Slide presentation
Early Chemical Discoveries and the Atomic Theory
General Chemistry • Atoms
Topic target
Early chemical discoveries built the atomic theory
Chemists did not begin by seeing atoms. They measured masses, compared substances, and looked for patterns that repeated every time.
Learning target: explain how conservation of mass, definite proportions, and multiple proportions support the idea that matter is made of atoms.
Why it matters
Atomic theory makes chemistry quantitative
Predict products
Balanced equations work because atoms are rearranged, not invented. For example, 2H2 + O2 → 2H2O conserves each element.
Identify substances
Pure compounds have fixed composition. Water is always made of hydrogen and oxygen in the same mass ratio.
Measure invisible particles
Stoichiometry, molar mass, and formulas all depend on the idea that matter consists of countable atoms.
Big idea: early chemistry turned observations into laws. Dalton’s atomic theory gave those laws a particle-level explanation.
Core model
Chemical reactions rearrange atoms
In a closed system, the same atoms present before a reaction are still present after the reaction. The atoms may be connected in new ways, but their total mass is conserved.
Vocabulary
The key terms behind early atomic theory
| Term | Meaning | Particle-level idea |
|---|---|---|
| Atom | The smallest unit of an element that keeps that element’s identity. | Atoms of one element are different from atoms of another element. |
| Element | A pure substance made of one type of atom. | Carbon contains carbon atoms; oxygen contains oxygen atoms. |
| Compound | A pure substance made from atoms of different elements chemically combined. | Water, H2O, contains hydrogen and oxygen atoms in a fixed ratio. |
| Chemical reaction | A process that changes how atoms are connected. | Atoms are rearranged; they are not created or destroyed. |
Three laws
The evidence came from mass patterns
Conservation of mass
In a closed system, total mass stays constant during a chemical reaction.
\(m_{\text{before}} = m_{\text{after}}\)
Definite proportions
A pure compound always has the same elements in the same mass ratio.
H2O has a fixed H:O mass ratio.
Multiple proportions
When two elements form more than one compound, the masses combine in small whole-number ratios.
CO and CO2 show a 1:2 oxygen ratio for the same carbon mass.
Interactive model
Test conservation of mass in a closed system
Move the sliders to change the starting masses for magnesium and oxygen forming magnesium oxide, MgO.
Dynamic relationship
Multiple proportions reveal whole-number atom ratios
Compare two compounds made from the same two elements. Keep one element’s mass fixed and compare how much of the other element combines with it.
| Compound pair | Fixed element mass | Other element masses | Simple ratio |
|---|---|---|---|
| CO and CO2 | 12.0 g C | 16.0 g O and 32.0 g O | 1:2 |
Small whole-number ratios make sense if compounds form from whole atoms, not arbitrary fractions of atoms.
Worked example
Use conservation of mass to find a missing product mass
Carbon reacts with oxygen to form carbon dioxide:
Known values: 12.0 g carbon reacts completely with 32.0 g oxygen.
Question: What mass of CO2 forms?
- Use the conservation relationship: \(m_{\text{reactants}} = m_{\text{products}}\).
- Substitute the known masses: \(12.0\ \mathrm{g} + 32.0\ \mathrm{g} = m_{\mathrm{CO_2}}\).
- Add the masses: \(m_{\mathrm{CO_2}} = 44.0\ \mathrm{g}\).
- Final answer: 44.0 g CO2 forms.
Common misconception
“Mass is lost because gas escapes” is not the same as conservation
The mistake
A student burns a substance in an open container and sees the measured mass decrease. The student concludes that atoms were destroyed.
The correction
The system was open. Gas particles may have escaped into the air. Conservation of mass applies to the complete closed system, including all gases.
Practice check
Can you connect the evidence to atoms?
A sample of pure water contains 2.0 g hydrogen and 16.0 g oxygen. Another sample of pure water contains 4.0 g hydrogen. How much oxygen should it contain?
Use the law of definite proportions: the mass ratio must remain the same.
Show answer
The first sample has an oxygen-to-hydrogen mass ratio of \(16.0\ \mathrm{g} : 2.0\ \mathrm{g} = 8.0 : 1.0\).
For 4.0 g hydrogen, oxygen mass is \(4.0\ \mathrm{g} \times 8.0 = 32.0\ \mathrm{g}\).
Answer: 32.0 g oxygen.
Application
How this topic supports later chemistry problems
Historical logic
Dalton’s atomic theory explained the laws
- Matter is made of tiny particles called atoms.
- Atoms of one element have characteristic properties.
- Compounds form when atoms of different elements combine in fixed whole-number ratios.
- Chemical reactions rearrange atoms.
Final summary
Most important takeaways
Mass is conserved
In a closed system, reactions rearrange atoms, so total mass stays constant.
Compounds are fixed
Pure compounds have definite proportions because their atoms combine in fixed ratios.
Ratios reveal atoms
Multiple proportions support the idea that atoms combine as whole particles.