Atomic Foundations of Matter · Lesson 8 of 14
Bonding by Electron Transfer — Ionic Bond
“Electron transfer creates cations and anions whose opposite charges attract.”
• Explain why losing electrons produces a cation. • Explain why gaining electrons produces an anion. • Define ions and ionic bonds. • Describe sodium chloride formation through electron transfer. • Predict common ions such as oxide, sulfide, magnesium and calcium ions. • Construct ionic combinations whose total charge is neutral.
Sharing is one route to a stable valence shell. Another route is electron transfer. An atom with only a small number of valence electrons can lose them, while another atom that is close to completing its octet can gain electrons. This creates charged particles.
Bonding by Electron Transfer — Ionic Bond
Formation of a Sodium Cation
Sodium has atomic number 11 and the electronic configuration 2, 8, 1. It can achieve the stable arrangement 2, 8 by losing its one valence electron. After losing that electron, the sodium species still contains 11 protons but only 10 electrons.
A positively charged ion formed when an atom has fewer electrons than protons, commonly because it has lost one or more electrons.
The positive charge arises from the imbalance between protons and electrons. Sodium has not gained a positive particle; it has lost a negatively charged electron.
Formation of a Chloride Anion
Chlorine has seven valence electrons. It can achieve a stable octet by gaining one electron. The chloride species then has one more electron than the neutral chlorine atom and therefore carries one unit of negative charge.
A negatively charged ion formed when an atom has more electrons than protons, commonly because it has gained one or more electrons.
A charged species. Cations and anions are collectively called ions.
Formation of an Ionic Bond
Once Na⁺ and Cl⁻ have formed, their opposite charges attract one another. This electrostatic attraction holds the ions together in sodium chloride.
The electrostatic force of attraction between oppositely charged ions that holds them together.
Ions With More Than One Unit of Charge
Some atoms gain or lose more than one electron. Sulfur has six valence electrons and can gain two to complete an octet, forming S²⁻. Oxygen can similarly form O²⁻. Magnesium can lose two valence electrons to form Mg²⁺, calcium can form Ca²⁺ and potassium can lose one to form K⁺.
Problem
Explain how magnesium and chlorine combine so that the total charge is neutral.
- 1.Magnesium loses two electrons and becomes Mg²⁺.
- 2.Each chlorine atom can gain only one electron and becomes Cl⁻.
- 3.One Mg²⁺ has total charge +2.
- 4.Two Cl⁻ ions have total charge −2.
- 5.The charges cancel, so one magnesium ion combines with two chloride ions, giving the ratio represented by MgCl₂.
Problem
Explain why sodium sulfide has the composition Na₂S.
- 1.Each sodium atom loses one electron and forms Na⁺.
- 2.Sulfur gains two electrons and forms S²⁻.
- 3.Two Na⁺ ions provide total charge +2.
- 4.One S²⁻ ion provides total charge −2.
- 5.The neutral combination therefore has two sodium ions for every sulfide ion, represented by Na₂S.
Charge comes from an imbalance between proton and electron numbers. Formula ratios then follow from total charge neutrality: total positive charge and total negative charge must cancel.
Quiz
Which description best matches Cation?
Which description best matches Anion?
Which term matches this description: A positively charged ion formed when an atom has fewer electrons than protons, commonly because it has lost one or more electrons.
Which term matches this description: A negatively charged ion formed when an atom has more electrons than protons, commonly because it has gained one or more electrons.
Which statement is a key takeaway from this lesson?
Practice Problems
- What ion is expected when oxygen gains two electrons?
- Show how potassium forms K⁺ and combines with chloride.
- Show how calcium forms Ca²⁺ and combines with chloride.
- Explain why one Mg²⁺ needs two Cl⁻ ions.
- Explain why losing an electron makes sodium positive rather than negative.
Key Takeaways
• Ionic bonding involves the transfer of electrons between atoms. • Loss of electrons forms positively charged cations, while gain of electrons forms negatively charged anions. • Oppositely charged ions attract each other through electrostatic force. • The final ionic compound must contain positive and negative charges in a balanced ratio.