Atomic Structure - Definitions

1.      Atom: is the smallest part of an element that can take part in a chemical change.

2.      Ion: is an atom or radical carrying an electrical charge. (A cation is positively changed, an anion is negatively charged.)

3.      Proton: is the electrically positive unit of the atom.

4.      Electron: is the electrically negative unit of the atom. (It’s mass is 1/1840 of a proton.)

5.      Neutron: is the electrically neutral unit of the atom, equal in mass to the proton.

6.      Molecule: is the smallest particle of an element or compound that can exist by itself. (It never breaks up except to chemically react)

7.      Electronic configuration: is the arrangement of electrons in the orbits of an atom.

8.      Nucleus: in the center of the atom is a heavy, positively-charged body that consists of all the protons and neutrons of the atom.

9.      Atomic weight: is the sum of the protons and neutrons in the nucleus of the atom of an element.

10.   Relative atomic weight: is the atomic weight of an element as compared to an atom of hydrogen (1), or as compared to an atom of carbon (12).

11.   Atomic number: of an element is the number of protons in its nucleus.

12.   Isotopes: are atoms of the same element, having the same atomic number but different atomic weights.

13.   Electrovalency: of an atom or radical; it it is the number of electrons which it transfers or gains in chemical reactions.

14.   Covalent, or molecular compound: the compound formed by sharing electron pairs between their atoms.

15.   Electrovalent, or ionic compound: the compound formed by the electrostatic attraction due to oppositely charged ions. Ions are formed by the transfer of electrons from one atom to another.

What Happens When...

1.      Mercuric oxide is heated?

Mercuric oxide is an orange powder. On heating, it becomes dark red owing to the formation of mercury (I) oxide. On further heating, the silver sublimate of mercury is formed. All the while, oxygen, a colorless, odorless, neutral gas escapes.

2HgO Δ→ (via Hg₂O) Δ→ 2Hg + O_2↑

2.      Lead dioxide is heated?

Lead dioxide is a chocolate brown powder, that on heating changes to a yellow substance that sticks to glass, lead monoxide. Oxygen, a colorless, odorless, neutral gas is evolved.

               2PbO₂ Δ→ 2PbO + O_2↑

3.      Ammonium dichromate is heated?

Ammonium dichromate is a bright orange, crystalline substance. When warmed gently, a vigorous action sets in, and chromium sesquioxide, a dirty green substance, starts pouring out. It is pushed out by nitrogen, a colorless, odorless, neutral, inert gas, and water vapor, a colorless, odorless gas. Only initial heating is required, since heat evolved during the reaction keeps it going.

               (NH₄)₂Cr₂O₇ Δ→ Cr₂O₃ + 4H₂O_↑  +  N_2↑  

4.      Copper nitrate is heated?

Copper nitrate is a deep blue crystalline substance. On heating, the crystals become a green mass after losing their water of crystallization. On further heating, nitrogen dioxide, a reddish-brown, acidic gas with a pungent odor is given off, along with oxygen, a colorless, odorless neutral gas. A black residue of copper (II) oxide remains.

               2Cu(NO₃)₂ Δ→ 2CuO + O_2↑  +  4NO_2↑  

5.      Lead nitrate is heated?

Lead nitrate is a white crystalline substance. On heating, lead monoxide, a yellow substance that sticks to glass is formed. Nitrogen dioxide, a reddish-brown acidic gas with a pungent smell, and oxygen, a colorless, odorless, neutral gas are evolved.

2Pb(NO₃)₂ Δ→ 2PbO + 4NO_2↑ + O_2↑

6.      Silver nitrate is heated?

Silver nitrate is a white substance. On heating, oxygen, a colorless, odorless, neutral gas, and nitrogen dioxide, a reddish-brown, acidic gas with a pungent odor, are evolved.

Metallic silver remains.

2AgNO₃ Δ→ 2Ag + 2NO_2↑ + O_2↑

7.      Ammonium nitrate is heated?

Ammonium nitrate is a white crystalline substance. On heating, a colorless, odorless neutral gas, water vapor, and a colorless, neutral gas with a pleasing odor are given off. There is no residue.

NH₄NO₃ Δ→ N₂O_↑ + 2H₂O_↑

8.      Potassium nitrate is heated?

It is a white substance. On heating, potassium nitrite remains, and oxygen, a colorless, odorless gas is given off.

2KNO₃ Δ→ 2KNO₂ + O_2↑

9.      Manganese dioxide is heated with concentrated Hydrochloric Acid?

Manganese dioxide is a black solid that oxidizes hydrochloric acid to chlorine, a greenish-yellow, acidic gas with a pungent odor. Manganese (II) chloride and water are also formed.

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl_2↑

10.   Ammonium chloride is heated?

 Ammonium chloride is a white, crystalline substance. On heating, dense white fumes of hydrogen chloride and ammonia are formed. When cooled, they reform the sublimate of ammonium chloride.

H₄Cl Δ↔ HCl + NH₃

11.   Trilead tetroxide is heated?

Red lead is an orange substance. On heating, a chocolate brown powder, lead dioxide is formed. It becomes yellow lead monoxide on heating. All the while, oxygen, a colorless, odorless gas is evolved.

               2Pb₃O₄ Δ→ (via PbO₂) Δ→ 6PbO + O₂     

12.   Potassium chlorate is heated?

Potassium chlorate is a white substance. On heating, a colorless, odorless, neutral gas is evolved, and potassium chloride remains.

     2KClO₃ Δ→ 2KCl + 3O_2↑

13.   Calcium carbonate is heated?

Calcium carbonate is a white substance. On strong heating, it decomposes to a calcium oxide, and carbon dioxide, a colorless, odorless and acidic gas.

     CaCO₃ Δ→ CaO + CO_2↑

14.   Iron is dipped in copper sulphate solution?

When iron is dipped in deep blue copper sulphate solution, the solution changes to a pale green, and a red deposit of copper is formed on the iron. (It becomes copper-plated.)
Fe + CuSO₄ → FeSO₄ + Cu

15.   Carbon dioxide is passed through limewater?

When carbon dioxide is passed through limewater, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

When more carbon dioxide is passed, the milkiness clears due to the formation of soluble calcium hydrogen carbonate.
CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂

16.   Sulphur dioxide is passed through limewater?

When sulphur dioxide is passed through limewater, it turns milky due to the formation of insoluble calcium sulphite.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

When excess sulphur dioxide is passed, the milkiness clears due to the formation of soluble calcium hydrogen sulphite.
CaSO₃ + H₂O + SO₂ → Ca(HSO₃)₂

17.   Ammonia is passed through copper sulphate solution?

When ammonia is passed through copper sulphate solution, a light blue precipitate of copper hydroxide is formed.
CuSO₄ + 2NH₃ + 2H₂O → Cu(OH)₂ + (NH₄)₂SO₄

In an excess of ammonia, an inky blue solution is formed due to the formation of tetramine copper sulphate.

               Cu(OH)₂ + (NH₄)₂SO₄ + NH₃ + H₂O → (Cu(NH₃)₄)SO₄ + H₂O

18.   Concentrated sulphuric acid is added to an aqueous solution of nitric acid and ferrous sulphate?

If concentrated sulphuric acid is added to an aqueous solution of iron (II) sulphate, nitric acid gets reduced to nitrogen monoxide.

               6FeSO₄ + 3H₂SO₄ + 2HNO₃ → 3Fe₂(SO₄)₃ + 4H₂O + 2NO

Nitric oxide forms an additional compound with excess iron (II) sulphate, which appears as a ring at the junction of the two liquids.

               FeSO₄ + NO → FeSO₄ · NO

When the test tube is shaken, the compound thermally dissociates due to heat liberated by the dilution of sulphuric acid.

19.   Chlorine and Ammonia are mixed?

In an excess of chlorine, hydrochloric acid and nitrogen trichloride are formed.

               NH₃ + 3Cl₂ → NCl₃ + 3HCl

In an excess of ammonia, the reaction produces ammonium chloride and nitrogen.

               8NH₃ + 3Cl₂ → 6NH₄Cl + N_2↑

20.   An amphoteric oxide is made to react with an acid and a base?

An amphoteric oxide behaves as both an acidic and basic oxide. It reacts with an acid or a base to form a salt and water only. Examples are zinc and aluminum oxide.

Zinc oxide reacts with sulphuric acid to form zinc sulphate and water.

ZnO + H₂SO₄ → ZnSO₄ + H₂O

Zinc oxide reacts with sodium hydroxide to form sodium zincate and water.

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

21.   Zinc reacts with sodium hydroxide?

Zinc reacts with sodium hydroxide to form the salt sodium zincate with the evolution of hydrogen, a colorless, odorless, neutral gas.

               Zn + 2NaOH → Na₂ZnO₂ + H_2↑

22.   Chlorine reacts with concentrated and dilute alkalies?

With a cold and dilute alkali solution, chlorine forms a chloride and hypochlorite of the alkali metal.

               2NaOH + Cl₂ → NaCl + NaClO + H₂O

               2KOH + Cl₂ → KCl + KClO + H₂O

With a hot and concentrated solution of an alkali, chlorine forms the chloride and chlorate of the alkali metal, and water.

               6NaOH + 3Cl₂ → NaClO₃ + 5NaCl + 3H₂O

               6KOH + 3 Cl₂ → KClO₃ + 5KCl + 3H₂O