SPM Chemistry Form 4 Notes – Chemical Bonds (Part 3)

by BerryBerryTeacher

in Berry Reference (Notes)

Covalent bond is the second type of chemical bonds that Berry Readers learn in SPM Form 4 Chemistry. Covalent bonds occurs when electrons are shared between atoms, rather than a complete transfer of electrons in ionic bonding. Typically, covalent bonds occur for non-metals when they bind together due to similar tendency for electrons (usually to gain electrons in the syllabus). When non-metals gain electrons, they will share electrons in order to fill up their valence shell, with one of the simplest example being the abundant hydrogen gas. Now that you know that covalent bonding is usually with non-metals and it is different from ionic bonding, the Berry Berry Easy notes of Part 3 of Chemical Bonding for SPM Chemistry Form 4 begins.

This Part 3 focuses on covalent bonds (definition), non-metals (and how they can form covalent bonds), covalent compound formula prediction, structure of covalent compounds and some tips on covalent bonding. So take note of the notes from Part 2 (Ionic bonding) and try to find parallels from it. This way you’ll better learn the two tips of bonds.

[Tips: In event that you forget how covalent bond works, think of the humble and simplest hydrogen gas. Why is it a good example, because it is easy to remember. As hydrogen atom (H) each has one valence electron in their first electron shell, they will 'prefer' to have a second electron to fill up the first electron shell. If you remember, the first electron shell has a capacity of 2 electrons. When the first H atom wants a second electron, another H atom also wants the same thing. As such, both hydrogen atoms will come together (or in chemistry, we call it 'react') to form H2, a gas compound. Thus, both atoms now 'enjoy' the stability afforded by full valence shell. So take out your pencil and draw the concept of covalent bonding a few times with the hydrogen atoms.]

SPM Chemistry Form 4 Notes – Chemical Bonds (Part 3)

 

Molecules

Molecules

 

Covalent Bonds

  • It is a chemical bond formed from the sharing of valence electrons between non-metal atoms to achieve the stable duplet of octet electron arrangement.
  • Each shared pair of electrons is as one covalent bond.
  • It produces molecules.
  • Usually the covalent bonds form between non-metal atoms from Group 15, 16 and 17 and sometimes can be formed from Group 14 (carbon and silicon) and hydrogen.
  • Covalent bond can be formed from atoms of the same element and atoms of different elements.

Example:

Non-metal + Non-metal –> Covalent compound
Bromine + bromine –> Bromine (Br2)
Nitrogen + nitrogen –> Nitrogen (N2)
Carbon + chlorine –> Tetrachloromethane (CCl4)
Hydrogen + oxygen –> Water (H2O)
Hydrogen + nitrogen –> Ammonia (NH3)

Types of covalent bond formed:

  • Single bond = one pair of electrons shared between two atoms.
  • Double bond = two pair of electrons shared between two atoms.
  • Triple bond = three pair of electrons shared between two atoms.

Non-metal

Group 15

  • A nitrogen atom with an electron arrangement of 2.5 needs three more electrons to achieve stable octet electron arrangement after it contribute (through sharing) three valence electrons to another atom (can be from Group 14, 15, 16, 17).
  • A phosphorus atom with an electron arrangement of 2.8.5 need three more electrons to achieve stable octet electron arrangement after it contribute (through sharing) three valence electrons to another atom (can be from Group 14, 15, 16, 17).

Group 16

  • An oxygen atom with an electron arrangement of 2.6 needs two more electrons to achieve stable octet electron arrangement after it contribute (through sharing) two valence electrons to another atom (can be from Group 14, 15, 16, 17).
  • A sulphur atom with an electron arrangement of 2.8.6 need two more electrons to achieve stable octet electron arrangement after it contribute (through sharing) two valence electrons to another atom (can be from Group 14, 15, 16, 17).

Group 17

  • A fluorine atom with an electron arrangement of 2.7 needs one more electron to achieve stable octet electron arrangement after it contribute (through sharing) one valence electron to another atom (can be from Group 14, 15, 16, 17).
  • A chlorine atom with an electron arrangement of 2.8.7 need one more electron to achieve stable octet electron arrangement after it contribute (through sharing) one valence electron to another atom (can be from Group 14, 15, 16, 17).

Predict the Formula of a Covalent Compound

  • Non-metal X atom (valence electron is a)
  • Combine with another non-metal Y atom (valence electron is b)
  • b = simplest ratio (n) and a = simplest ratio (m)
  • Formula of a covalent compound formed, XnYm

Example:

The electron arrangement of atom X is 2.8.6 and atom Y has four valence electrons. Which of the following is the formula of the compound formed between X and Y?

(A) Y4X
(B) Y2X
(C) YX
(D) YX2

Solution:

  • X has 6 valence electrons, it needs to share 2 electrons to achieve the stable octet electron arrangement.
  • Y has 4 valence electrons, it needs to share 4 electrons to achieve the stable octet electron arrangement.
  • Therefore, the formula of the covalent compound is X4Y2 = Y2X4 = simplest ratio YX2.

Answer: D

Some common covalent compound

  • Hydrogen molecule, H2 (single bond)
  • Chlorine molecule, Cl2 (single bond)
  • Bromine molecule, Br2 (single bond)
  • Fluorine molecule, F2 (single bond)
  • Water molecule, H2O (single bond)
  • Nitrogen trifluoride molecule, NF3 (single bond)
  • Tetrachoromethane / carbon tetrachloride, CCl4 (single bond)
  • Ammonia molecule, NH3 (single bond)
  • Oxygen molecule, O2 (double bond)
  • Carbon dioxide molecule, CO2 (double bond)
  • Nitrogen molecule, N2 (triple bond)
  • Ethyne molecule, C2H2 (triple bond)

Structure of covalent compounds

  • Can be simple molecular structure or giant molecular structure.
  • The atoms in the molecule are joined together by strong covalent bond but intermolecular forces are weak by weak van der Waals’ forces.

Berry Important Notes:

In the diagram of ionic compound, always shows

  • The outermost shells of all atoms must achieve a stable duplet or octet electron arrangement through sharing.
  • The outermost shells of each atom must overlap.
  • Label all atoms clearly.

The next part, Part 4 in Berry Berry Easy‘s notes on Chemical Bonding for SPM Form 4 Chemistry will be on the differences between ionic compound and covalent compound (in terms of particle, electrons, forces, state, melting point, volatility, solubility in water, solubility in organic solvent, electricity conductor) and also the uses of covalent compound as solvent. So stay tuned.

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