Edexcel IGCSE Group 7 (halogens)

GROUP 7 : Halogens ( Salt Forming)

GROUP 7 : Halogens Physical Properties

• They are non metals
• They gain an electron to form -1 ions.
• They have low melting and boiling points
• Their melting point increases down the group due to increases in intermolecular forces.
• They are found in pairs and exist as diatomic molecules (X₂)
• They are poisonous and smelly
• Their reactivity increase down the group
• Their density increases down the group.
• They are poor conductors of heat and electricity

HALOGEN REACTION X[F,Cl,Br,I]

REACTION WITH HYDROGEN

They react with hydrogen to form hydrogen halides.

X₂(s) + H₂(g)                  2HX(g)

Reactivity decreases down the group so fluorine and chlorine reacts explosively and bromine and iodine reacts at higher temperature in the presence of catalyst.

REACTION WITH METALS

They react with metals to form ionic compounds. In Ionic compounds, halogens gain one electron from the metals to form -1 ions and attain noble gas electronic configurations.

2Na(s) + Cl₂(g)                  NaCl(s)

Mg(s) + Cl₂(g)                  MgCl₂(s)

DISPLACEMENT REACTION

The more reactive halogen displaces the less reactive halogen from its salt

As the reactivity decreases down the group, the halogen at the top can take the position of the halogen at the bottom in its compounds and will displace the less reactive halogen.

Cl₂    +               2NaBr                               2NaCl + Br₂(yellow solution formed)

(more               (salt of less reactive        (chlorine being more reactive has taken the reactive)     halogen than Chlorine)     position of less reactive bromine in its

compounds)

Cl₂ +2NaF                    No reaction

F2 – can displace all halogens

Cl₂ – can displace all Halogen except Fluorine

Br₂ – can displace only Iodine

F          Cl         Br           I

decreasing reactivity

Group 1 Alkali metal (2)

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WHY REACTIVITY OF GROUP 7 DECREASES DOWN THE GROUP ?

The Reactivity of Group 7 decreases down the group as the electron affinity or tendency to gain the electron decreases down the qroup. – React by gaining electron

FACTORS AFFECTING TENDENCY TO GAIN AN ELECTRON

• To gain an electron, smaller nuclear charge smaller size and less shielding is required
• Nuclear Charge – Smaller the size of the atom, greater will be the force of the nucleus as the electron will be closer to the nucleus.
• Shielding – Less electrons and shells, smaller will *be the shielding which will in turn increa the nuclear charge.
• Size of Atom – Greater the size of the atom, the outer electron will become further away from th nucleus resulting in decreases in nuclear charge
• Down the group the atom size increases due to increase in number of electron shells. As a result the nuclear charge decreases.
• The size of the atom also increases down the group which makes the nuclear charge weaker
• The electron shells also increases which decreases the effective nuclear charge on the incoming electron.
• Due to all these factors, the nuclear charge decreases which decreases the tendency of gaining electrons down the group of halogen making them less reactive.

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References:

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