Questions on the Halogens

Group (VII) (The Halogens)

1.      Explain the trend in atomic size in descending the group.

Each time the group is descended by one, a new quantum level is added. Therefore, the distance from the nuclear charge to the outer electrons increases and so does the screening effect, therefore, the attraction on the outer electrons is reduced and the atoms get bigger.

2.      What trend in electron affinities would you predict?

As the atomic size (distance from outer electrons to the nuclear charge) and the screening effect increases going down the group, the attraction on an added electron from the nuclear charge will decrease. Therefore, I would expect the electron affinity to decrease going down the group.

3.      Based on number 2, what trends in reactivity would you expect? Give reasons.

The halogens have seven electrons in their outer shell and so react by obtaining one more (either by sharing in covalent bonding or gaining one in ionic bonding). One of the factors influencing their reactivity will be their ability to gain an electron. This will be greatest when they are smallest with the least screening  (greatest attraction). Therefore, the reactivity should decrease going down the group or increase going up the group.

4.      The actual values for the electron affinity are; F_(g)   -328 (KJ/mol), Cl_(g)   

-349, Br_(g) -324 and I_(g) -245

What is surprising is that Cl has a higher value (more heat released) than F. This is because that F is such a small atom that the electrons in the outer quantum level are closely packed. This causes there to be more repulsion for an incoming electron and therefore, the overall heat energy released is actually less than for Cl. However, F is still more reactive than Cl and this is because in a reaction other things are occurring, such as: Atomisation and hydration energy (if its in a solution). Flourine has the lowest atomization energy and the greatest hydration energy. This compensates for the slightly lower electron affinity value. Therefore, overall fluorine is the most reactive of the halogens.

You may find the atomisation data for fluorine and chlorine surprising as they are both gases at room temperature. Therefore, atomisation involves breaking the bond between the atoms. Look at the following link which includes bond energy data, and all should be clear to you.

Chem Guide Group 7 properties

Homework The Halogens

The Halogens

Please make sure that all your observations for the reaction with iron wool and with the solutions; KI_(aq), KBr_(aq) and KCl_(aq) are recorded. Please write equations for the reactions and explain the trend in reactivity.

E.g. Cl₂ + 2I^- →  I₂ +  2Cl^-

Or showing the two half equations:  Cl₂ + 2e^- → 2Cl^-    (reduction)

                                                                  2I^- → I₂ + 2e^-        (oxidation)

This illustrates that chlorine can oxidise iodide ions and that chlorine is more reactive than iodine.

Period three reactions with water

Period three reactions with water

The oxidising ability of the halogens

look at the following link:

The Oxidising power of the halogens

Electron Affinity Trends

look at this link on electronegativity trends. Notice how these trends sometimes fall down, e.g. the value for F(g) is lower than the trend would suggest. Can you think why this is so?

Electron Affinity Trends


Note: This is not the same as electronegativity. Electronegativity values relate to the ability of an atom within a covalent bond to pull the bonding electrons towards itself. A significant difference in electronegativity values leads to a polar bond. This may or may not lead to a polar molecule, depending if the dipoles cancel due to symmetry in the molecule. If they cancel the molecule will be overall non polar, if they don't it will be polar.

Periodic Patterns

This link gives you a menu to select physical and chemical properties and the trends in the Periodic Table:

Trends in the Periodic Table

Ionisation Energy Trends

This is a useful link that explains the trend in ionisation values in the Periodic Table:

Trends in ionisation energy

Group 1 Metals Reacting With Water

Can you spot the "interesting science" in this video?

Explain carefully the trend in reactivity.

Notes from Atomic Structure Test

Notes from atomic Structure test:

The following are points that you need to clarify in order to improve your understanding and marks on this unit:

·      Copper has the electronic configuration of [Ar] 3d¹⁰ 4s¹ (remember also that Cr has a half filled d subshell)

·      Although the 4s orbital fills before the 3d. The 4s electrons are removed first.

·      The definition of RAM! The weighted average of an atom relative to a 1/12 of carbon-12

·      The sequence in a mass spectrometer is: vaporization, ionization, acceleration, deflection and detection

·      Acceleration is caused by a very high positive potential difference

·      Deflection is caused by an electromagnet around the main tube, causing the positive ions to move in a curved path

·      The amount of deflection depends on the mass to charge ratio (m/z). The higher the mass and lower the charge the bigger the m/z ratio and the less the deflection. The lower the m/z ratio, the smaller the mass and or the bigger the charge, the greater the deflection

·      Big jumps in ionization energy relate to breaking into the next quantum level. This value is much higher as its nearer the nucleus and the quantum level is full

Atomic Structure Summary notes

This is a useful summary and links to the syllabus. You can download it via facebook, or just see it through this link:

Atomic Structure Summary

Video clip on orbitals

You may find this useful. here is a video clip of another teachers's lecture on this. What do you think? Does it make more sense than mine? The clip below shows the orientation of the orbitals.

Internal Assessments

You need to look carefully at the assessment criteria given in the interactive IB site. A link to this is on this blog. In addition you may find the following link useful.

Internal assessment

Link to Assessment criteria