Atoms of most elements react to form bonds. The atoms either lose, gain or share electrons to form full outer shells like the electronic configurations of noble gases.
Atoms of noble gases do not form bonds because they have a very stable arrangement of electrons in the outer shell. Other electrons react and bond to archieve this.
full outer shell of 2 electrons – stable
full outer shell of 8 electrons – stable
outer shell of 8 electrons – stable
Atoms bond with each other in order to gain a stable arrangement of
outer-shell electrons, like the atoms of Group 0. In other words, they bond in order to gain 8 electrons in their outer shell (or 2, if they have only one shell).
Ionic bond is a bond formed when metals combine with non-metals. When this type of bond is formed, electrons are transferred from the metal atoms to the non-metal atoms during the chemical reaction. In doing this, the atoms become more stable by getting full outer shells.
Sodium has just one electron in its outer shell (Na 2,8,1). Chlorine has seven electrons in its outershell (Cl 2,8,7). When these two elements react, the outer electron of each sodium atom is transferred to the outer energy level of a chlorine atom. In this way both the atoms obtain full outer shells and become like the noble gas configuration. The sodium atom has become a sodium ion which has
an electron configuration like neon (Ne 2,8). The chlorine atom becomes a chloride ion with an electron configuration like argon (Ar 2,8,8).
Only the outer electrons are important in bonding, so we can simplify the diagrams by leaving out the inner shells.
The charges on the sodium ions (Na+)and chloride ions (Cl–) are equal but opposite. They balance each other and
the resulting formula for sodium chloride is NaCl. These oppositely charged ions attract each other and are bonded to one another by strong electrostatic forces. This type of bonding is called
ionic bonding. The alternative name, electrovalent
bonding, is derived from the fact that there are electrical charges on the atoms involved in the bonding.
The word equation for the reaction is:
sodium + chlorine → sodium chloride
Always when a metal reacts with a non-metal to form an ionic compound the metal atoms lose electrons and the non-metal atoms gain them. Since the number of electrons lost by the metal is equal to the number of electrons gained by the non-metal, the compound has no overall charge.
A magnesium atom has 2 outer electrons and an oxygen atom has 6. When magnesium burns in oxygen, each magnesium atom loses its 2 outer
electrons to an oxygen atom.
When magnesium burns in chlorine, each magnesium atom donates two
electrons to the chlorine atoms. But since each chlorine atom has 7 outer electrons it has room for only one of two electrons donated by the magnesium. Therefore one magnesium atom reacts with two chlorine atoms to form MgCl2 which is made up of Mg2+ ions and Cl2- ions. Each ion now has 8 outer electrons.
When two non-metal atoms react together, both need to gain electrons to achieve stable outer shells. They manage this by sharing electrons. Atoms can share only their outer (valence)
electrons. Thats why most of our diagrams only show outer electron only
A hydrogen atom has only one shell, with one electron. This shell can hold a maximum of two electrons since it is the first shell. When two hydrogen atoms meet, their shells overlap until they can share electrons. So each has gained a full shell of two electrons to achieve a full outer shell like helium atoms. Both positively charged hydrogen nuclei attract the shared electrons producing a force of attraction holds the two atoms together. This force of attraction is called a covalent bond.
A single covalent bond is formed when atoms share two electrons.
Two chlorine atoms need to share one electron each, to obtain a stable outer shell of eight electrons each like this:
Since only one pair of electrons is shared, the bond between the atoms is called a single bond which can also be shown by a single line, like this:
Carbon has four valence electrons so in order to fill its outer shell it makes 4 single covalent bonds with hydrogen to form methane. Each single bond is made up of a pair of shared electrons to make them 8 in the outer shell of carbon and 2 in the outer shell of each hydrogen atom.
A molecule is a group of atoms held together by covalent bonds. Since it is made up of molecules, hydrogen is a molecular element. Its formula is H2 which means the molecule is made up of two hydrogen atoms in this case. Elements made up of molecules containing two atoms are called diatomic. Examples of other diatomic elements are; iodine, I2; oxygen, O2; nitrogen, N2; chlorine, Cl2.
a) Carbon dioxide is a compound of carbon and oxygen, two non-metallic elements.
(i) State the name of the type of bonding between a carbon atom and an oxygen atom. …
(ii) Describe, in terms of electrons, the bonding between a carbon atom and an oxygen atom. ….
(b) The compound sodium chloride contains sodium ions and chloride ions.
Describe, in terms of electrons, the formation of ions from the atoms. …
- Covalent Bond
- The outer shell electrons are shared between carbon and oxygen atoms.
b) sodium loses one electron from the outer shell and becomes a positively charged ion. chlorine gains one electron and become a negatively charged chloride ion.
Comparing ionic and covalent compounds
A metal and a non-metal react together to form an ionic compound.
Ionic compounts exist in form of charged ions held together by strong
electrostatic forces. Non-metals react together to form covalent compounds. Covalent compounts exist as uncharged molecules held together by weak forces of attraction.
The properties of ionic compounds
- Ionic compounds have high melting and boiling points because the
ionic bonds are very strong and take up a lot of energy to break.
- Ionic compounds are solid at room temperature.
- Ionic compounds are usually soluble in water.
- Ionic compounds conduct electricity, when molten or when dissolved in water but a solid ionic compound will not conduct electricity.
The properties of covalent compounds
- Covalent compounds have low melting and boiling points because
the attraction between the molecules is low and so it does not take much energy to separate them from each other.
- Many covalent compounds are liquids or gases at room temperature.
- Covalent compounds are generally insoluble in water but they do
dissolve in some solvents such as tetrachloromethane and ethanol
- Covalent compounds do not conduct electricity because they dont have
Naming the compounds
The following is a guide on naming binary compounds. Binary compounds are compounts which contain only 2 elements.
When the compound contains a metal
and a non-metal:
The name of the metal is given first followed by the name of the non-metal, but ending with -ide.
Examples: sodium chloride (sodium and chlorine), magnesium oxide (magnesium and oxygen), iron sulphide (iron and sulphur), sodium bromide (sodium and bromine).
When the compound is made of two non-metals:
If one of the elements is hydrogen, that is named first else the one with the lower group number comes first followed by the name of the other non-metal, ending with -ide.
Examples: hydrogen chloride (hydrgen and chlorine), carbon dioxide (carbon dioxide), hydrogen sulphide (hydrogen and sulphur).
But some compounds have ‘everyday’ names that give you no clue about
the elements in them. Water (H2O), methane (CH4), and ammonia (NH3) are examples. You just have to remember their formulae! .
Complete the following sentences using words from the list.
Each word may be used once, more than once or not at all.
- The smallest parts of an element are ……. 
- All …….. contain atoms joined by sharing pairs of electrons in
……… bonds. 
- Atoms which lose or gain electrons form particles called ……..
- Elements in …….. cannot be separated by simple physical processes. Substances in …….can be separated by simple physical processes. 
- molecules, covalent
- compounds, mixture