Atomic Foundations of Matter · Lesson 4 of 14
How Atoms Combine?
“Atoms form stable groupings by sharing or transferring valence electrons, creating chemical bonds.”
• Define a molecule as used in the chapter. • Distinguish a molecule of an element from a molecule of a compound. • Explain why helium can exist as individual atoms. • Relate valence-shell stability to bonding. • Distinguish electron sharing from electron transfer. • Define a chemical bond.
An isolated atom is not always the most stable arrangement available. Hydrogen atoms commonly occur in pairs, oxygen occurs as molecules, and different elements can join to form substances such as hydrogen chloride and water. To understand these combinations, the electron arrangement in the outermost shell becomes central.
How Atoms Combine?
An electrically neutral entity consisting of more than one atom that is capable of independent existence and shows the properties of that substance.
Molecules of Elements and Compounds
Atoms of the same element can combine to form a molecule of that element. A hydrogen molecule contains two hydrogen atoms. Atoms of different elements can combine to form a molecule of a compound. Hydrogen chloride, for example, contains hydrogen and chlorine atoms joined together.
Not every element must form a multi-atom molecule. Helium has a stable outer shell with two electrons in its K shell, so helium can exist as individual atoms.
Valence Shell Stability
Atoms with eight electrons in the outermost shell are stable in the treatment used here; when the K shell is the outermost shell, two electrons complete it. Atoms with incomplete valence shells can become more stable by sharing, losing or gaining valence electrons.
Two General Ways of Combining
| Method | What happens |
|---|---|
| Sharing of electrons | Atoms share some or all of their valence electrons with another atom. |
| Transfer of electrons | One atom loses valence electrons and another accepts them. |
When atoms combine successfully, the total energy of the resulting arrangement is lower than the sum of the energies of the separate atoms. The resulting arrangement is therefore more stable.
The force that holds atoms together in a stable combined arrangement.
Do not decide the bond type from the formula alone at first. Begin with valence electrons: ask whether stability is reached by sharing electrons or by transferring electrons. That electron process explains the bond.
Quiz
Which description best matches Molecule?
Which description best matches Chemical Bond?
Which term matches this description: An electrically neutral entity consisting of more than one atom that is capable of independent existence and shows the properties of that substance.
Which term matches this description: The force that holds atoms together in a stable combined arrangement.
Which statement is a key takeaway from this lesson?
Practice Problems
- Give one example of a molecule of an element and one molecule of a compound.
- Why can helium remain as individual atoms?
- Explain the difference between sharing and transferring valence electrons.
- What is meant by a chemical bond in this chapter?
- Why does a lower-energy combined arrangement correspond to greater stability?
Key Takeaways
• Atoms combine to form more stable arrangements through chemical bonding. • Valence electrons are mainly responsible for the way atoms combine. • Atoms may achieve stable outer-shell arrangements by sharing or transferring electrons. • These processes lead to covalent and ionic bonding.