Atomic Foundations of Matter · Lesson 6 of 14
Molecules of Compounds
“Different elements share electrons to form hydrogen chloride, water and other covalent molecules.”
• Explain covalent bonding in hydrogen chloride. • Explain how two hydrogen atoms combine with one oxygen atom in water. • Interpret subscripts in molecular formulae. • Apply electron sharing to carbon dioxide, hydrogen sulfide and ammonia. • Explain why neon does not need to share or transfer electrons.
Electron sharing is not limited to atoms of the same element. Atoms of different elements can share electrons when the arrangement allows each atom to move toward a stable valence shell. The resulting neutral entity is a molecule of a compound.
Molecules of Compounds
Formation of Hydrogen Chloride
Hydrogen has one electron in its K shell and needs one more to complete a duplet. Chlorine has seven electrons in its valence shell and needs one more to complete an octet. The two atoms can therefore share one electron each, creating one shared pair.
Because one pair is shared, hydrogen and chlorine are connected by a single covalent bond. The molecule is written HCl and can be shown structurally as H—Cl.
Formation of Water
Oxygen has six valence electrons, so it needs access to two more electrons. Each hydrogen atom needs access to one additional electron. One oxygen atom can therefore share one electron with one hydrogen and another electron with a second hydrogen. Two separate shared pairs are formed, one between oxygen and each hydrogen.
Reading a Molecular Formula
A chemical formula communicates composition. In H₂O, the subscript after H tells us that two hydrogen atoms are present. Oxygen has no written subscript, which means one oxygen atom. A subscript belongs to the symbol immediately before it unless brackets are used.
Formation of Other Covalent Molecules
The same valence-electron reasoning can be applied to carbon dioxide, hydrogen sulfide and ammonia. Begin with each atom's valence electrons, determine how many additional electrons are needed for a stable outer shell, and then arrange shared pairs so that the requirements are satisfied.
Problem
Use valence requirements to reason about carbon dioxide.
- 1.Carbon has four valence electrons and needs access to four more for an octet.
- 2.Each oxygen has six valence electrons and needs access to two more.
- 3.One carbon atom can share two pairs with one oxygen and two pairs with the other oxygen.
- 4.The composition contains one carbon and two oxygen atoms, giving CO₂.
Problem
Reason about the composition of hydrogen sulfide.
- 1.Sulfur has six valence electrons and needs two additional electrons for an octet.
- 2.Each hydrogen needs one additional electron for a duplet.
- 3.Sulfur shares one pair with each of two hydrogen atoms.
- 4.The molecule therefore contains two hydrogen atoms and one sulfur atom, H₂S.
Problem
Reason about the composition of ammonia.
- 1.Nitrogen has five valence electrons and needs access to three more electrons.
- 2.Each hydrogen needs one additional electron.
- 3.Nitrogen forms one shared pair with each of three hydrogen atoms.
- 4.The composition is NH₃.
Why Neon Does Not Need to Bond
Neon has atomic number 10 and the electronic configuration 2, 8. Its outer shell is already complete, so it does not need to transfer or share valence electrons merely to achieve an octet. This is why the chapter uses neon as an example of an already stable atom.
The chapter also notes that enormous energy can be released when atomic nuclei split or combine. Nuclear energy is used in electricity generation, medicine, scientific research and space exploration. It highlights Raja Ramanna's contribution to the development and peaceful use of nuclear-energy work in India.
Quiz
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Which another statement is also a key takeaway from this lesson?
Which expression represents Hydrogen Chloride?
Practice Problems
- Explain HCl formation using the electron needs of hydrogen and chlorine.
- Explain why one water molecule contains two hydrogen atoms.
- Use electron sharing to describe the composition of CO₂.
- Use electron sharing to describe the composition of NH₃.
- Why does neon not need to gain, lose or share electrons to achieve the stable configuration described here?
Key Takeaways
• Molecules of compounds contain atoms of different elements chemically bonded together. • Covalent compounds form when atoms share electrons. • A molecular formula identifies the elements present and the number of atoms of each element. • Stable outer electron arrangements help explain why atoms form particular molecules.