General Trends

Atomic Structure is arranged in the order of their
atomic number.

The atomic radii decrease across a period because the
increasingly positive nuclear charge pulls the electrons
in the outer shell closer to the nucleus.

The atomic radii of noble gases are measured from
their vander wall’s forces. This is found by measuring
the distance between the nuclei of two neighboring
touching atoms which are not chemicall bonded. The
distance is then divided by 2 to get atomic radii.

Shielding effect remains reasonably constant across the period.

The cation radii decrease across the period as the positively charged ion has lost electron from
the outer shell. The increasing nuclear charge attracts the electrons in the outermost shell.
Cations are much smaller than their respective atom.

The anions are larger than their respective elements.
The anion size decreases from phosphorus to argon.
Anions are larger than cations in period 3. Each atom has
gained electron in the outerhsell. This increases the
repulsion between the electrons, while the nuclear
charge remains constant.

Electrical conductivity increases from sodium and
aluminum as number of delocalized electron increases.

Melting point increases from sodium to aluminum as
sodium donates 1 electron into the seas of electrons where aluminum donates 3. So metallic
bonding increases across the period.

Silicon has the highest melting point because it has a giant covalent structure. Every silicon atom
is held to its neighboring silicon atom by strong covalent bond.

Silicon is metalloid. Its electrical conductivity is lower than metals as there are no free electrons.
Elements to the right of silicon are nonmetallic. They exist as S8 and P4, although there are
strong covalent bonds between atoms the have weak instantaneous dipole- induced dipole.

At room temperature phosphorus and sulfur are solids while chlorine is a gas. Argon is a
monatomic element with weak intermolecular forces.

Ionization energy increases across period because the nuclear charge increases and atomic radii
decreases which make the attractive forces between the nucleus and the outer electrons
greater.
Chemical Reaction with oxygen
Sodium reacts vigorously in air – burns with bright yellow flame – white solid of sodium oxide formed.
Magnesium reacts vigorously with air – with bright white flame forming white solid magnesium oxide.
Aluminum protected by aluminum oxide layer. It burns in oxygen with white flame. Powdered aluminum
does not react well.
Silicon reacts very slowly with oxygen.
Phosphorus reacts vigorously with white flame and clouds of phosphorus oxide are produced.
Sulfur powder once ignited burns gently with blue flame. Toxic fumes of sulfur dioxide gas are produced.
Further oxidation of sulfur dioxide gives sulfur trioxide.
Chlorine and argon do not react in oxygen.
Chemical Reaction with chlorine
Sodium, magnesium, and aluminum react vigorously forming their metal chloride.
Silicon reacts slowly and phosphorus reacts very slowly with excess chlorine gas.
Argon does not form chloride.
Chemical reaction of metals with water
Sodium reacts vigorously with cold water melting into ball of molten metal. It moves across the surface
of the water, giving off hydrogen gas. It leaves a strong alkaline solution.
Fresh magnesium reacts very slowly with cold water and is weakly alkaline. Magnesium hydroxide is
slightly soluble therefore a lower concentration of hydrogen ions enters the solution. Sodium hydroxide
is soluble in water. Magnesium reacts vigorously with steam.
Effect of water on oxides and hydroxides
The oxides of sodium and magnesium react with water to form hydroxides. The presence of excess
hydroxide ions makes the solution alkaline.
Magnesium oxide and hydroxide are used in indigestion remedies.
Aluminum oxide does not react with water – protective oxide layer.
Aluminum oxide is amphoteric (behaves both as acid and base) so it reacts with acid and hot
concentrated alkali.
Silicon dioxide is also insoluble in water. But it reacts with hot concentrated sodium hydroxide forming
sodium silicate.
Phosphorus V oxide reacts vigorously with to form acidic solution of phosphoric acid.
Oxides of sulfur react with water forming acidic solution with pH 1.
The Electronegativity is the power of an atom to attract the electrons in covalent bond towards itself.
Electronegativity increases across the period and decreases down the group. Across the period,
electrons are more strongly attracted to the nucleus due increasing nuclear charge.
Greater the difference in Electronegativity between period 3 element and oxide, more likely for the
oxide to have ionic bond
Due to their high melting points, Magnesium oxide is used to line the inside of the furnace and
aluminum oxide and silicon oxide to make ceramics.
Aluminum oxide is not purely ionic or covalent.
The oxidation state of period 3 elements in their chlorides is positive because chlorine has the higher
Electronegativity than any one of them.
Sodium and magnesium chloride do not react with water but dissolve in water as its polar to it breaks
down their giant structure. The metals ions formed in solution are hydrated metal ions.
AlCl3 – suggests that it has ionic bonding – solid hydrated aluminum chloride crystals.
Al2Cl6 – dimer – two molecules of AlCl3 joined together by a covalent bond.
On adding water the dimers are broken and aluminum ions and chloride ions enter the solution. Small
and highly charged aluminum ion is hydrated and causes water molecule bonded to it to lose H+ ions.
The solution of Aluminum chloride is acidic.
Silicon chloride reacts with water to forms an off white ppt of silicon dioxide and presence of some of
HCl in dissolved in water makes the solution acidic.
Phosphorus chloride reacts with water to produce HCl and phosphoric acid. Both are highly acidic and
soluble in water.