Chemical bond is the force that holds atoms together in a molecule or compound.
The electron arrangements in the noble gases are very stable. They are stable because they have full electron energy levels.
Noble gas
Symbol
Atomic number
Electron arrangement
Helium
He
2
2
Neon
Ne
10
2:8
Argon
Ar
18
2:8:8
Krypton
Kr
36
2:8:18:8
Xenon
Xe
54
2:8:18:18:8
Radon
Rn
86
2:8:18:32:18:8
Therefore, other atoms combine in order to be stable as one of these gases.
Atoms can be stable by:
Sharing electrons.
Atoms share electrons in a way that allows both atoms to achieve a stable electronic configurations.
Losing electrons.
Atoms that have more electrons in the outermost shell than a noble gas can lose electrons to achieve a stable electronic configuration.
Gaining electrons.
Atoms that have fewer electrons than noble gases in their outer most shell can gain/receive electrons from other atoms to fill their outermost shell.
Generally, in order for an atom to be stable, it can: → transfer electron(s) (lose/gain) or → share electrons.
There are two types of stability
Octet stability – in which there are eight electrons in the outermost shell eg. neon, argon etc.
Duplet stability- in which there are two electrons in the outermost shell of an atom eg. helium.
Chemical bonding is the union between atoms of the same element or between atoms of different elements to form molecules or compounds.
The outermost electrons in atoms (valence electrons) are used in making these bonds.
Ion formation
ION is an electrically charged atom or group of atoms.
When an atom loses an electron to acquire a stable electron arrangement, the lost electron is transferred to another atom which then becomes stable. Both atoms become ions.
When an atom loses one or more electrons, it becomes a positively charged ion called cation. When an atom gains one or more electrons, it becomes a negatively charged ion, or anion.
There are two types of ions:
Cation is a positively charged ion. All metals have a tendency of losing electrons to become cation.
Anion is a negatively charged ion. All non-metals have the tendency of gaining electrons to become anion.
An ion has a charge because it has an unequal number of protons and electrons in an atom.
→ For cation, this happens because the atom now has more protons than electrons, giving it an overall positive charge.
→ For anion, this happens because the atom has more electrons than protons, this gives an overall negative charge.
The magnitude of the charge on an ion depends on the number of electrons gained or lost. For example, calcium loses two electrons and become an ion with a +2 charge.
The process of ion formation.
Na has 2:8:1 electronic configuration. In order to be stable like noble gases it needs to lose 1 electron from the outermost shell.
Na₋e →Na+
This is usually written as
Na →Na+ + e⁻
Chlorine has 2:8:7, electronic configuration. Rather than losing all 7 electrons to be stable, it is better to gain only one electron to get an electronic configuration similar to that of noble gas (Argon).
Cl + e-→Cl-
Types of chemical bonding
There are several types of chemical bonding, though for this level we will discuss two types of bonding:
Ionic /electrovalent bonding. This occurs when one atom donates an electron to another atom to form a stable compound. This usually occurs between metals and non-metals, whereby a metal transfers/donates electrons to a non-metal atom.
Covalent bonding This occurs when two atoms share electrons in order to achieve a stable electronic configuration. Each atom donates electrons towards the bond formation.
Ionic compounds
Covalent compounds
They are crystalline solids at room temperature.
They exist mainly as gases, liquids and sometimes as soft solids at room temperature.
Gases eg. Carbon dioxide and oxygen.
Liquids like petrol and water.
Soft solid like candle wax.
They have high melting and boiling points. Because ions in the ionic compounds are held by very strong electrostatic force.
They have low melting and boiling points. This because the covalent bonds are weak and require very little heat to break.
They conduct electricity when in molten or aqueous solution state.
They do not conduct electricity.
They are soluble in water but insoluble in organic solvents.
They are insoluble in water but soluble in organic solvents (non-polar solvents).
OCTET RULE /DUPLET RULE.
"Atoms tend to form stable electronic configurations by acquiring a full valence shell of 8 electrons/ 2 electrons."
Why noble gases are said to be stable?
Noble gases are said to be stable because they have completely filled valence electron shell, meaning that their outermost shell is filled with the maximum number of electrons allowed.
REMEMBER, the maximum number electrons that can be held by each shell is determined by 2n2 formula.