Test Paper for Atoms and Molecules

Law of mass conservation states that, mass can neither be created nor destroyed in a chemical reaction or a physical transformation.

According to the Law of constant proportion, a chemical compound always has its compounding elements in definite proportion by mass, irrespective of the source and the type of chemical reaction.

In modern science, John Dalton widely used it and he can be said to be responsible for coining it, but a more factually correct answer would be a Greek philosopher who goes by the name of Lucretius.

Atom is the smallest particle of matter which can take part in a chemical reaction.

A molecule is a group of atoms bonded together which represents the most fundamental unit of a chemical compound capable of taking part in a chemical reaction.

Atomicity is the number of atoms in a molecule.

The Atomic mass unit is the sum of the atomic masses of all the atoms in a molecule.

An ion is an atom or a molecule with an electric charge (+ve or -ve), caused by gain or loss of a single or multiple electrons.

Avogadro’s Constant (6.022 x 10²³) is the number of atoms of an element present in one mole of that element.

(a) Atoms of most elements don’t exist independently they exist as molecules, for molecules are more stable. However, atoms of inert gasses are chemically unreactive and they exist independently. E.g. helium.
(b) Atomicity is the number of atoms in a molecule. E.g. The atomicity of H₂O = 3.
(c) A polyatomic ion is an ion composed of multiple atoms acting as a single charged unit.

Atomic mass is the unit in which the mass of an atom is expressed, where one atomic mass unit is 1/12^ththe mass of a carbon-12 atom.
Gram atomic mass is the atomic mass of an element expressed in grams.
The mass of an atom or a molecule is expressed in ‘u’, whereas, the molar mass is expressed in ‘gm’.

One mole of atoms, molecules, or particles is that amount of the particle(atoms, molecules and ions) whose mass is equal to that particle's atomic or molecular mass in grams. 1 mole = 6.022 x 10²³ particles of that substance.
Importance of a mole: -

• Atoms and molecules are very small. So, it gets bothersome weighing them in grams and trying to count them. Mole concept, however, allows us to count atoms and molecules by weighing macroscopic amounts of materials.
• It gives us a universally accepted standard of mass.
• It provides a standard for reaction stoichiometry.
  

• H₂O represents water
• H₂O is a single molecule of water
• H₂O is a single mole of water. Thus, it contains 6.022 x 10²³molecules of water.
• H₂O contains 2 atoms of hydrogen and 1 atom of oxygen.
• H₂O has a molar mass of 18g

Dalton was the first scientist to use the symbol for the name of the elements a specific sense but it was difficult to memorize and in uses so Dalton's symbol are not used in chemistry

2O represents 2 atoms of oxygen, and it is not possible for it to exist independently.
O₂represents an oxygen molecule which has two constituent oxygen atoms.
O₃represents a single ozone molecule and it does exist independently.

we know that density of water is 1gm/ml.
Hence, 1 gm water will = 1 ml water.
Now, we have molecular mass of water $H_2O$ = 1x2 + 16 = 18 gm
18 gm of water contain $6.022 \times 10^{23}$ molecules
1 gm of water will contain = $\frac {6.022 \times 10^{23}}{18} = 0.33 \times 10^{23}$ molecules
So, the no. of molecules of water in 1ml of water = $3.3 \times 10^{22}$