Metalloids

Category : 10th Class

*       Metalloids

 

There are those elements which behave as metals as well as non-metals. They have four valence electrons.

e.g.

Carbon (C-6) : 2,4

Silicon (Si-14) : 2, 8, 4

 

 

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        Which one of the following is the poorest conductor of electricity?

(a) Copper

(b) Aluminium

(c) Lead

(d) Silver 

(e) None of these

 

Answer: (c)

 

 

        Sodium is a metal because:

(a) It is an element

(b) It has 11 electrons          

(c) It has one valence electron

(d) It has 11 protons           

(e) None of these          

 

Answer: (c)

Explanation 

It is one valence electron. So it can donate one electron and form cations.

 

 

*             Physical Properties of Metals

  • Most metals are solid at room temperature except mercury and gallium, which are liquid at room temperature.
  • Uncorrode or freshly cut metals have shiny surface which is called metallic luster.
  • Metals are hard except sodium and potassium, which are soft and can be cut with knife.
  • Metals are sonorous i.e. they produce sound when hit against hard surface.
  • Metals have high melting and high boiling point. Sodium, potassium, mercury and gallium have low boiling point.
  • Metals are malleable i.e. they can be beaten into sheets. However Zinc, antimony, bismuth and arsenic are not malleable.
  • Metals are ductile i.e. they can be drawn into wires except zinc, antimony, bismuth and arsenic.
  • Metals are good conductor of heat and electricity. Silver is the best conductor while Bismuth, lead and tungsten do not conduct heat through them.
  • Metals have high tensile strength.

 

 

*             Physical properties of non-metals

  • Non-metals exist in all the three physical states. For example carbon, phosphorous and sulphur are solid, while bromine is liquid, rest all are gaseous at room temperature.
  • Most of the non-metals are soft, except diamond, which is an allotrope of carbon.
  • They have low density except bromine, iodine which are quite heavy.
  • Non-metals are neither malleable nor ductile.
  • They have low melting and boiling points.
  • Non-metals have low tensile strength except carbon fibres.
  • Non-metal are poor conductor of heat and electricity except graphite.

 

 

*             Chemical Properties of Metals

  • Metals are electropositive elements i.e. they have tendency to form cation by loosing electrons.
  • Reaction with Oxygen

Metals + Oxygen \[\to \] Metal oxides

Metal oxides are basic oxides and they react with acids to form salt and water

 \[4Na+{{O}_{2}}\to 2N{{a}_{2}}O\]

\[N{{a}_{2}}O+{{H}_{2}}O\to 2NaOH\]

\[3Fe+2{{O}_{2}}\to F{{e}_{3}}{{O}_{4}}\]

Reactivity of metals with oxygen is : \[K>Na>Mg>Zn>Fe>Cu\]

Metal oxides, which are water soluble, are called alkali, e.g. \[N{{a}_{2}}O,\,\,{{K}_{2}}O\]

  • Reaction with Water

Metals + Water \[\to \] Metal oxide + Hydrogen gas

\[2Na+2{{H}_{2}}O\to 2NaOH+{{H}_{2}}\]

\[Mg+{{H}_{2}}O\to MgO+{{H}_{2}}\]

\[3Fe+4{{H}_{2}}O\to F{{e}_{3}}{{O}_{4}}+4{{H}_{2}}\]

  • Reaction with Acids

Metals above hydrogen in reactivity series react with dilute acids to liberate hydrogen gas.

Metal + Acid \[\to \] Metal salt + Hydrogen gas

\[2Na+2HCl\to 2NaCl+{{H}_{2}}\]

 \[Zn+{{H}_{2}}S{{O}_{4}}\to ZnS{{O}_{4}}+{{H}_{2}}\]

  • On reaction with Nitric acid no hydrogen gas is liberated. It is because nitric acid is an oxidizing agent and it oxidize hydrogen gas to water and itself get reduced to \[N{{O}_{2}}\] or \[NO\].

\[3Fe+8HN{{O}_{3}}\to 3Fe{{(N{{O}_{3}})}_{2}}+4{{H}_{2}}O+2NO\]

  • Magnesium and manganese react with very dilute nitric acid (about 1%) to liberate hydrogen gas.

\[Mg+2HN{{O}_{3}}\to Mg\,{{(N{{O}_{3}})}_{2}}+{{H}_{2}}\]

(1 % Conc.)

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