IAA ACADEMY – Lesson 6: Covalent, Coordinate Covalent, and Metallic Bonds

3.2.2 Covalent Bond (کوویلنٹ بانڈ – *Covalent Bond*)

In our last lesson, we learned about the ionic bond, where atoms achieve stability by completely *transferring* electrons. But what if atoms don’t want to lose or gain electrons entirely? What if they just want to share? That’s where the Covalent Bond comes in!

Energy Minimization through Electron Sharing (الیکٹران کی شراکت سے توانائی کم کرنا – *electron ki sharākat se tawanāi kam karnā*)

Just like in ionic bonding, atoms still want to lower their energy and become more stable. However, some atoms can do this by mutually sharing their electrons (باہمی اشتراک سے الیکٹران بانٹنا – *bāhamī ishtirāk se electron bāntnā* – sharing electrons through mutual cooperation).

Think of it like two friends who both need a specific toy. Instead of one friend giving the toy to the other, they decide to share it and play with it together. Both friends are happy and stable!

Forces Acting During Bond Formation (بانڈ بننے کے دوران کام کرنے والی قوتیں – *bond ban’ne ke daurān kām karnay walī quwwatain*)

When two atoms approach each other to form a bond, the same attractive and repulsive forces we discussed earlier are at play:

• The electrons belonging to one atom are attracted (کشش میں آنا – *kashish mein ānā* – to come under attraction) by the nucleus of the other atom. This new force of attraction helps to lower the energy (توانائی کم کرنا – *tawanāi kam karnā*).
• At the same time, the electrons of one atom repel (دفع کرنا – *dafa karnā* – to push away) the electrons of the other atom. Similarly, the nuclei of both atoms (being positive) also repel each other. These repulsive forces tend to increase the energy (توانائی بڑھانا – *tawanāi baṛhānā*).

The two atoms will adjust themselves at a specific distance where the attractive forces dominate (کشش کی قوتیں غالب آ جائیں – *kashish kī quwwatain ghālib ā jāen* – attractive forces become dominant) the repulsive forces. At this ideal distance, the total energy becomes minimum (کل توانائی کم سے کم ہو جاتی ہے – *kul tawanāi kam se kam ho jātī hai*), and a stable molecule is formed. This balance is crucial for covalent bond formation.

Definition of Covalent Bond (کوویلنٹ بانڈ کی تعریف – *Covalent Bond kī ta’rīf*)

A covalent bond is therefore a bond formed by the mutual sharing of an electron pair (الیکٹران کے جوڑے کا باہمی اشتراک – *electron ke joṛe kā bāhamī ishtirāk*) provided by the bonded atoms. When one pair of electrons is shared, it’s called a single covalent bond.

Types of Covalent Bonds (کوویلنٹ بانڈز کی اقسام – *Covalent Bonds kī aqsām*)

Atoms can share more than one pair of electrons, leading to different types of covalent bonds:

• In some compounds, atoms share two electrons each (یعنی کل چار الیکٹران) to form a double covalent bond. This involves two shared electron pairs.
• In others, atoms share three electrons each (یعنی کل چھ الیکٹران) to form a triple covalent bond. This involves three shared electron pairs.

Representation of Bonds (بانڈز کی نمائندگی – *Bonds kī numāindagī*)

In chemical structures, we use lines to represent covalent bonds:

• Single covalent bond: one line (—)
• Double covalent bond: two lines (=)
• Triple covalent bond: three lines (≡)

The mutually shared electrons themselves can also be shown by a dot (•) or a cross (x) in what are called “Lewis dot structures.”

(Imagine Fig 3.5 here, showing examples like H-H for single, O=O for double, and N≡N for triple bonds, with dots/crosses representing shared electrons.)

Formation of Covalent Compounds (کوویلنٹ کمپاؤنڈز کی تشکیل – *Covalent Compounds kī tashkīl*)

Let’s look at how some common covalent compounds are formed:

• Water Molecule (H₂O)

  A water molecule is formed when two hydrogen atoms share their electrons separately with the electrons of one oxygen atom. Oxygen needs two electrons to complete its octet, and each hydrogen needs one to complete its duplet. So, oxygen shares one electron with each hydrogen, and each hydrogen shares one electron with oxygen. This forms two single covalent bonds.

  H — O — H  (Each line represents a shared pair of electrons)
                  

• Carbon Dioxide (CO₂)

  A carbon dioxide molecule is formed when an atom of carbon shares its four electrons with two oxygen atoms. Each oxygen atom also shares two electrons with carbon. This results in two double covalent bonds. Carbon completes its octet, and each oxygen also completes its octet.

  O = C = O  (Each double line represents two shared pairs of electrons)
                  

Exercise: Covalent Bond

Let’s check your understanding of covalent bonds:

Exercise: Draw Electron Dot and Cross Structures

For the following compounds, describe how you would draw their electron dot and cross structures, indicating shared and unshared pairs. (Note: Since we can’t draw directly here, describe the process for each.)

    H
    |
H — Si — H
    |
    H
                    

    Cl
    |
Cl — P — Cl
    (lone pair on P)
                    

O = S — O  (with lone pairs on S and O atoms)
                    

    O
    ||
O = S = O
                    

3.2.3 Coordinate Covalent Bond (کوآرڈینیٹ کوویلنٹ بانڈ – *Coordinate Covalent Bond*)

Now, let’s look at a special type of covalent bond called a Coordinate Covalent Bond (also known as a Dative Bond). It’s still about sharing electrons, but with a twist!

Definition and Formation (تعریف اور تشکیل – *ta’rīf aur tashkīl*)

• A coordinate covalent bond is a type of covalent bond in which the shared electron pair is donated by one atom only (مشترکہ الیکٹران کا جوڑا صرف ایک ایٹم کی طرف سے عطیہ کیا جاتا ہے – *mushtarakah electron kā joṛā sirf ek atom kī taraf se atiyah kiyā jātā hai*).
• This bond is formed when one molecule (or atom) has an unshared electron pair (غیر مشترکہ الیکٹران کا جوڑا – *ghair mushtarakah electron kā joṛā* – also called a lone pair) to donate to another molecule (or atom) that has an empty orbital (جگہ – *jagah* – space) to accept it.
• The molecule donating the pair is called a donor (عطیہ کرنے والا – *atiyah karnay wālā*).
• The molecule accepting it is called an acceptor (قبول کرنے والا – *qabūl karnay wālā*).
• This type of bond is represented by an arrow head (→) pointing toward the acceptor.

(کوآرڈینیٹ کوویلنٹ بانڈ میں، الیکٹران کا جوڑا جو شیئر کیا جاتا ہے، وہ صرف ایک ایٹم کی طرف سے دیا جاتا ہے۔ جو ایٹم الیکٹران دیتا ہے اسے ‘ڈونر’ اور جو قبول کرتا ہے اسے ‘ایکسیپٹر’ کہتے ہیں۔ اسے تیر کے نشان سے دکھایا جاتا ہے۔)

Example 1: Hydronium Ion (H₃O⁺) (ہائیڈرونیم آئن – *Hydronium Ion*)

Acids often provide protons (H⁺) (پروٹون – *proton*) when dissolved in water. A proton is essentially a hydrogen atom that has lost its only electron, so it has an empty outer shell (خالی بیرونی شیل – *khāli beroonī shell*).

A water molecule (H₂O) has two lone pairs of electrons on its oxygen atom. This oxygen atom can donate one of its lone pairs (اپنے ایک غیر مشترکہ جوڑے کو عطیہ کرنا – *apne ek ghair mushtarakah joṛe ko atiyah karnā*) to the empty shell of the proton. As a result, a hydronium ion (H₃O⁺) is formed.

(Imagine Fig 3.7 here, showing H₂O with lone pairs on Oxygen, and H⁺ approaching. An arrow goes from O’s lone pair to H⁺, forming H₃O⁺.)

After the formation of the hydronium ion, the positive charge covers the whole ion. Interestingly, once the bond is formed, there doesn’t remain any practical difference between this coordinate covalent bond and the other regular covalent bonds in the hydronium ion. All three bonds of oxygen behave exactly alike.

Example 2: Reaction Between NH₃ and BF₃ (NH₃ اور BF₃ کے درمیان رد عمل – *NH₃ aur BF₃ ke darmiyān rad-e-amal*)

A reaction between ammonia (NH₃) and boron trifluoride (BF₃) is another classic example of a coordinate covalent bond.

• Ammonia (NH₃) has a lone pair of electrons on its nitrogen atom.
• Boron trifluoride (BF₃) has an incomplete octet on its boron atom, meaning it has a partially empty outer shell that can accept electrons.
• An electron pair from the nitrogen of ammonia fills the partially empty outer shell of boron in boron trifluoride, forming a coordinate covalent bond.

(Imagine Fig 3.8 here, showing the lone pair on N in NH₃ pointing an arrow towards the empty space on B in BF₃.)

Additional Example: Ammonium Chloride and Protonated Ethyl Alcohol

In ammonium chloride (NH₄Cl), a coordinate covalent bond links the nitrogen of ammonia (NH₃) and a proton (H⁺). The positive charge is then spread all over the ammonium ion (NH₄⁺). All four bonds between nitrogen and hydrogen in the ammonium ion behave exactly alike.

This proves an important point: the difference between a covalent bond and a coordinate covalent bond lies mainly in the way they are formed (ان کی تشکیل کا طریقہ – *un kī tashkīl kā tarīqah*). Once these bonds are formed, there often doesn’t remain any practical difference in their behavior.

3.3 Metallic Bond and Electropositive Nature of Metals (دھاتی بانڈ اور دھاتوں کی الیکٹرو پوزیٹو نوعیت – *Dhātī Bond aur Dhāton kī Electropositive Nau’iyat*)

We’ve talked about ionic bonds (electron transfer) and covalent bonds (electron sharing). Now, let’s explore a very unique type of bond found in metals: the Metallic Bond.

What Type of Atoms Form Metallic Bonds?

Metallic bonds are formed between metal atoms. These atoms have a strong tendency to lose electrons easily (الیکٹران آسانی سے کھونے کا رجحان – *electron āsānī se khonay kā rujhān*), forming positive ions (cations).

But here’s the special part: In a metallic bond, electrons are not shared or transferred between specific atoms. Instead, the valence electrons from all the metal atoms become delocalized (ایک جگہ پر نہ رہنا – *ek jagah par na rehnā* – not staying in one place) and form a “sea of free-moving electrons” (آزاد حرکت پذیر الیکٹران کا سمندر – *āzād harkat pazīr electron kā samundar*) that surrounds the positively charged metal ions (مثبت چارج والے دھاتی آئن – *musbat charge walay dhātī ion*).

Imagine a crowd of people (positive metal ions) and a bunch of balloons floating freely above their heads (the electrons). Everyone can grab any balloon! This “sea” of electrons acts like a glue, holding the positive metal ions together. This unique structure gives metals their special properties like:

• Conductivity (برق اور حرارت کی بہترین ترسیل – *barq aur harārat kī behtareen tarseel* – excellent conduction of electricity and heat)
• Malleability (قابلِ پٹائی – *qābil-e-piṭāī* – ability to be hammered into sheets)
• Ductility (قابلِ تار سازی – *qābil-e-tār sāzī* – ability to be drawn into wires)

Comparison of Metallic Bond and Ionic Bond (دھاتی بانڈ اور آئنک بانڈ کا موازنہ – *Dhātī Bond aur Ionic Bond kā muwāzanah*)

Let’s compare metallic bonds and ionic bonds to highlight their differences:

Interesting Information! (دلچسپ معلومات – *dilchasp ma’lūmāt*)

Metals are incredibly useful and are extensively used in many industries such as:

• Machinery (مشینری)
• Automobiles (گاڑیاں)
• Railways (ریلوے)
• Aircrafts (ہوائی جہاز)
• Rockets (راکٹ)
• Construction (تعمیرات)
• Electronics (الیکٹرانکس)
• Jewelry (زیورات)
• Electric wires (بجلی کی تاریں)

All thanks to their unique metallic bonding!

Final Exercise: Chemical Bonding Review

Let’s recap what we’ve learned about different types of chemical bonds. Click to reveal the answers!