Science - Combined (0653) | The Periodic Table

🔬 Comprehensive Study Notes: C8 The Periodic Table 🖼

Welcome to one of the most important chapters in Chemistry! The Periodic Table is often called the 'map of chemistry' because it organises all the elements we know into a logical structure. Understanding this structure helps you predict how elements will behave, which is incredibly useful.
Don't worry if it looks complicated—by the end of these notes, you’ll be navigating this map like a pro!

C8.1 Arrangement of Elements in the Periodic Table

The Periodic Table is arranged based on the properties of elements. There are two main ways elements are organised:

1. Arrangement by Atomic/Proton Number

The fundamental rule of the modern Periodic Table is that elements are arranged in order of increasing Proton Number (or Atomic Number).
Remember: The proton number is the number of protons found in the nucleus of an atom.

2. Groups and Periods: The Map Coordinates

The grid structure of the table uses two labels: Groups and Periods.

• Groups (Vertical Columns):
  These run down the table (like columns in a building). Elements in the same group have the same number of outer-shell electrons. Because chemical reactions primarily involve outer electrons, elements in the same group share similar chemical properties.
• Periods (Horizontal Rows):
  These run across the table (like lines of text). Elements in the same period have the same number of occupied electron shells.

💡 Analogy: Think of the Periodic Table like an apartment building.
The Group is the Street Number (tells you what kind of neighbours you have—similar chemistry).
The Period is the Floor Number (tells you how many electron shells/floors the apartment has).

3. Trends Across a Period: Metallic to Non-Metallic Character

As you move from left to right across any period, the character of the elements changes systematically:

• Elements on the far left (Groups I and II) are metals.
• Elements in the middle sometimes show properties of both metals and non-metals (metalloids).
• Elements on the far right (Groups VII and VIII) are non-metals.

Key Takeaway: Metallic character decreases and non-metallic character increases as you move from left to right across a period.

C8.2 Group I: The Alkali Metals (Core & Supplement)

Group I elements (Lithium, Sodium, Potassium, etc.) are known as the Alkali Metals. They are very reactive metals.

General Properties of Group I Elements

• They are relatively soft metals. You can often cut them with a knife!
• They have low densities (Lithium, Sodium, and Potassium even float on water).
• They have relatively low melting points compared to other metals like Iron or Copper.

General Trends Down Group I

When observing trends going down the group (from Lithium to Caesium):

1. Melting Point Decreases: As atoms get larger, the metallic bonds become weaker, requiring less energy to melt the metal.
2. Density Increases: The atoms are heavier and packed more closely together as you move down the group.
3. Reactivity Increases: This is the most important trend!

🧩 Why does reactivity increase down the group?
Alkali metals react by losing their single outer electron to form a positive ion (\(Li^{+}\), \(Na^{+}\)). As you go down the group, the outer electron is further away from the nucleus and shielded by more electron shells. This makes the outer electron easier to lose, making the element more reactive.

Reaction with Water

Group I metals react vigorously with cold water to produce hydrogen gas and a metal hydroxide (which dissolves to form an alkaline solution).

Example (Sodium):
\(2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\)

The vigour of this reaction increases dramatically down the group (Potassium is more reactive than Sodium).

Predicting Properties (Supplement)

If you know the properties of Lithium (Li), Sodium (Na), and Potassium (K), you can predict the properties of Rubidium (Rb) or Caesium (Cs).
For example: Since melting point decreases down the group, Rubidium will have a lower melting point than Potassium. Since reactivity increases, Caesium will react even more violently with water than Potassium.

C8.3 Group VII: The Halogens (Core & Supplement)

Group VII elements (Fluorine, Chlorine, Bromine, Iodine) are known as the Halogens. They are reactive non-metals.

General Properties of Halogens

• They exist as diatomic molecules (meaning their atoms pair up), e.g., $Cl_2$, $Br_2$, $I_2$.
• They react by gaining a single electron to achieve a full outer shell, forming negative ions (halide ions, e.g., $Cl^{-}$).

Physical Appearance at Room Temperature (r.t.p.)

Halogens show a clear change of state down the group at r.t.p.:

• Chlorine (\(Cl_2\)): A pale yellow-green gas.
• Bromine (\(Br_2\)): A red-brown liquid.
• Iodine (\(I_2\)): A grey-black solid.

💭 Memory Aid: G L S (Gas, Liquid, Solid) when going down the group!

General Trends Down Group VII

When observing trends going down the group:

1. Density Increases: The elements become heavier and more closely packed.
2. Reactivity Decreases: This is opposite to Group I!

🧩 Why does reactivity decrease down the group?
Halogens react by gaining an electron. As you move down the group, the outer shell is further from the positive nucleus. The nucleus’s ability to attract and pull in an extra electron decreases, making the element less reactive.

Displacement Reactions (Supplement)

A more reactive halogen can displace a less reactive halide ion from a solution of its salt.

Think of it as a competition: The more reactive halogen is "stronger" and kicks the weaker halide ion out of its compound.

Reactivity Order (Most Reactive to Least): Chlorine > Bromine > Iodine

Example 1: Chlorine displacing Bromine
When Chlorine water ($Cl_2$) is added to Potassium Bromide solution ($KBr$), Chlorine (more reactive) displaces Bromine (less reactive):

$Cl_2(aq) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(aq)$
Observation: The solution changes from colourless to red-brown (due to the formation of liquid Bromine).

Example 2: Iodine with Potassium Bromide
Iodine ($I_2$) is less reactive than Bromine ($Br^-$), so no reaction occurs.

Predicting Properties (Supplement)

The halogen Astatine (At) is below Iodine. We can predict it would be a solid at r.t.p. (below Iodine, which is solid), have a higher density than Iodine, and be even less reactive than Iodine.

C8.4 Transition Elements (The Central Block)

The Transition Elements are the large block of metals found in the middle of the Periodic Table (Groups III to VII in the main part). Elements like Iron, Copper, and Gold are in this group.

Distinguishing Properties of Transition Elements (Metals)

Unlike Group I metals, Transition Elements have very specific characteristics:

• High Densities: They are typically very heavy (e.g., Iron).
• High Melting Points: They require a lot of energy to melt (e.g., Tungsten, used in light bulb filaments).
• Form Coloured Compounds: This is a crucial distinguishing feature. Most compounds of Group I and II metals are white, but transition metal compounds are vibrantly coloured.
  Examples: Copper(II) compounds are blue/green, Iron(II) compounds are green, and Iron(III) compounds are red/brown.
• Often Act as Catalysts: They (or their compounds) speed up chemical reactions without being used up themselves.
  Example: Iron is used as a catalyst in the Haber process (making ammonia). Platinum is used in catalytic converters in cars.

C8.5 Group VIII (or Group 0): The Noble Gases

Group VIII elements (Helium, Neon, Argon, etc.) are found on the far right of the Periodic Table. They are often called the Noble Gases.

Properties and Electronic Configuration

• Unreactive: They are chemically inert (they do not easily react with other elements).
• Monatomic Gases: They exist as single atoms (not paired like $O_2$ or $Cl_2$).
• Full Outer Shell: This is the key to their lack of reactivity. All noble gases have a complete outer electron shell (except Helium, which has 2 electrons filling its only shell; the others have 8).

Explanation of Unreactivity: Atoms react in order to achieve a stable electronic configuration—usually a full outer shell. Since Noble Gases already have a full outer shell, they do not need to lose, gain, or share electrons. This makes them extremely stable and unreactive.

Did You Know?

The name "Noble" suggests they are highly unreactive, just like noble metals (like gold) which resist corrosion. They are used when an inert atmosphere is required. For example, Argon is used in light bulbs to stop the hot filament from reacting with oxygen.