Why does reactivity decrease as you descend the column of Halogens?

Question

Please only answer if you know what you're talking about. Why is Fluorine more reactive than Chlorine, for example?

Why is fluorine so much more reactive than the rest of the elements in the Halogens?

Answer 1

The exact answer to you question is: the general reactivity of halogens (to form ionic compounds) decreases down the group.

when those elements goes down the group, the number of filled electron shells increases. due to the screening effect, the attraction between the nucleus and the coming electron (donated by the cation, eg, Na donates an electron to Br, forming Na+ and Br-) is reduced down the group.

since electrons are less easy to be accepted by the halogen atoms, the tendency for the atoms to form ions decreases down the group. therefore the reactivity decreases down the group.

these sites contain alot of information about halogens if u want to improve ur knowledge
http://www.chemguide.co.uk/inorganic/group7/properties.html
http://www.chemtopics.com/elements/halogen/halogen.htm

As the atomic structure of the halogens becomes more complex with increasing atomic weight there is a gradiation in physical properties. For example: Fluorine is a pale green gas of low density. Chlorine is a greenish-yellow gas 1.892 times as dense as fluorine. Bromine is a deep reddish-brown liquid which is three times as dense as water. Iodine is a grayish-black crystalline solid with a metallic appearance. And astatine is a solid with properties which indicate that it is somewhat metallic in character.

properties
The halogens show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point. Not all halogens react with the same intensity. Fluorine is actually the most reactive and combines all of the time. As you move down the column, reactivity decreases.

Answer 2

For a Group 7 non-metal atom to react means that that non-metal atom has to gain an electron to become a negative ion.
The easier for that electron to be gained, the higher is its reactivity.

That negatively-charged electron gained must fall into the outermost (valence) orbit and orbit around the positive nucleus under the latter's attractive pull.

Fluorine has only two orbits with the following electronic configuration [2,7]; while chlorine has three orbits [2,8,7]

It is easier for the electron to fall into the 2nd orbit of fluorine than for that electron to fall into the 3rd orbit of chlorine as the 2nd orbit is nearer to the attractive pull of the positive nucleus.

So the reactivity of group 7 decreases DOWN the group.

We say that fluorine gas is able to displace chlorine gas out of a solution containing chloride ions.
Similarly, chorine is more reactive than bromine - hence chlorine gas is able to displace bromine out of a solution containing bromide ions.

Answer 3

Halogens donate electron density by resonance and withdraw electron density by induction. TThis classes them as electron withdrawing and therefore deactivating. Read up on resonance structures to find out more. Halogens are also meta-directors and they increase acidity.

Answer 4

As you go down the group the atomic radius (and ionic radius) increases due to additional levels of electrons. These levels of electrons act as a "shield" between the valence electrons and the pull of the positive nucleus. It is called the "shielding effect." This reduces the ionization energy and the electron affinity of the ions, therefore reducing the reactivity.