How Atoms Join to Form Molecules and Compounds
Atlas stands at a cluttered lab bench covered with colorful ball-and-stick molecule models, holding up a water molecule made of one red ball and two white balls snapped together, grinning as he compares it to separate jars of hydrogen gas and oxygen gas on the shelf behind him.
- Explain the difference between an atom, a molecule, and a compound using specific examples.
- Identify the fixed ratio of elements in a compound from its chemical formula.
- Compare the properties of a compound with the properties of the elements that form it.
- Predict whether a substance is a pure element or a compound based on its chemical formula.
- Describe why atoms form bonds using the idea of stability.
Key terms
- Molecule
- Two or more atoms bonded together as a single particle.
- Compound
- A substance of two or more different elements bonded in a fixed ratio.
- Fixed ratio
- The unchanging recipe of atoms in a compound, like 2 to 1 in water.
- Chemical formula
- A shorthand showing which atoms and how many are in a substance.
- Stability
- The lower-energy state atoms reach by filling their outer electron shells.
Molecule Versus Compound
When two or more atoms bond together they form a molecule, like individual building blocks snapped into one piece. A compound is a special kind of substance in which atoms of different elements join in a fixed, unchanging ratio. Water is always H2O, exactly two hydrogen atoms to one oxygen atom, every single time. Not every molecule is a compound, though. Oxygen gas, written O2, is a molecule of two atoms, but because both atoms are the same element it is not a compound.
New Properties From Bonding
The remarkable result of bonding is that a compound has properties completely unlike the elements that form it. Hydrogen is a flammable gas and oxygen supports burning, yet combined in a 2 to 1 ratio they form water, a liquid that puts out fires. Sodium is a metal that reacts violently with water and chlorine is a toxic gas, but together they form harmless table salt. Atoms bond because filling their outer electron shells makes them more stable.
Worked examples
Is O2 a compound? Explain your reasoning.
- A compound requires atoms of different elements bonded together.
- O2 is two oxygen atoms, which are the same element.
- Same-element atoms cannot form a compound, though they do form a molecule.
Answer: No, O2 is a molecule but not a compound, because both atoms are the same element.
State the carbon to hydrogen to oxygen ratio in glucose, C6H12O6.
- Read the subscripts in the formula directly.
- The subscripts are 6 for carbon, 12 for hydrogen, and 6 for oxygen.
- So the fixed ratio is 6 carbon to 12 hydrogen to 6 oxygen atoms.
Answer: 6 : 12 : 6, which is the same in every glucose molecule.
Activity
Build three different molecules by dragging atom cards into the correct bonding slots and label each as molecule-only or compound.
Practice
Decide whether table salt, NaCl, is a compound or a molecule-only substance and explain.
Compare the properties of water with the properties of hydrogen and oxygen gases.
Common mistakes to avoid
- A compound keeps its elements' properties.Atoms in a compound do not keep their original properties, so water is nothing like hydrogen or oxygen gas.
- Any two bonded atoms make a compound.A compound needs different elements, so two atoms of the same element form a molecule but not a compound.
Check your understanding
Water (H₂O) is made of hydrogen and oxygen. Which statement BEST describes water compared to hydrogen and oxygen alone?
A student sees the formula O₂ on a label. Which of the following correctly describes this substance?
The compound glucose has the formula C₆H₁₂O₆. What is the fixed ratio of carbon to hydrogen to oxygen atoms in every molecule of glucose?
Recap
A molecule is two or more atoms bonded together, while a compound is two or more different elements bonded in a fixed ratio. Compounds gain entirely new properties, which is why flammable hydrogen and oxygen form water that puts out fire.
Reflect
How can you tell from a formula whether a molecule is also a compound?