Summary of what I learnt (Chemical Bonding)

Why do atoms bond?

Noble gases, such as helium, neon and argon, are monoatomic because their valence shells are fully occupied by electrons. Thus, noble gases are stable and do not undergo bonding with other atoms. Since atoms with electronic configuration of noble gases are stable, atoms bond to achieve electronic configuration of a noble gas. Atoms do so by transfer or sharing of electrons with other atoms.

By having an electronic configuration of noble gases, atoms will achieve stability.

-When you have 2 electrons in the 1^st shell, you have a duplet structure

-When you have 8 electrons in the rest of the shells, you have an octet structure

Chemical bonds:

There are three ways of forming chemical bonds between atoms:

1) Ionic bonding

2) Covalent bonding

3) Metallic bonding (To be covered in Sec. 3)

Ionic bonding:

Ionic bonding is the electrostatic force of attraction between oppositely charged ions in an ionic compound. It is usually formed between metals and non-metals.

Naming of ionic compounds:

(name of cation) (space) (name of anion)

Formation of ionic bonds:

1) Using the formation of sodium chloride (refer to the pictures above) as an example:

The sodium atom loses 1 electron to form a positive ion (cation) in order to obtain an octet structure. The chlorine atom gains the electron from sodium atom to form a negative ion (anion) in order to obtain an octet structure. The ions have opposite charges and therefore attract each other to form an ionic compound known as sodium chloride. This attraction force is called ionic bond.

Sodium atom = 11 protons and 11 electrons

Sodium ion = 11 protons and 10 electrons

Therefore, the sodium ion carries an overall positive charge of 1+ and is written as Na⁺.

2) Using the formation of magnesium chloride (refer to the pictures above) as an example:

The magnesium atom gives up 2 electrons to form a magnesium ion, Mg²⁺. These 2 electrons are transferred to 2 chlorine atoms to form 2 chloride ions, Cl^-. The magnesium chloride has the formula MgCl_2-.

Chlorine atom = 17 protons and 17 electrons

Chloride ion = 17 protons and 18 electrons

Therefore, the chloride ion carries an overall positive charge of 1- and is written as Cl^-.

Steps to draw ‘dot-and-cross’ diagrams of ionic compounds:

1) Draw the valence electrons of the elements using ‘Dot and Cross’ only.

2) Electron(s) is/are transferred from the valence shell of the metal to the valence shell of the non-metal.

3) The anion that had gained electron(s) from the cation will now have 2 types of electrons: 1 originally from its valence shell, another 1 from the cation, and the 2 different type of electrons are differentiated by ‘dots’ and ‘crosses’.

4) Indicate the charges of the ions

Examples of ‘dot-and-cross’ diagram can be found in the pictures above.

Covalent bonding:

A covalent bond is a bond formed by the sharing of a pair of electrons. Some atoms form bonds by sharing electrons to gain electronic configuration of a noble gas. The bonds formed are known as covalent bonds. Generally, covalent bonds are formed between atoms of non-metal. It can also be formed between atoms of same elements or between atoms of different elements. Compounds which contain covalent bonds are known as covalent compounds.

Single covalent bond- 1 pair of shared electrons between 2 atoms

Double covalent bond – 2 pairs of shared electrons between 2 atoms

Triple covalent bond – 3 pairs of shared electrons between 2 atoms

Covalent bonding in elements:

1) Hydrogen molecules, H₂ (Refer to the pictures above)

A hydrogen atom has 1 valence electron. It requires 1 more electron to obtain the electronic configuration of a noble gas.

In order to obtain the electronic configuration of helium, 2 hydrogen atoms can share a pair of electrons between themselves to form hydrogen molecules, H₂. In order to differentiate the 2 electrons, the electron of 1 atom is represented by a ‘cross’ while the other electron of another atom is represented by a ‘dot’.

2) Oxygen molecules, O₂ (Refer to the pictures above)

In order to obtain the electronic configuration of a noble gas, each oxygen atom requires 2 more electrons. Instead of sharing a pair of electrons, 2 oxygen atoms can share 2 pairs of electrons to form a double bond and obtain an octet structure.

Covalent bonding in compounds:

[Refer to the pictures above]

The carbon atom has 4 valence electrons and it needs 4 more electrons to gain an octet structure. The carbon atom can share its 4 electrons with 4 other hydrogen atoms, forming a single covalent bond with each of the hydrogen atom.

Valency:

The number of electrons an atom uses to form bonds is called its valency.

Hydrogen: 1 valency

Oxygen: 2 valencies

Aluminum: 3 valencies

Carbon: 4 valencies