8/7/2019 CHEM- Elements, compounds and molecules

1/2

Recap! Elements, compounds, molecules ~~~

1. ElementsA pure substance that cannot be split up into 2 or more simpler substances by physical orchemical processes (Explanation: If you take a copper wire and cut it into trillion pieces and

observe these pieces under a very powerful scanning electronmicroscope, you will find nothing

else - no iron, no sulfur, no carbon etc except the element copper, ie. copper itself)

(a) All found in the Periodic Table.

(b) All the atoms in one element are identical (Eg. Copper is an element-made up of copper

atoms only); however, atoms of one element are different from the atoms of another element.

For eg. If I say that an atom in the element iron can be represented by a triangle, then all the

atoms in iron must look like triangles! However, the atoms (triangles) in iron will be differentfrom the atoms (say, circles) in carbon.

(c) Just like our fingerprints or DNA, Each and every element is identified by its atomic number.

No two elements in the Periodic Table have the same atomic number!

(d) Elements can be classified into 2 classes: metals and non-metals. Follow the zig- zag line!

2.Molecules

A molecule is a group (2 or more) of atoms chemically combined together by covalent bonding.

That is, the atoms that combine chemically together must be non-metals.

Egs. of molecules - oxygen molecule O2, sulphur molecule S8, water molecule H2O , glucose

C6H12O6.

Note: the atoms that combine chemically together can be from the same element or from

different elements

There are 2 types of molecules:

a) Molecules of elements (All the atoms in the group are the same)For eg., in the case of an oxygen molecule, the 2 atoms contain the same element, oxygen

b) Molecules of compounds (At least 2/more different types of atoms are present in the group)

For eg., Water H2O is a molecule - it has got 3 atoms; it is also a compound - it consists of 2

different elements , ie. Hydrogen & oxygen.

8/7/2019 CHEM- Elements, compounds and molecules

2/2

3. Compounds

A substance which contains 2 or more atoms chemically combined together and these atoms

must be from at least 2 different elements.

They are chemically combined either by covalent or ionic bonding.

An oxygen molecule does NOT classify itself as a compound as the 2 atoms are of the same

element, oxygen. However, water H2O is a compound. It contains 2 hydrogen atoms chemically

combined with 1 oxygen atom & the elements are both different: hydrogen & oxygen.

Smallest particle of an element is an atom. Smallest particle of a compound is a molecule. A molecule is the smallest particle of an element or compound which can exist

independently.

IMPORTANT:

NOT all compounds are composed of molecules

o Water H2O is a compound (2 diff elementshydrogen and oxygen); it is composed ofmolecules (each molecule consists of 3 atoms and all 3 atoms are non-metals).

o Sodium chloride NaCl is a compound (2 diff elements sodium and chloride). Butsodium chloride is NOT composed of molecules. NaCl has 2 atoms but to be considered

a molecule, the atoms in it must be covalently bonded (all the atoms must be non

metals). Cl is a non-metal but Na is a metal. Hence, sodium chloride is composed of

oppositely charged ions Na+ and Cl-.

Bottom line: Not all compounds are made up of molecules, some are made up of metallic and

non-metallic ions. Compounds that contain oppositely charged ions (cations and anions) are

called IONIC compounds. Examples of ionic compounds are CaO, NaCl, MgF2, CuSO4 etc.Observe the formulae at least one atom present in the compound is a metal (colored blue).

Contrast this with molecules where all the atoms must be non-metals.