent this electron dot structure,
Anions and cations have opposite charges. They attract one another by strong electrostatic forces. An ionic bond is a strong electrostatic attraction between positively charged metal ions and negatively charged non-metal ions. Compounds that consist of ions joined by electrostatic forces are called ionic compounds. The total positive charge of the cations must be equal to the total negative charge of the anions because ionic compounds are electrically neutral as a whole.
For each of the following pairs of atoms, use electron dot and electron cross structures to write the equation for the formation of an ionic compound:
Problem Solving Strategy:
Solution:
(a) Na is a metal and Cl is a non-metal.
A Mg atom has two electrons in the outermost shell. It losses two electrons to form Mg Ion. Since a Fatom has seven electrons in the outermost shell, so it gains one electron to Form Fion.
2Na + Cl₂ → 2NaCl (Sodium Chloride)
(b) Mg is a metal and F is a non-metal.
A Mg atom has two electrons in the outermost shell. It losses two electrons to form Mg Ion. Since a Fatom has seven electrons in the outermost shell, so it gains one electron to Form Fion.
Mg + F₂ → MgF₂ (Magnesium Fluoride)
for every Mg +2 ion you need two Fions
Recognize the following compounds as having ionic bonds.
(a) MgO
Mg is metal and O is non-metal. A Mg atom has two electrons in outermost shell. So it loses two electrons to form Mg ion. Since an O atom has six electrons in outermost shell, so it gains two electrons to form O' ion. In this way both the atoms acquire nearest noble gas configuration. For every Mg ion you need one O' ion. Chemical formula of resulting compound is MgO. Therefore MgO is an ionic compound.
(b) Na is metal and F is non-metal. A Na atom has one electron in outmost shell. So it loses one electron to form Na' ion. Since a F atom has seven electrons in outermost shell, so it gains one electron to form F ion. Na atom by losing one electron and F atom by gaining one electron acquire nearest noble gas electronic configuration. You need one F ion for each Na' ion. Therefore, NaF is an ionic compound.
Recognize the following compounds as having ionic bonds:
Nonmetal atoms tend to share electrons with each other or with other nonmetal atoms, forming a chemical bond called a covalent bond. A chemical bond formed by mutual sharing of electrons between two atoms is called a covalent bond. General representation of a covalent bond is given below.
Consider the formation of a covalent bond between two hydrogen atoms. A hydrogen atom has one valence electron. Two hydrogen atoms share their valence electrons to form a diatomic molecule.
H + H → H:H (shared pair of electrons)
In the formation of this molecule, each hydrogen atom reaches the electronic configuration of the noble gas helium with two valence electrons. An electron pair in the region between two atoms attracts both hydrogen nuclei. This creates a strong electrostatic attraction between the shared electrons and the two nuclei. This means that the situation is more stable than in individual atoms. Because of this stability, the two atoms form a covalent bond.
In a covalent bond, a strong electrostatic forve of attraction between the bonding electrons and two atomic nuclei binds them together.
A covalent bond between two atoms can be represented by using electron-dot and electron-cross symbols for the atoms and the resulting molecule. As already discussed valence electrons are represented by dots. Just to understand sharing, we represent valence electrons in one atom by dots and in the other atom by crosses. However, remember that all the electrons are identical and cannot be differentiated.
A shared pair of electrons is also represented by a dash (-) in a molecule. Consider the formation of a bond between two fluorine atoms. Fluorine belongs to Group VilA, so it has seven electrons in the valence shell. It needs ane more electron to attain the electron configuration of a noble gas. Thus two F-atoms share an electron pair and achieve electron cguration of Ne. For sharing each F-atom contributes one electron to complete the octet.
Pairs of valence electrons that are not shared between atoms are called lone pairs or lone pairs. A covalent bond formed by sharing one pair of electrons is called a single covalent bond. So both H, and F, molecules contain single covalent bond.
Can you explain the formation of covalent bond between H-atom and a F-atom?
Sometimes atoms may share two or three electron pairs to complete an octet. Double covalent bonds are the bonds that are formed by sharing of two electron pairs.
Triple covalent bonds are the bonds that involve three shared pairs of electrons.
Consider the formation of O, molecules. Oxygen is in Group VI A, so it has 6 electrons in the valence shell. It needs two electrons to complete its octet. So for sharing each O-atom contributes two electrons.
Can you explain the formation of N, molecules?
Draw electron cross and dot structures for (a) CH, that is a major component of natural gas (b) H.O that covers about 80% of the earth crust.
(a) CH₄
(1) C has four electrons in the valence shell and needs four electrons to complete its octet. H has only one valence electron and needs one electron to complete the duplet. So C can form four single bonds with four H-atoms. C is the central element.
(ii) Connect the atoms with a dot and a cross
(b) H₂O
(i) O has six valence electrons O and each hydrogen atom has one valence electron. H So O-atom needs two electrons to complete the octet. Each H needs one electron to complete duplet.
(ii) O is central atom and will form two single bonds with H-atoms.
(iii) Arrange H-atoms around O and connect them by a pair of electrons (one dot and one cross)
Draw electron cross and dot structures for the following molecules:
(a) NH,
(b) HCI
(c) CH,OH
Draw electron cross and dot structures for the following molecules:
(a) CO₂
(1) C has four electrons in the valence shell. It needs four electrons to complete octet.
(fi) Each oxygen atom has six valence electrons and needs two electrons to have an octet.
(iii) C is central atom, arrange O-atoms around it.
(iv) Since C needs four electrons and there are only two oxygen atoms. So it will share its two electrons with each oxygen atom.
(b) HCN
(1) H has one, Chas four and N has five electrons.
(ii) C needs four and N needs three electrons. So C shares one electron with H to form a single bond and three electrons with N to form a triple bond. This will satisfy octet rule.
Draw electron cross and electron dot structures for the following molecules:
A covalent bond may form between two similar atoms such as in H₂, N₂, O₂, Cl₂, etc. It can also occur between two different atoms, as in HCl, H₂O, NH₃, HCN, CO, etc. When two identical atoms share electron pairs, both atoms exert the same force on the shared electron pairs. Such a covalent bond is called a non-polar covalent bond. For example, bonds H-H, O=O, etc. are non-polar covalent bonds.
On the other hand, when two different atoms share an electron pair, both atoms exert different forces on the shared electron pair. The more electronegative atom pulls the shared electron pairs towards itself with a greater force than the other atom. Thus, the more electronegative atom attracts some of the electron density towards itself. This makes it partially negatively charged and the other atoms partially positively charged. Such a covalent bond is called a polar covalent bond. The forces of attraction between molecules are called intermolecular forces. For example, H-C
A coordinate covalent bond is a type of covalent bond where the shared electron pair comes from a single atom (called donor). Atoms are held together because both nuclei attract a pair of electrons. Once a covalent bond is formed, it is impossible to distinguish the origin of the electrons. Such bonding is usually observed when metal ions bind to ligands. However, nonmetals can also participate in this bond. The reaction between a Lewis acid and a base is a covalent coordinate bond.
1. Differentiate between polar and non-polar covalent bonds.
2. How is coordinate covalent bond different from normal covalent bond?