In the particle model of a water molecule, the oxygen atom (O) is shown as a larger red circle in the center, and two hydrogen atoms (H) are shown as smaller white circles. Each H is connected to the O by a single line (bond), forming one bent H–O–H molecule. Which chemical formula matches this model?
- HO₂
- H₂O (correct answer)
- H₃O
- O₂H
Explanation: This question tests understanding of how to use particle-level models to represent molecules, which are groups of atoms bonded together. A molecule is formed when two or more atoms chemically bond together in a specific arrangement—each type of atom is represented by its element symbol (H for hydrogen, O for oxygen, C for carbon, N for nitrogen), and the chemical formula uses subscripts to show how many atoms of each type are in one molecule (for example, H₂O means 2 hydrogen atoms bonded to 1 oxygen atom, while CO₂ means 1 carbon atom bonded to 2 oxygen atoms). For water H₂O: In the model shown, there are 2 hydrogen atoms (smaller circles, often white or labeled H) bonded to 1 oxygen atom (larger circle, often red or labeled O), giving a total of 3 atoms per water molecule. The bonds are shown as lines connecting each hydrogen to the central oxygen, and the bent arrangement is characteristic of water—this composition of 2 H and 1 O defines water at the molecular level. Choice B is correct because it accurately counts the atoms shown: 2 H and 1 O, giving the right chemical formula with proper subscripts matching the atom count. Choice C incorrectly counts 3 hydrogen atoms when the model clearly shows 2, making an atom counting error—carefully counting each atom type separately is essential for determining molecular composition. To interpret molecular models: (1) identify each atom type by its label, color, or size (H = small/white, O = larger/red, C = black, N = blue typically), (2) count how many atoms of each type are present in one molecule, (3) write the formula with subscripts showing the count (2 H atoms = H₂, 1 O atom = O, giving H₂O), (4) verify bonds connect atoms (lines between circles), and (5) remember that each molecule is a discrete unit—if diagram shows 3 water molecules, count atoms in just one to get H₂O, not all combined. Common mistakes: (a) counting atoms from multiple molecules instead of one, (b) confusing atom types (check labels/colors carefully), (c) writing formulas with wrong subscripts (count carefully: 2 H means H₂ not H2), (d) forgetting atoms without subscripts (H₂O has 1 oxygen even though no subscript after O), and (e) counting bonds as atoms (4 bonds doesn't mean 4 atoms necessarily).