4.4.2 Reactivity

This topic explains reactivity — the ability of an element to react with other elements to form new compounds.

Definition

The capability of an element to react with other elements to form new compounds is called its reactivity.

Simple explanation: Reactivity is how easily and how vigorously an element reacts with other elements.

Main Idea

Reactivity of elements generally follows two trends in the periodic table:

• Down a group: reactivity increases.
• Across a period: reactivity varies — elements on the left and right sides are highly reactive, but in different ways.

Simple explanation: Elements on the far left and far right of the periodic table are the most reactive — but for different reasons.

Core Concept

Reactivity Down a Group

Reactivity of elements generally increases as you move down a group.

• This is due to the increase in atomic size.
• The outermost electrons are farther from the nucleus.
• They experience weaker attractive forces.
• It becomes easier for them to participate in chemical reactions.

Simple explanation: Bigger atoms hold their outer electrons more loosely, so they react more easily.

Reactivity Across a Period

Reactivity tends to vary across a period.

• Elements on the left side of a period (Group 1 and 2) are highly reactive due to their strong tendency to lose electrons and form positive ions.
• Elements on the right side of a period (Group 16 and 17) are highly reactive as well but tend to gain electrons to form negative ions.

Simple explanation: Both ends of a period are reactive — left side metals lose electrons; right side non-metals gain electrons.

Detailed Explanation

Why Reactivity Increases Down a Group

• Atomic size increases.
• Shielding effect increases.
• Valence electrons are farther from the nucleus and feel a weaker attraction.
• This makes them easier to lose (for metals) or easier to share/gain (for non-metals).
• So reactivity increases.

• Li reacts with water steadily.
• Na reacts more vigorously.
• K reacts violently.
• Rb and Cs react explosively.

Simple explanation: The farther down the group, the more violently the metal reacts with water.

Why Reactivity Varies Across a Period

Across a period, elements show different types of reactivity:

• Strong tendency to lose electrons.
• Form positive ions.
• Highly reactive with non-metals (like oxygen, chlorine).

• Strong tendency to gain electrons.
• Form negative ions.
• Highly reactive with metals (like sodium, potassium).

• Less reactive in the same way.
• Often form covalent compounds.

Simple explanation: Metals on the left want to give away electrons; non-metals on the right want to take electrons. Both are reactive — just in opposite ways.

Reactivity and Metallic Character

Reactivity is closely related to metallic character:

• For metals, more metallic = more reactive.
• For non-metals, more non-metallic = more reactive.

Intermediate Level

Why Reactivity Increases Down a Group

• Ionization energy decreases → easier to lose electrons.
• Atomic size increases → outer electrons farther from nucleus.
• Shielding effect increases → weaker attraction between nucleus and valence electrons.
• These factors make the element more reactive.

Why Metals and Non-Metals Are Reactive in Different Ways

Simple explanation: Metals react by giving away electrons; non-metals react by taking electrons. Both are reactive, but in opposite ways.

Reactivity Series of Metals

The reactivity series of metals arranges metals from most reactive to least reactive:

Most reactive: K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au :Least reactive

• Group 1 metals (K, Na) are the most reactive.
• Transition metals (Fe, Cu, Ag, Au) are less reactive.
• Gold (Au) is very unreactive.

Reactivity of Halogens

The halogens (Group 17) show a clear trend:

Simple explanation: Fluorine is the most reactive halogen; iodine is the least. Reactivity decreases down the group for halogens.

Note on Halogen Reactivity

While reactivity for metals increases down a group, reactivity for halogens decreases down a group. This is because halogens react by gaining electrons, and the ability to gain electrons decreases as atomic size increases.

Simple explanation: For metals, bigger = more reactive. For halogens, smaller = more reactive.

Important Points

• - Reactivity: capability of an element to react with other elements to form new compounds.
• - Down a group: reactivity increases (for metals).
• - Across a period: reactivity varies — left and right sides are highly reactive in different ways.
• - Metals (left side) → lose electrons → form positive ions.
• - Non-metals (right side) → gain electrons → form negative ions.
• - Group 1 metals → very reactive, increasing down the group.
• - Group 17 halogens → very reactive, decreasing down the group.
• - Noble gases (Group 18) → very unreactive.
• - Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au.

Remember

• - Reactivity = how easily an element reacts.
• - Down a group → reactivity increases (for metals).
• - Across a period → reactivity varies.
• - Metals lose electrons; non-metals gain electrons.
• - Group 1 → very reactive (increases down).
• - Group 17 → very reactive (decreases down).
• - Noble gases → unreactive.
• - Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au.

Common Mistakes

• 1. Thinking reactivity increases down a group for all elements. It increases for metals, but decreases for halogens.
• 2. Thinking noble gases are reactive. They are very unreactive (full valence shells).
• 3. Confusing reactivity with metallic character. They are related but not identical — reactivity depends on the type of reaction.
• 4. Thinking all metals have the same reactivity. They vary widely — K is much more reactive than Au.
• 5. Forgetting that non-metals are also reactive. They react by gaining electrons, not losing them.
• 6. Thinking reactivity only applies to metals. Non-metals (like F₂, Cl₂) are also highly reactive.

Quick Revision

• Reactivity: capability of an element to react with other elements to form new compounds.
• Down a group: reactivity increases (due to increase in atomic size; outer electrons farther from nucleus; weaker attraction).
• Across a period: reactivity varies — left side (Groups 1–2) highly reactive metals; right side (Groups 16–17) highly reactive non-metals.
• Metals react by losing electrons; non-metals react by gaining electrons.
• Group 1 metals: very reactive; reactivity increases down the group.
• Group 17 halogens: very reactive; reactivity decreases down the group.
• Noble gases (Group 18): very unreactive.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au.

60-Second Revision

• Reactivity = how easily an element reacts.
• Down a group → increases (for metals).
• Across a period → varies.
• Metals lose electrons; non-metals gain electrons.
• Group 1 → reactive (increases down).
• Group 17 → reactive (decreases down).
• Noble gases → unreactive.
• Warning: reactivity does not increase down a group for halogens — it decreases.