Atoms join to reach a stable outer shell, either by giving and taking electrons or by sharing them.
An electrovalent bond forms by the transfer of electrons from a metal atom to a non-metal atom, giving oppositely charged ions that attract each other. Each ion ends up with a stable outer shell.
Show how sodium chloride forms in terms of electrons.
Na (2, 8, 1) loses one electron to become Na⁺ (2, 8); Cl (2, 8, 7) gains it to become Cl⁻ (2, 8, 8). The two ions attract
How many electrons does magnesium lose in forming MgCl₂, and why are two chlorine atoms needed?
Mg (2, 8, 2) loses two electrons; each Cl atom accepts only one, so two Cl atoms are needed
Why does solid sodium chloride not conduct electricity, while molten sodium chloride does?
In the solid the ions are fixed in the lattice; on melting they become free to move and carry current
A covalent bond forms by the sharing of electron pairs between atoms. Sharing one pair gives a single bond, two pairs a double bond and three pairs a triple bond. Covalent compounds usually have low melting and boiling points and do not conduct electricity.
How many electron pairs are shared in a nitrogen molecule, N₂?
Three pairs, so there is a triple bond. Each nitrogen atom has 5 valence electrons and needs 3 more
Why do covalent compounds generally have low melting points?
Their molecules are held together by weak forces, which need little energy to overcome
In a coordinate bond, both electrons of the shared pair come from one atom, the donor, which has a lone pair. The ammonium ion forms when ammonia donates its lone pair to a hydrogen ion.
How is the ammonium ion, NH₄⁺, formed from NH₃?
The nitrogen atom of NH₃ donates its lone pair to an H⁺ ion, forming a coordinate bond
1. Define an electrovalent bond.
2. How many electrons does Mg lose when it forms MgCl₂?
3. Why is molten NaCl a conductor of electricity?
4. How many electron pairs are shared between the atoms of an oxygen molecule, O₂?
5. Which of these has covalent bonds: NaCl, CH₄, MgO, KCl?
6. Why do covalent compounds have low melting points?
7. What is a coordinate bond?
8. Name an ion that contains a coordinate bond.
9. How many lone pairs does the nitrogen atom in NH₃ have?
10. Why can an aqueous solution of NaCl conduct electricity but solid NaCl cannot?