PROPERTIES AND TRENDS ACROSS A PERIOD - Form 2 Chemistry Notes

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Introduction:

  • A period is a vertical row of elements in the periodic table.
  • Elements in the same period have same number of energy levels.
  • There are 7 periods in the periodic table except for lanthanides and actinides which are not assigned periods.
  • Periods 1 - 3 have fewer elements because they lack the d-block elements and have only the s-block elements and the p-block elements.
    • s-block elements: group 1 and 2
    • d-block elements: transitional elements
    • p-block elements: groups III to VIII.
  • To understand trends and properties across a typical period of the periodic table, we shall use period 3 as the reference.

The Period Three of the Periodic Table.

  • Consists of elements with three energy levels.
  • Consists of the 8 elements from sodium to argon.
  • It is only made of s-block and p-block elements and lacks any element in the d-block group of elements.

 

 

 

 

 

 

 

 

 

Part of Periodic Table Showing Period 3 of the Periodic Table.

periodic table showing periods

Trends in Physical Properties of Elements in Period 3.

Electrical Conductivity.

  • Sodium, magnesium and aluminium are good conductors of electricity, because they all have giant metallic structures with delocalized electros which conduct electricity;
  • The electrical conductivity increases from sodium to aluminium as electrical conductivity increases with increase in the number of delocalized electrons; thus aluminium with the highest number of delocalized electrons (3) in each atom will have the highest electrical conductivity;
  • In the metals the electrical conductivity decreases with increase in temperature because increase in temperature distorts the alignment of electrons thus preventing their easy flow and hence poor conductivity;
  • Phosphorus, sulphur, chlorine and argon do not conduct electric current, as they all have molecular structures and all the electrons in the atoms are used in bonding; thus they lack delocalized electron or free ions for electrical conductivity.
  • Silicon conducts electric current, and its electrical conductivity increases with increase in temperatures.
  • It is a semi-conductor; making it a very unique element in this period.
  • A semi-conductor is a crystalline material which only conducts electricity under certain conditions.

The Atomic and Ionic Radii.

  • The atomic radii of period 3 elements decrease across the period since for the same number of energy levels the number of protons in the nucleus increases across the period; this leads to the increase in nuclear charge while the shielding effect remains the same hence decrease in atomic radius across the period.

 

Melting and Boiling Points

  • Sodium, magnesium and aluminium have very high melting and boiling points because they have giant metallic structures with strong metallic bonds which need a lot of energy to break.
  • The boiling and melting points increase from sodium to aluminium since as you move across the period from sodium to aluminium, the nuclear charge increases while the energy levels remain the same hence decrease in atomic radius; the smaller the atomic radius (for metals) the stronger the metallic bonds.
  • Silicon, though a non-metal also has a very high melting and boiling points because it has a giant atomic structure with strong covalent bonds throughout the structure, which need a lot of heat energy to break.
  • Phosphorus, Sulphur, Chlorine and argon have low melting and boiling points as they all have molecular structures with strong covalent bonds between the atoms (except in argon) but weak van der waals forces between the molecules which are easy to break.
  • Note that argon exist as atoms and thus a monoatomic molecule.
  • The melting and boiling points decreases from phosphorus to argon because as we move across the period from phosphorus to argon, the size of the atoms decreases leading to smaller atoms and hence molecules, which lead to decrease in the strength of the van der Waals (across the period)
  • Phosphorus and sulphur exists as solids at room temperature while chlorine and argon exists as gases at room temperature because phosphorus and sulphur have giant molecular structures while chlorine and argon have simple molecular structures.

Summary: Some Physical Properties of Elements in Period 3.

 Property  Na  Mg  Al  Si  P(white)  S(monoclinic)  Cl  Ar
 Physical state and appearance  Silver  Silver solid  Silver solid  Black solid  White solid  Yellow solid  Green yellow gas  Colourless gas
 Electron arrangement  2.8.1  2.8.2  2.8.3  2.8.4  2.8.5  2.8.6  2.8.7  2.8.8
 Valency  1  2  3  4  5  6  7  8
 Atomic radius  0.157  0.136  0.125  0.117  0.110  0.104  0.09  0.192
 MP (oC)  98  650  660  1410  44  119  -101  -189
 BP (oC)  890  1110  2470  2360  280  445  -35  -186
 Structure  Giant metallic  Giant metallic  Giant metallic  Giant atomic  Molecular  Molecular  Simple molecular  Simple molecular /atomic
 Bond type  metallic  metallic  metallic  Covalent  Covalent  Covalent  Covalent  Van der waals

Trends in Chemical Properties of the Elements in Period 3.

Trends in Reactivity.

  • The reactivity among the metallic elements decreases across the period from sodium to aluminium since there is a continuous increase in nuclear charge from sodium to aluminium which leads to increase in ionization energies hence increasing difficulty in removing an electron from the outermost energy level.
  • Among the non-metallic elements, the reactivity increases across the period from phosphorus to chlorine since there is increase in nuclear charge from phosphorus to chlorine, hence an increase in ease of electron gain (electro affinity) since non-metals react by gaining electrons.
  • Argon is unreactive and can only react under very special conditions, because it s a noble gas with a stable octet configuration.

Reaction of period 3 elements with oxygen.

  • All period three elements react with (burn in) oxygen with the exception of argon.


Experiment: To investigate the reactions between period 3 elements and oxygen

Procedure:

  • A small piece of the element is placed in a deflagrating spoon and warmed gently until it catches fire.
  • It is then lowered into a gas jar full of oxygen.
  • The flame colour and the colour of the product are noted.
  • 10 cm3 of water containing universal or litmus indicator is added into the gas jar with the products.

Sodium:

  • Burns vigorously in oxygen with a golden yellow flame ; to produce white solid of sodium oxide.

Equation:

Na(s) + O2(s) → Na2O(s)

  • The resultant sodium oxide dissolves in water to form sodium hydroxide.

Equation:

Na2O(s) + H2O(l) → 2NaOH(aq) ;

  • The sodium hydroxide is alkaline in nature and thus turns litmus indicator blue;

Magnesium:

  • Burns vigorously in oxygen with a bright white light ; to produce white solid of magnesium oxide.

Equation:

2Mg(s) + O2(s) → 2MgO(s)

  • The resultant magnesium oxide is slightly soluble in water to form magnesium hydroxide.

Equation:

MgO(s) + H2O(l) → Mg(OH)2(aq) ;

  • The magnesium hydroxide is alkaline in nature and thus turns litmus indicator blue;

Aluminium:

  • Aluminium (foil) is usually coated with a thin layer of aluminium oxide, Al2O3 ; which prevents reaction with the oxygen.
  • When polished, it reacts slowly with oxygen to form a white solid of aluminium oxide.

Equation:

Al(s) + 3O2(s) → Al2O3(s)

  • The resultant aluminium (III) oxide is insoluble in water.

Silicon:

  • Silicon powder can only bur in oxygen at very high temperatures of about 450oC to form solid silicon (IV) oxide.

Equation:

Si(s) + O2(s) → SiO2(s)

  • The resultant silicon (IV) oxide is insoluble in water.

Phosphorus.

  • Burns readily in oxygen with a bright orange flame ; to produce dense white fumes (solid) of phosphorus (V) oxide.

Equation:

P4(s) + 5O2(s) → 2P2O5(s)

  • sulphur exists and therefore reacts as molecules of P4 .
  • The resultant phosphorus (V) oxide readily dissolves in water to form phosphoric (V) acid.

Equation:

P2O5(s) + 3H2O(l) → 2H3PO4(aq) ;

  • The phosphoric acid is acidic in nature and thus turns litmus indicator pink / red;

Sulphur

  • Burns in oxygen with a blue flame ; to form a colourless gas of sulphur (IV) oxide.

Equation:

S(s) + O2(s) → SO2(s)

  • The resultant sulphur (IV) oxide readily dissolves in water to form sulphuric (IV) acid.

Equation:

SO2(s) + H2O(l) → H2SO3(aq) ;

  • The phosphoric acid is acidic in nature and thus turns litmus indicator pink / red;
  • The sulphuric (IV) acid is unstable and thus easily gets oxidized by oxygen to the stable sulphuric (VI) acid.

Equation:

H2SO3(aq) + O2(g) → H2SO4(aq) ;

Chlorine.

  • Burns in oxygen only under certain conditions to form acidic oxides.

Equation:

2Cl2(s) + O2(s) → 2Cl2O(s)


Argon.

  • Argon is unreactive.

Conclusion:

  • Metallic elements burn in oxygen to form basic oxides.
  • Non-metallic oxides burn in oxygen to form acidic oxides.

Reaction of period 3 elements with water.

Sodium.

Procedure:

  • A small piece of sodium metal is cut and dropped into a trough containing water;
  • The resultant solution is tested with litmus paper;

Diagram of apparatus:

reaction of sodium with water

Observations and explanations:

  • The metal floats on the water surface; because it is less dense than water;
  • A hissing sound is produced; due to production of hydrogen gas;
  • It vigorously melts into a silvery ball then disappears because reaction between water and sodium is exothermic (produces heat). The resultant heat melts the sodium due to its low melting point.
  • It darts on the surface; due to propulsion by hydrogen;
  • The metal may burst into a golden yellow flame; because hydrogen may explode into a flame which then burns the sodium;
  • The resultant solution turns blue; because sodium hydroxide solution formed is a strong base;

Reaction equations.

Equation I

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) ;

Equation II

4Na(s) + O2(g) → 2Na2O(s) ;

Equation III:

Na2O(s) + H2O(l) → 2NaOH(aq)

Effect of resultant solution on litmus paper;

  • Litmus paper turns blue; sodium hydroxide formed is highly soluble in water; releasing a large number of hydroxyl ions which result into alkaline conditions // high pH;

Magnesium.

Procedure:

  • A small piece of magnesium ribbon is cut and dropped into a trough containing water;
  • The resultant solution is tested with litmus paper;

 

Diagram of apparatus:

magnesium reaction with water

Observations and explanations:

  • The metal sinks into the water surface; because it is denser than water;
  • It reacts slowly with water leading to slow evolution of hydrogen gas.
  • The resultant solution turns blue; because sodium hydroxide solution formed is a strong base;

Reaction equation.

Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g) ;

Effect of resultant solution on litmus paper;

  • Litmus paper turns blue; magnesium hydroxide formed is highly soluble in water; releasing a large number of hydroxyl ions which result into alkaline conditions // high pH;

Aluminium.

  • Aluminium does not normally react with cold water or steam since it is usually coated with a thin coating of aluminium oxide which prevents further reaction.
  • However at very high temperatures of about 700oC steam can react with aluminium to form aluminium oxide and hydrogen gas.

Equation:

2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g) ;

  • Due to its inability to react with water and air aluminium is preferable for making cooking utensils like sufurias and coking pans.

Silicon, phosphorus and sulphur.

  • These non-metals do not displace hydrogen from water and thus do not react with water.

Chlorine.

  • Dissolves in water to form chlorine water, which is a mixture of chloric (I) acid and hydrochloric acid.

Equation:

Cl2(g) + H2O(l) → HOCl(aq) + HCl(aq)

Reaction of period 3 elements with acids

Procedure:

  • A piece of the element is dropped into 5 cm3 of an acid in a test tube.
  • Any gas produced is tested.

Sodium:

  • Reacts explosively with acids to form salts and hydrogen and thus reactions of sodium with acids should not be tried in the laboratory.

Magnesium:

  • Reacts with both dilute hydrochloric and dilute sulphuric acid to form magnesium salts and hydrogen gas.

With hydrochloric acid:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

With sulphuric acid:
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)

Aluminium:

  • It does not readily react with dilute acids due to presence of a thin aluminium oxide coating that prevents contact hence reaction with the acids.
  • When polished to remove the oxide coating it reacts with both dilute hydrochloric and dilute sulphuric acid to form aluminium salts and hydrogen gas.

With hydrochloric acid:
2Al(s) + 6HCl(aq) → 3AlCl3(aq) + 3H2(g)
With sulphuric acid:
2Al(s) + 3H2SO4(aq) → Al2(SO4)2(aq) + 3H2(g)

Silicon, phosphorus, sulphur and chlorine.

  • They do not react with dilute acids.

Summary: Chemical Properties of Period 3 Elements.

 Element  Na  Mg  Al  Si  P  S  Cl
 Reaction with air or oxygen  Readily reacts with air.
- Burns brightly in oxygen with a golden yellow flame to form Na2O
 Reacts slowly with air.
- Burns in oxygen with a bright white flame to form MgO
 - Forms a protective coating of Al2O3 when it burns in oxygen.  Si powder burns at temperatures above 950oC to form SiO2  White phosphorus
smolders in air;
- P burns in air with a bright orange flame to form P2O3 and P2O5
 Burns in air or oxygen with a blue flame to form SO2 gas  No reaction with air or oxygen under normal conditions.
 Reaction with water  Reacts vigorously to produce H2 and NaOH  Slow reaction with cold water to form Mg(OH)and H2 ;
- Reacts faster with steam to form MgO and H2
 No reaction  No reaction  No reaction  No reaction  Dissolves in water to form chlorine water
 Reaction with dilute acids  Violent reaction giving out H2 and a sodium salt  Rapid evolution of Hgas and a Mg salt is formed  Reacts slowly to give Hand an Al salt  No reaction  No reaction  No reaction  No reaction

 


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