The atomic-molecular model
Lavoisier's and Proust's laws describe what happens in a reaction, but they do not explain why it happens. The answer came with one idea: all matter is made of atoms, and in a reaction those atoms do not disappear, they are simply rearranged.
Atoms that regroup
When an electric current is passed through liquid water, two different gases are obtained (hydrogen and oxygen), but the combined mass of the two gases is exactly equal to that of the water used up. How is it possible for one substance to turn into completely different ones without a single gram too many or too few?
According to the atomic-molecular model, elements are made of atoms, and these join together to form molecules. In a chemical reaction, the bonds between atoms are broken and new ones are formed, but the atoms themselves are neither created nor destroyed.
Complete the sentence with the missing words: 'According to the atomic-molecular model, all matter is made up of $\dots$. These group together to form $\dots$ and, in a…
Complete the sentence with the missing words: 'According to the atomic-molecular model, all matter is made up of $\dots$. These group together to form $\dots$ and, in a chemical reaction, they do not disappear: they simply $\dots$ to form new substances.'
When ice melts, the molecules remain $H_2O$; when water is decomposed by an electric current, $H_2$ and $O_2$ are produced. The figure shows the particles in both…
When ice melts, the molecules remain $H_2O$; when water is decomposed by an electric current, $H_2$ and $O_2$ are produced. The figure shows the particles in both processes.
Use the atomic-molecular model to explain which of the two changes is a chemical change and why.
Reading a chemical formula
A formula such as $H_2O$ is not just a name: it is an exact recipe showing which atoms, and how many, make up the molecule.
The formula of water is $H_2O$. The figure shows its space-filling model. a) How many hydrogen atoms does a water molecule have? b) How many oxygen atoms? c) How many…
The formula of water is $H_2O$. The figure shows its space-filling model.
- a) How many hydrogen atoms does a water molecule have?
- b) How many oxygen atoms?
- c) How many atoms are there in total in the molecule?
The formula of carbon dioxide is $CO_2$. State which elements form its molecule and how many atoms of each element it contains.
The formula of carbon dioxide is $CO_2$. State which elements form its molecule and how many atoms of each element it contains.
Match each chemical symbol to its element: $H$, $O$, $C$ and $N$, with the names hydrogen, carbon, nitrogen and oxygen.
Match each chemical symbol to its element: $H$, $O$, $C$ and $N$, with the names hydrogen, carbon, nitrogen and oxygen.
Joined atoms and loose atoms
Writing $O_2$ is not the same as writing $2\ O$, even though both contain two oxygen atoms. The difference lies in whether they are joined together to form a molecule or are separate.
Look at the figure: Explain the difference between writing $O_2$ and writing $2\ O$: how many oxygen atoms are there in each case and how are they arranged (bonded…
Look at the figure:
Explain the difference between writing $O_2$ and writing $2\ O$: how many oxygen atoms are there in each case and how are they arranged (bonded together or separate)?
We want to build a model of the water molecule using plasticine balls: we use red balls for the oxygen atoms and white balls for the hydrogen atoms. a) How many balls of…
We want to build a model of the water molecule using plasticine balls: we use red balls for the oxygen atoms and white balls for the hydrogen atoms.
- a) How many balls of each colour do you need for a water molecule?
- b) Describe how the balls would be joined together.
Count carefully: a) How many hydrogen and oxygen atoms are there in $3\ H_2O$? b) How many carbon and oxygen atoms are there in $5\ CO_2$?
Count carefully:
- a) How many hydrogen and oxygen atoms are there in $3\ H_2O$?
- b) How many carbon and oxygen atoms are there in $5\ CO_2$?
Balanced chemical equations
A chemical equation is a sentence written with formulae: the reactants on the left, the products on the right, and an arrow meaning "turn into". To obey Lavoisier's law, it has to be balanced: the same number of atoms of each element on both sides.
Move the coefficients until the table below says "balanced" for every element. Notice: the subscripts of a formula are never changed, only the coefficients in front.
In the formation of water, two hydrogen molecules ($2\ H_2$) react with one oxygen molecule ($O_2$) to form two water molecules ($2\ H_2O$), as shown in the particle…
In the formation of water, two hydrogen molecules ($2\ H_2$) react with one oxygen molecule ($O_2$) to form two water molecules ($2\ H_2O$), as shown in the particle diagram.
Count the hydrogen and oxygen atoms before and after the reaction. Has any atom disappeared?
The formula of ammonia is $NH_3$. a) How many atoms are there in an ammonia molecule? b) How many atoms of each element are there in $2\ NH_3$?
The formula of ammonia is $NH_3$.
- a) How many atoms are there in an ammonia molecule?
- b) How many atoms of each element are there in $2\ NH_3$?
The combustion of methane is written as follows: $CH_4 + 2\ O_2 \rightarrow CO_2 + 2\ H_2O$. The diagram represents the molecules before and after. Count the atoms of…
The combustion of methane is written as follows: $CH_4 + 2\ O_2 \rightarrow CO_2 + 2\ H_2O$. The diagram represents the molecules before and after.
Count the atoms of each element before and after the reaction and verify that they are conserved.
Counting atoms with coefficients
When an equation has coefficients and subscripts at the same time, you have to multiply them to count the atoms correctly.
In a ball model, black balls are carbon atoms, white balls are hydrogen atoms, and red balls are oxygen atoms. A black ball joined to four white balls represents…
In a ball model, black balls are carbon atoms, white balls are hydrogen atoms, and red balls are oxygen atoms. A black ball joined to four white balls represents methane, $CH_4$.
- a) Which molecule is represented by a black ball joined to two red balls?
- b) Describe the water molecule using the balls from this model.
The formation of water is represented as follows: $2\ H_2 + O_2 \rightarrow 2\ H_2O$. a) How many molecules are there before and after the reaction? b) How many atoms of…
The formation of water is represented as follows: $2\ H_2 + O_2 \rightarrow 2\ H_2O$.
- a) How many molecules are there before and after the reaction?
- b) How many atoms of each type are there before and after?
- c) Use the atomic-molecular model to justify the law of conservation of mass.
When an electric current is passed through water, it decomposes: $2\ H_2O \rightarrow 2\ H_2 + O_2$. The figure shows the experimental apparatus. a) Count the atoms of…
When an electric current is passed through water, it decomposes: $2\ H_2O \rightarrow 2\ H_2 + O_2$. The figure shows the experimental apparatus.
- a) Count the atoms of each element before and after.
- b) In the experiment, twice the volume of hydrogen is collected compared to oxygen: relate this to the number of molecules of each gas formed.
- c) Is mass conserved in this decomposition? Justify your answer.
Back to the start: Mass is conserved (Lavoisier) because the reacting atoms just change partners without being destroyed, and proportions are fixed (Proust) because each molecule is always formed with a constant, identical number of each type of atom.