Periodic Table: Trends in Group 17 Elements (halogens) Chemistry Tutorial

Key Concepts

• The vertical columns in the periodic table of the elements are known as groups.
• Group 17 elements lie in the second last column from the right of the periodic table.
• Elements in Group 17 are:

  Group 17 Elements (Halogens)
  Name	Symbol
  fluorine	F
  chlorine	Cl
  bromine	Br
  iodine	I
  astatine (1)	At

• Group 17 elements are also known as:

  (a) halogens⁽²⁾ (name still in common use)

  (b) Group VIIA (name no longer used)

• Group 17 elements are non-metals
• Group 17 elements exist as diatomic molecules (X₂) when not combined with other elements.
• Group 17 elements can combine with nearly all the elements in the periodic table. These compounds can be:

  (i) ionic (eg, metal + halogen → metal halide)
  (ii) covalent (eg, non-metal + halogen → non-metal halide)

• All Group 17 (group VIIA or halogen) elements have 7 valence electrons (7 electrons in the valence shell or highest energy level).
• Atomic radius increases down Group 17 from top to bottom.
• Electronegativity decreases down group 17 from top to bottom.
• Chemical reactivity of group 17 elements decreases down group 17 from top to bottom.
• First ionization energy decreases down group 17 from top to bottom.
• Melting point and boiling point increase down Group 17 from top to bottom.
• Elements become darker in colour going down group 17 from top to bottom.
• Metallic character of the group 17 elements increases down the group from top to bottom.

Summary of trends in the properties of Group 17 elements is shown below:

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Data Table for Group 17 Elements (Halogens)

Inspect the data in the table below for Group 17 elements.

Can you identify any patterns, or trends, in the data?

Trends in the Physical Properties of Group 17 Elements

Consider the melting points and boiling points of the Group 17 elements as shown in the table below:

As you go down group 17 from top to bottom, the melting point of the elements increases and the boiling point increases.

We can use the melting point and boiling point to determine the state of each element at standard temperature and pressure (25°C, 100 kPa).

If element is a:

• gas: boiling point < 25°C
• liquid: melting point < 25°C < boiling point
• solid: 25° < melting point

So let's take another look at the melting points (M.P.) and boiling points (B.P.) of the group 17 elements and decide which are gases, liquids and solids:

We can see a trend in the states of matter. Going down Group 17 from top to bottom the elements change from gaseous state to liquid to solid.

The melting point of a substance reflects the amount of energy required to weaken the forces of attraction between molecules (intermolecular forces), the higher the melting point the stronger the forces of attraction between the molecules.
We can infer that there is a gradation in the intermolecular forces acting between the molecules such that the strongest forces of attraction act between iodine molecules and the weakest forces of attraction act between fluorine molecules.

There is also a gradation in the colour of the elements going down group 17 from top to bottom:
pale yellow to greenish-yellow to reddish-brown to gray

This is why we say that the properties of group 17 elements become more metallic in character as you go down the group from top to bottom, even though all the elements in group 17 are non-metals.

Trends in the Chemical Properties of Group 17 Elements

As you go down group 17 from top to bottom the number of occupied electron shells (or energy levels) increases as shown in the table below:

As each successive atom gains an additional electron shell (energy level), the radius of the atom increases as shown in the table below:

The negatively charged valence electrons are getting further away from the positively charged nucleus so they feel less of a "pull" towards the nucleus.
Successive filled electron shells (energy levels) are said to "shield" the valence electrons.
This means that you as go down the group from top to bottom an electron is easier to remove from the atom because it is less strongly attracted towards the nucleus.
The energy required to remove an electron from a gaseous atom is known as its "first ionisation energy".
We can see the trend in first ionisation energy of the group 17 elements decreases as we go down the group from top to bottom as shown in the table below:

While it is easier to pull an electron off an iodine atom compared to pulling an electron off an atom of fluorine, these numbers are all quite high.
It requires quite a lot of energy to remove an electron from any of these atoms.
Losing an electron is not the preferred method by which a halogen would form a compound.

If we take a look at the electronic configuration of the group 17 element atoms, we can see something that doesn't change down the group:

The atoms of group 17 elements all have 7 electrons in the valence shell (highest energy level).

So, what if a halogen atom gained an electron instead of losing an electron, as shown in the equations below:

F + e^- → F^-

Cl + e^- → Cl^-

Br + e^- → Br^-

I + e^- → I^-

What would the electronic configuration of these "halide" ions look like?
We've done this in the table below:

And this is what people mean when they refer to an "atom" completing its "octet" of electrons ("oct" means 8).
This is a stable electronic configuration.
In fact, each of these electronic configurations is now the same as a Noble Gas (very unreactive group 18 element).
We say that each halide ion is isoelectronic with its neighboring Noble Gas (Group 18) element as shown in the table below:

There is supporting evidence for this desire of a halogen atom to pull an electron towards itself from the values for electronegativity:

Firstly, all the halogen atoms are very electronegative, they are all very capable of pulling an electron towards themselves.
Remember, gaining an electron is favourable for halogens because it enables them to form an anion with the same electron configuration as a stable Group 18 (Noble Gas) element.

Secondly, this ability to attract electrons towards the nucleus of the atom decreases as you go down group 17 from top to bottom, fluorine is more electronegative than chlorine which is more electronegative than bromine which is more electronegative than iodine.
Halogen atoms are all capable of gaining an electron to form the negatively charged halide ion (general formula X^-), but fluorine will do this more "completely" than iodine for example.

As the radius of the atom increases down group 17 from top to bottom, and the valence shell electrons are increasingly shielded, the positively charged nucleus exerts less of an attractive force on the electrons so it has less ability to attract electrons towards itself, hence, electronegativity decreases down the group from top to bottom.

And this means the chemical reactivity of the group 17 elements also decreases going down the group from top to bottom!
We expect fluorine to be more reactive than chlorine, and chlorine to more reactive than bromine, and bromine to be more reactive than iodine.

So let's consider the reaction between halogens and hydrogen to produce hydrogen halides.
These hydrogen halides are all covalent molecules and exist in the gaseous state at room temperature and pressure.

There is a trend in the reactivity of the halogens, they become less reactive as you go down group 17 from top to bottom.

Halogens react with most non-metals to form covalent halides, and the reaction with fluorine is always the most vigorous!

Indeed, fluorine is so reactive that it reacts with most substances vigorously!
The ONLY reason we can store it in containers like steel is because it forms a fluoride coating on the metal surface which prevents any further reaction taking place with the fluorine.⁽⁴⁾

Halogens (group 17 elements) react with most metals to form an ionic metal halide, and the reactions are more vigorous with fluorine and least vigorous with iodine.

The halogens also react with water. Ofcourse the reaction with fluorine is vigorous, while the reaction with other halogens is much less so...

but fluorine reacts vigorously with water to produce hydrogen fluoride and oxygen gas:

2F_2(g) + 2H₂O_(l) → 4HF_(aq) + O_2(g)

while the other halogens react with water only to a slight extent to produce hypohalous acids (HOX) ⁽⁵⁾:

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