Atoms & Models
What is the smallest unit of matter that makes up all substances?
Atom
Who created the first periodic table of elements?
Dmitri Mendeleev
What is a molecule?
Two or more atoms bonded together.
Give one example of a physical property that helps identify a substance.
Examples: melting point, density, solubility, boiling point.
What is a solution?
A mixture where one substance dissolves in another.
Why do scientists use models when studying atoms?
Atoms are too small to see; models help visualize structure and behavior.
Why did they leave blank spaces in the periodic table?
To predict elements not yet discovered.
What is a compound?
A molecule made of different types of atoms.
Why is melting point more reliable for identifying a substance than color or shape?
It doesn’t change based on size or shape; it’s unique to each substance.
What happens when a solvent (like water) cannot dissolve any more solute (like sugar)?
It becomes saturated — no more solute can dissolve.
What kind of model helps scientists understand how atoms bond to form molecules?
Ball-and-Stick model (or molecular model)
What property was originally used to organize the elements?
Atomic mass (modern table uses atomic number).
Why do two substances made of the same atoms sometimes have different properties?
Their atoms are arranged differently or in different ratios.
What happens to the particles of a substance when it melts?
Particles move faster and spread farther apart.
What can you do to make more solute dissolve in a saturated solution?
Add more solvent or heat the solution.
What does a model of an atom help scientists understand about matter that they cannot see directly?
It shows how atoms are arranged, move, and combine to form substances. (things too small to observe directly)
How is the modern periodic table arranged differently from Mendeleev’s version?
Modern table is arranged by increasing atomic number, not mass.
A molecule has 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom. Write its chemical formula.
C₂H₆O (ethanol).
Which property could you test to identify which sample of white powder is sugar and which is salt?
Test the melting point — it’s a unique property.
Why does oil float on top of water?
Oil is less dense than water.
Explain why models are useful but also have limitations when showing atomic structures.
Models help visualize size, shape, and bonding but cannot show true scale, energy, or motion accurately.
Predict what scientists could learn if a new element fits perfectly into a blank space on the periodic table.
Scientists can predict the element’s properties before discovering it.
Compare a simple molecule (like water) to an extended structure (like salt). How are they different?
Simple molecules have a fixed number of atoms; extended structures repeat in patterns (lattice).
Explain how temperature can affect the physical properties of a substance, such as density or state.
Increasing temperature usually makes particles move faster and farther apart — lowering density and changing the state (solid → liquid → gas).
How are molecular spacing and density related to whether something sinks or floats?
The closer particles are packed, the greater the density; less dense materials float.