Atoms, Elements and Compounds

Table of Contents

  • Atomic structure
  • Bonding: the structure of matter
  • Ions and ionic bonds
  • Molecules and covalent bonds
  • Macromolecules
  • Metallic bonding



Atoms, Elements and Compounds

Element is the smallest part of a substance that cannot be broken further into its simpler substances. Elements consists of atoms (basic building blocks) that have the complete representation of the element. When two or more elements are chemically combined they form a compound.[/quote]

Atomic structure

What is an atom made up of?

Atoms consist of three particles

1. Protons

  • Nucleus contains protons
  • Each proton carries +1 charge on them.
  • Relative mass of 1
  • Symbol representing :- p

2. Electrons


  • Each electron carries -1 charge on them.
  • Relative mass of 1/1840
  • Symbol representing :- e

3. Neutrons

  • Nucleus contains neutrons
  • Neutron are neutral.
  • Relative mass of 1
  • Symbol representing :- n

Atoms are electrically neutral when their protons and electrons are equal in number bring a neutral on the atom. The number of protons in an atrevisoion notes chemistry elements compounds, sodium, atomic number, mass number neutrons, protons, electronsom is called as its Proton number / Atomic number. Different atoms have different proton numbers. The number of protons is always equal to the number of electrons in a neutral atom. When neutrons and protons are added, It is called as its Nucleon Number / Mass Number.


Atoms of the same element having same Proton number but different Nucleon number. For example Chlorine 35 and Chlorine 37. Both have same number of protons that is 17 but different nucleon numbers ( 35 & 37).

As protons numbers are same that means Isotopes have same chemical properties but different physical properties as they are dependent upon nucleon number.

chemistry igcse gcse gce revsion notes atomic structureElectron Arrangement

Electrons in an atom move around the nucleus, (that is very tiny compared to the whole atom) in regions known as shells that is capable to hold only a limited number of electrons.

  1. First Shell (K shell)
    • closest to the nucleus
    • Maximum  2 Electrons
    • Filled First
  2. 2nd Shell (L shell), 3rd shell (M Shell) & subsequent.
    • Maximum 8 Electrons
    • Filled in the shell order

Valence Shell / electron

As shown in the diagram above, the farthest and the last shell is the valence shell and the number of electrons in this outermost shell are called as Valence electrons. This is useful in deciding to which group the element belongs to:-

No. of valence electrons = Elements Group No.

Noble Gases

The last Group / Group 8 in the periodic table are all gases and have a special structure called noble gas structure. Electrons in these groups are arranged in such a way that all the shells are completely filled that does not allow them to form ions and bond with other elements.


An ion is a charged particle. Atoms are electrically neutral initially but they either gain or lose electrons to complete their shells. This process is known as ionization.

Metals always prefer to lose an electron and form a positive ion, A Cation; and, non-metals gain to make a negative ion, An Anion.



Forming Ionic Bonds

Ionic bond is formed between a metal and a non-metal, where a metal usually loses an electron and the non-metal gains it. Both partners should lose and gain equal number of elctrons as in case of :- Nacl

Sodium loses one electron and chlorine gains one electron due to electrostatic attraction between them. Sodium becomes sodium ion (2,8,1) → (2,8), chlorine becomes a chloride ion (2,8,7) → (2,8,8) and both combine to form Sodium Chloride.

Ionic bonding

Properties Of  Ionic bonds

  1. Ionic Bonds are found in Ionic Compounds
  2. They have (crystal lattice) giant ionic structures
  3. Contain Ions
  4. Usually have high melting point and boiling point
  5. Insoluble in organic solvents
  6. Water Soluble
  7. Aqueous and Molten states conduct electricity.

Covalent Bonding

This type of bonding helps us to bond non-metals with non-metals as in this ionic bonding (electron transfer) won’t work. Covalent bonding makes use of electron sharing (one given from each partner) between two atoms to help them reach noble gas configuration and bond with each other simultaneously. These bonds can be formed within 2 atoms of the same as well as different element. 3 different type of covalent bonds are:

  • Single Covalent Bond:- 1 pair of shared electrons → 2 electrons
  • Double Covalent Bond:- 2 pair of shared electrons → 4 electrons
  • Triple Covalent Bond:- 3 pair of shared electrons → 6 electrons

For example

Chlorine molecule covalent bonds

Covalent Compounds have simple and giant molecular structures.

Properties of Simple Molecular Structures.

  • Volatile
  • Water insoluble but soluble in organic solvents
  • Low Melting point and Boiling Point
  • Don’t conduct electricity

Giant Molecular structures.

Giant covalent structures have nonmetal atoms joined by covalent bonds to adjacent atoms. In the examples there are diamond, graphite ( allotropes of carbon) and Silicon (IV) oxide.

giant molecular structures chemistry diamond graphite silicon oxide silica

Metallic Bonding

Metals atoms are held strongly to each other due to metallic bonding. A metallic bond is defined as the force of attraction between “sea of delocalized electrons” and positive ions.

Sea of delocalized electrons includes those valence electrons that no longer belong to any atom after the metals lose them.

Properties of Metals ( Giant Metal Lattice)

  • Contains metal ions ‘and sea of electrons
  • Non -Volatile
  • High M.P & B.P
  • Able to conduct electricity in all states
  • Insoluble in both organic solvents and water.

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