Unit 12: Non-metals, Noble Gases & Halogens
Introducing the diverse elements of the upper right periodic table and their physical characteristics.
12.1 Introduction to Non-metals
Non-metals are a group of elements located on the upper right-hand side of the periodic table. Unlike metals, they do not possess metallic bonding. Instead, their atoms are held together by strong covalent bonds, or in the case of noble gases, they exist as individual atoms.
Their physical properties are highly variable and depend on their structure:
- Simple Molecular Structures (e.g., $O_2, N_2, Cl_2$): These elements exist as discrete molecules held together by weak intermolecular forces (van der Waals forces). Consequently, they have low melting and boiling points and are often gases at room temperature. They are soft, brittle, and poor conductors of electricity.
- Giant Covalent Structures (e.g., Carbon (diamond), Silicon): In these structures, atoms are linked by a continuous network of strong covalent bonds. This results in very high melting and boiling points, extreme hardness, and being poor electrical conductors (with exceptions like graphite).
In general, non-metals are characterized by being poor conductors of heat and electricity, having a dull appearance (non-lustrous), and being brittle when solid.
Solved Examples:
-
Explain why chlorine ($Cl_2$) is a gas at room temperature while carbon in
the form of diamond is a solid with a very high melting point.
Solution: Chlorine has a simple molecular structure. Strong covalent bonds exist within the $Cl_2$ molecules, but only weak intermolecular forces exist between them. Little energy is needed to overcome these weak forces, so it's a gas. Diamond has a giant covalent structure where each carbon atom is bonded to four others by strong covalent bonds. A vast amount of energy is needed to break this extensive network, giving it a very high melting point. -
Is sulfur (S) expected to be a good conductor of electricity?
Explain.
Solution: No. Sulfur is a non-metal that exists as simple molecules (e.g., $S_8$ rings). Its electrons are held tightly within covalent bonds and are not free to move and carry an electric current.
12.2 The Noble Gases (Properties & Uses)
The noble gases (Group 18: He, Ne, Ar, Kr, Xe, Rn) are a unique family of non-metals. Their defining characteristic is their chemical inertness (unreactivity). This is because they have a full outer shell of valence electrons, making them extremely stable.
Physical Properties
- They exist as individual, unbonded atoms (monatomic gases).
- They are colorless, odorless, and non-flammable.
- They have very low melting and boiling points because the only forces between the atoms are extremely weak van der Waals forces.
- Boiling points and densities increase down the group as the number of electrons per atom increases, leading to stronger van der Waals forces.
Uses
Their unreactivity is key to their applications:
- Helium (He): Used in balloons and airships because it is much less dense than air and is non-flammable (unlike hydrogen).
- Neon (Ne): Used in advertising signs, as it glows with a distinct red-orange color when an electric current is passed through it.
- Argon (Ar): Used to provide an inert atmosphere for welding and in incandescent light bulbs to prevent the hot filament from reacting with oxygen.
Solved Examples:
-
Explain the trend in boiling points of the noble gases as you go down the
group.
Solution: As you go down Group 18, the number of electrons in each atom increases. This leads to stronger temporary dipoles and therefore stronger van der Waals forces of attraction between the atoms. More energy is required to overcome these stronger forces, resulting in an increase in boiling points. -
Why is argon used in light bulbs?
Solution: The filament in a light bulb gets extremely hot. If it were surrounded by air, it would quickly react with oxygen and burn out. Argon is used because it is chemically inert and will not react with the hot filament, allowing it to last much longer.
12.3 Halogens: Appearance & Physical Properties
The halogens (Group 17: F, Cl, Br, I) are a group of reactive non-metals. In their elemental form, they exist as diatomic molecules ($F_2, Cl_2, Br_2, I_2$) with a single covalent bond between the atoms.
Physical Trends Down the Group
| Halogen | State at Room Temp. | Color | Melting Point (°C) | Boiling Point (°C) |
|---|---|---|---|---|
| Fluorine ($F_2$) | Gas | Pale yellow | -220 | -188 |
| Chlorine ($Cl_2$) | Gas | Greenish-yellow | -101 | -35 |
| Bromine ($Br_2$) | Liquid | Red-brown | -7 | 59 |
| Iodine ($I_2$) | Solid | Grey-black | 114 | 184 |
- State of Matter: The halogens are the only periodic group to contain elements in all three states at room temperature.
- Color: The color of the halogens becomes darker and more intense down the group.
- Melting & Boiling Points: These increase down the group. Like the noble gases, this is due to an increase in the number of electrons per molecule, which leads to stronger van der Waals forces between the diatomic molecules.
Solved Examples:
-
Explain why iodine is a solid at room temperature while chlorine is a
gas.
Solution: An iodine molecule ($I_2$) has many more electrons (106) than a chlorine molecule ($Cl_2$) (34). This results in significantly stronger van der Waals forces between iodine molecules compared to chlorine molecules. More energy is required to overcome these forces in iodine, giving it a much higher melting and boiling point, making it a solid at room temperature. -
What is the physical state of bromine at 25 °C?
Solution: Bromine is a liquid at room temperature (25 °C), as this temperature is between its melting point (-7 °C) and boiling point (59 °C). - Predict the physical state of Astatine (At), the halogen below
iodine.
Solution: Following the trend, the intermolecular forces in astatine would be even stronger than in iodine. Therefore, astatine is predicted to be a solid at room temperature with an even higher melting point than iodine. - Why do halogens exist as diatomic
molecules?
Solution: Halogen atoms have seven valence electrons. By forming a single covalent bond with another halogen atom, each atom can share an electron and achieve a stable outer shell of eight electrons (an octet). - Describe the appearance of iodine
vapor.
Solution: When solid grey-black iodine is gently heated, it sublimes to form a purple vapor. - What holds the diatomic molecules together in solid
iodine?
Solution: Weak intermolecular forces of attraction (van der Waals forces) hold the discrete $I_2$ molecules together in the crystal lattice.