Electron Sharing Reactions

students, when atoms react, they are trying to become more stable ⚛️. In many chemical changes, that stability comes from sharing electrons rather than fully transferring them. In this lesson, you will learn how electron sharing reactions work, how they are represented in chemistry, and why they are a key part of organic chemistry, bonding, and the broader IB Chemistry HL topic of Reactivity 3 — What Are the Mechanisms of Chemical Change?. By the end, you should be able to explain the main terms, interpret examples, and connect electron sharing to reaction pathways in real life and in exams.

What does electron sharing mean?

Electron sharing happens when atoms form covalent bonds by using a pair of electrons together. This is common in molecules made mostly of nonmetals, such as $\mathrm{H_2}$, $\mathrm{Cl_2}$, $\mathrm{CH_4}$, and $\mathrm{H_2O}$. Each atom contributes one or more electrons to the shared pair, and the shared pair is attracted to both nuclei.

A covalent bond is often shown with a line, such as $\mathrm{H-H}$ or $\mathrm{H-Cl}$. The line represents a shared pair of electrons. Electron sharing allows atoms to reach more stable electron arrangements, often a noble-gas-like outer shell.

In IB Chemistry HL, this is important because many reactions are explained by changes in electron density rather than full electron loss or gain. For example, in organic chemistry, molecules often react because a region becomes slightly electron-rich or slightly electron-poor. These regions help determine where new bonds form.

Key vocabulary

Covalent bond: a bond formed by sharing a pair of electrons.
Shared electron pair: the two electrons used in a covalent bond.
Electron density: the probability of finding electrons in a region of space.
Polar bond: a covalent bond where electrons are shared unequally.
Electronegativity: the ability of an atom to attract shared electrons.

For example, in $\mathrm{H-Cl}$, chlorine is more electronegative than hydrogen, so the shared electrons are pulled closer to chlorine. That makes chlorine slightly negative ($\delta^-$) and hydrogen slightly positive ($\delta^+$). This small charge separation is very important in reaction mechanisms.

How electron sharing affects reactivity

Not all covalent bonds are equally strong or equally reactive. The way electrons are shared affects how easily a molecule reacts. When electrons are shared equally, the bond is non-polar, like in $\mathrm{Cl_2}$. When electrons are shared unequally, the bond is polar, like in $\mathrm{H_2O}$ or $\mathrm{HBr}$.

Polar bonds create partial charges, and those partial charges guide reactions. A positively polarized atom can attract electron-rich species, while a negatively polarized atom can repel them. This is why electron sharing is central to mechanistic explanations in chemistry.

For example, consider hydrogen bromide, $\mathrm{HBr}$. The bond is polar because bromine is more electronegative than hydrogen. In water, $\mathrm{HBr}$ can ionize, but in organic reactions it can also take part in electrophilic addition reactions. The bond polarity makes the hydrogen atom electron-poor, so it can be attacked by an electron-rich species.

In many reactions, chemists describe electrons moving using curly arrows. A curly arrow starts at the source of an electron pair and ends where the pair is going. This is not a literal path traced by one electron; it is a model that shows movement of an electron pair. In IB exams, these arrows are very important for showing how bonds break and form.

Example: simple covalent bond formation

When two hydrogen atoms form $\mathrm{H_2}$, each atom contributes one electron:

$$\mathrm{H\cdot + \cdot H \rightarrow H-H}$$

The shared pair becomes the bond between the two atoms. This is the simplest example of electron sharing.

Example: polar sharing in hydrogen chloride

In $\mathrm{H-Cl}$, the electrons are shared unequally because chlorine is more electronegative. The molecule is polar, so the $\mathrm{H}$ end is $\delta^+$ and the $\mathrm{Cl}$ end is $\delta^-$. This polarity influences how the molecule reacts with water, bases, and nucleophiles.

Electron sharing in organic reaction pathways

Electron sharing is especially important in organic chemistry, where reactions are often explained by the movement of electron pairs between molecules. Many organic mechanisms begin when an electron-rich species attacks an electron-poor center.

Two important terms are:

Nucleophile: an electron-pair donor.
Electrophile: an electron-pair acceptor.

A nucleophile usually has lone pairs, negative charge, or a region of high electron density. Examples include $\mathrm{OH^-}$, $\mathrm{CN^-}$, and $\mathrm{NH_3}$. An electrophile often has a positive charge or a partial positive charge, such as the carbon in $\mathrm{C=O}$ groups or the hydrogen in $\mathrm{HBr}$.

Example: addition to an alkene

Alkenes contain a $\pi$ bond, which is a region of electron density above and below the plane of the molecule. Because the $\pi$ electrons are relatively exposed, alkenes react readily with electrophiles.

For example, in the addition of $\mathrm{HBr}$ to ethene, $\mathrm{CH_2=CH_2}$, the $\pi$ bond can attack the hydrogen atom in $\mathrm{HBr}$. The bond in $\mathrm{HBr}$ breaks unevenly, and the electrons move toward bromine.

A simplified sequence is:

$$\mathrm{CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br}$$

This reaction shows electron sharing changing during the process. A new $\mathrm{C-H}$ bond forms while the original $\mathrm{H-Br}$ bond breaks. The mechanism is better understood when you track electron pairs rather than memorizing only the overall equation.

Example: nucleophilic substitution

In a reaction such as $\mathrm{CH_3Br}$ with $\mathrm{OH^-}$, the nucleophile $\mathrm{OH^-}$ donates an electron pair to the carbon atom attached to bromine. At the same time, the $\mathrm{C-Br}$ bond breaks and bromide leaves.

$$\mathrm{CH_3Br + OH^- \rightarrow CH_3OH + Br^-}$$

This reaction is a clear example of electron sharing and bond breaking happening together. The carbon atom is slightly positive because bromine pulls electron density toward itself, so the nucleophile is attracted to it.

How to describe electron sharing with IB reasoning

IB Chemistry HL often asks students to explain reactions using structure, polarity, and mechanism. A strong answer usually does more than state that a reaction happens. It explains why it happens.

When analyzing an electron sharing reaction, ask these questions:

Where is the electron-rich region?
Where is the electron-poor region?
Which bond is likely to break?
Which new bond is likely to form?
What evidence shows this pattern?

This reasoning helps with both organic and general chemistry. For example, in a carbonyl compound such as ethanal, $\mathrm{CH_3CHO}$, the $\mathrm{C=O}$ bond is polar because oxygen is more electronegative than carbon. The carbon atom becomes electrophilic. That is why nucleophiles attack the carbonyl carbon in many reactions.

Real-world example: why alcohols are useful

Ethanol, $\mathrm{C_2H_5OH}$, can form hydrogen bonds because oxygen has lone pairs and polar bonds. Those electron-sharing features influence boiling point, solubility, and reactions. In industry and biology, the way electrons are shared affects how molecules interact with water, enzymes, and other substances.

Real-world example: polymers

Many plastics are made by addition polymerization. Monomers such as ethene join together by changing where electrons are shared. The double bond opens and new single bonds form, producing long chains. This is an example of how electron sharing can build large useful materials.

Electron sharing and mechanisms across Reactivity 3

Electron sharing is not isolated from the rest of Reactivity 3. It connects to acid-base chemistry, redox, and organic reaction pathways.

In acid-base chemistry, electron sharing explains why a base can accept a proton. A base donates a lone pair to form a new bond with $\mathrm{H^+}$. For example, in water and ammonia chemistry, the lone pair on nitrogen in $\mathrm{NH_3}$ helps form $\mathrm{NH_4^+}$.

In redox chemistry, electron transfer is the main idea, but many reactions still involve bond changes that can be understood by looking at electron density. In other words, even when the official focus is electron transfer, the detailed reaction pathway may still include polar bonds and electron sharing.

In organic mechanisms, electron sharing is the core idea. Curly arrows, nucleophiles, electrophiles, and bond polarity all describe how electrons move through a reaction.

So, electron sharing reactions matter because they help explain:

why some molecules are reactive and others are stable,
why bonds form in particular places,
how reaction mechanisms are drawn,
and how chemistry links to structure and properties.

Conclusion

Electron sharing reactions are a foundation of chemistry because they explain how covalent bonds form and how molecules react. students, if you can identify where electrons are concentrated, where they are pulled away, and how a new bond can form, you are already thinking like a chemist 🧪. In IB Chemistry HL, this skill helps you understand organic reaction pathways, predict reactivity, and write clear mechanistic explanations. Electron sharing is one of the main ideas that connects structure, bonding, and reaction change across Reactivity 3.

Study Notes

Covalent bonds form when atoms share electron pairs.
Electron sharing may be equal or unequal; unequal sharing creates polar bonds.
Electronegativity determines how strongly an atom attracts shared electrons.
Partial charges are shown as $\delta^+$ and $\delta^-$.
Nucleophiles are electron-pair donors; electrophiles are electron-pair acceptors.
Curly arrows show the movement of electron pairs in mechanisms.
Alkenes react easily because their $\pi$ electrons are exposed.
Polar bonds help explain many organic reactions, including addition and substitution.
Electron sharing connects bonding, acidity, redox ideas, and organic pathways in Reactivity 3.
Always explain reactions by linking structure, polarity, and electron movement.