MS-PS1-1: Modeling Atoms in Molecules and Crystals
Also called NGSS MS-PS1-1 · Topic: Structure and Properties of Matter · Physical Science
The Official Wording
Develop models to describe the atomic composition of simple molecules and extended structures.
Clarification statement: Emphasis is on developing models of molecules that vary in complexity. Examples of simple molecules could include ammonia and methanol. Examples of extended structures could include sodium chloride or diamonds. Examples of molecular-level models could include drawings, 3D ball and stick structures, or computer representations showing different molecules with different types of atoms.
Assessment boundary: Assessment does not include valence electrons and bonding energy, discussing the ionic nature of subunits of complex structures, or a complete description of all individual atoms in a complex molecule or extended structure is not required.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-PS1-1 Means in Plain English
Middle schoolers build and draw models that show which atoms make up a substance. They model small molecules, like water or ammonia, with a fixed number of atoms, and compare them with structures like table salt or diamond, where atoms repeat in a pattern.
Models show which atoms and how many, not how electrons hold them together, and students need not map every atom in a big structure. This builds on 5-PS1-1, where matter is made of particles too small to see, and leads to chemical reactions in MS-PS1-2.
MS-PS1-1 I Can Statements
- I can build a model that shows the atoms in a molecule.
- I can tell a single molecule from a repeating crystal structure.
- I can use a model key to name each kind of atom.
MS-PS1-1 Worked Example
A gumdrop build a class can finish in one period, plus a pinch of table salt.
How do the atoms in ammonia compare with the atoms in a grain of salt?
Set It Up: Gumdrops in five colors, toothpicks, a pinch of table salt, a hand lens and a color key.
- Write a key: yellow is H, red is O, blue is N, green is Cl, purple is Na.One color per kind of atom
- Build water from one red and two yellow gumdrops, then ammonia from one blue and three yellow.H₂O has 3 atoms, NH₃ has 4
- Look at some salt grains through the hand lens.Tiny cubes with flat faces
- Build a cube of salt with green and purple gumdrops alternating, then stack a second layer on top.The grid can keep growing
- Ask whether one more gumdrop on the water model still makes water.Water stops at 3 atoms, salt keeps repeating
Water and ammonia are molecules with a set count of atoms, while salt is a grid of Na and Cl that grows in every direction.
Why it works: A molecule is a closed group of a few atoms, but in an extended structure like salt the pattern repeats over and over, which is why salt grains form cubes.
Common Mistakes with MS-PS1-1
Mixing up atoms and molecules
“Water is one atom.”
Try this: An atom is one building block. A water molecule is three atoms joined: one oxygen and two hydrogen, so count each gumdrop.
Treating salt as one small molecule
A salt model with just one sodium and one chlorine
Try this: Salt has no single small unit. Show a grid where sodium and chlorine alternate and the pattern repeats in every direction.
A model with no key
Colored gumdrops with no labels
Try this: Add a key that names the element each color stands for, so anyone can read which atoms the model contains.
Where MS-PS1-1 Fits
Practice MS-PS1-1
Quick Check
- How many atoms are in one molecule of ammonia, NH₃?
- Why do salt grains look like tiny cubes?
- Diamond is pure carbon. Is it one small molecule?