**What is an atom,molecule and atomicity of a substance?**

**Atoms: **The atom is the smallest indivisible particle of a substance that can’t exist independently, atom is also composed of subatomic particles protons, neutrons, and electrons. The subatomic particles proton, neutron, and electron were discovered by Goldstein, Chedwik, and J.J Thompson respectively. Although atoms and its subatomic particles can’t be seen directly by a device, scientists experiment on the observation of the different kinds of phenomena due to the content of matter shows the existence of atom and its subatomic particles. Atoms are as small as that even the tip of a needle can accommodate more than 1 lakh atoms.

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**Molecules: **A molecule is the smallest particle of a substance that exists independently and shows the characteristic of the substance. Molecules are composed of two or more than two atoms. In a molecule, atoms are bonded to each other with a covalent bond.

As an example, are the molecules of carbon dioxide, oxygen, and hydrogen respectively.

In the equation of chemical reaction molecules of a substance are represented for showing the involvement of the substance in the chemical reaction.

As an example

The size of molecule depends on the number of atoms available in it

**What are Surds and how to compare them?**

**The atomicity:** Atomicity of a substance is the number of atoms in its molecule. The atomicity of oxygen, hydrogen, Carbon dioxide and water is 2,2,3 and 3 respectively. If we know the molecular formula of a substance then we can easily calculate the atomicity of a substance.

Example: What are the atomicity of the following substance

(a) Sulphur (b) Phosphorus (c) Helium (d) Sodium Chloride (e) Potassium Chlorate (f) Calcium hydroxide (g) Nitric acid (f) Sulphuric acid

Ans. (a) Sulphur molecule has 8 atoms so its atomicity is 8

(b) Phosphorus molecule has 4 atoms so its atomicity is 4

(c) The formula of Sodium chloride (NaCl) shows that it has one atom of Na and 1 atom of Cl, so its atomicity is 2.

(d) The formula of Potassium Chlorate (KClO3) shows that it has one atom of K , 1 atom of Cl and 3 atoms of oxygen ,so its atomicity is 5.

(e) The formula of Calcium hydroxide Ca(OH)2 shows that it has one atom of Ca ,2 atoms of O, 2 atoms of H, so its atomicity is 5.

(f) The formula of Sulphuric acid (H2SO4) shows that it has 2 atoms of H and 1 atom of S and 4 atoms of O, so its atomicity is 2.

(h) The formula of aluminum sulphate Al2(SO4)3 shows that it has 2 atoms of Al, 3 atoms of sulphur, and 12 atoms of oxygen so its atomicity is 17.

Related questions on molecular mass, molar mass and moles.

Q1. What is the molecular mass of sodium nitrate?

Q2. How many moles are in 440 gm of carbon dioxide?

Q3. How many carbon atoms are in 1 kg of carbon dioxide?

Q4. How many sulphate ions are in 400 gm of aluminum sulphate?

Q5. What is the molar mass of Sodium carbonate?

Q6. What is the atomicity of calcium chloride?

Q7. How many moles are in electrons.

Q8. How many electrons are in 64 gm of oxygen gas?

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