Exercises: The atomic-molecular model

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.'

The formula of water is $H_2O$. The figure shows its space-filling model.

1. a) How many hydrogen atoms does a water molecule have?
2. b) How many oxygen atoms?
3. 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.

Match each chemical symbol to its element: $H$, $O$, $C$ and $N$, with the names hydrogen, carbon, nitrogen and oxygen.

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 modeling clay balls: we use red balls for the oxygen atoms and white balls for the hydrogen atoms.

1. a) How many balls of each color do you need for a water molecule?
2. b) Describe how the balls would be joined together.

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?

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.

Count carefully:

1. a) How many hydrogen and oxygen atoms are there in $3\ H_2O$?
2. b) How many carbon and oxygen atoms are there in $5\ CO_2$?

The formula of ammonia is $NH_3$.

1. a) How many atoms are there in an ammonia molecule?
2. 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 each element before and after the reaction and verify that they are conserved.

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, $CH_4$.

1. a) Which molecule is represented by a black ball joined to two red balls?
2. b) Describe the water molecule using the balls from this model.

In air, $N_2$ and $O_2$ molecules are separate and mixed together; in water, hydrogen and oxygen atoms are joined together forming $H_2O$ molecules, as shown in the figure.

Explain the difference between a mixture and a compound using the atomic-molecular model.

Explain, using the atomic-molecular model, why mass is conserved in a chemical reaction (Lavoisier's law): what happens to the atoms during the reaction and what happens to their mass?

Water is $H_2O$ and hydrogen peroxide is $H_2O_2$. The figure shows the model of both molecules.

1. a) Do they contain the same elements?
2. b) Are they the same substance?
3. c) Relate your answer to the law of definite proportions.

When water boils, the $H_2O$ molecules separate from one another, but do not break apart.

1. a) Why is steam still water?
2. b) What would have to happen to the molecules for steam to stop being water?

The formation of water is represented as follows: $2\ H_2 + O_2 \rightarrow 2\ H_2O$.

1. a) How many molecules are there before and after the reaction?
2. b) How many atoms of each type are there before and after?
3. c) Use the atomic-molecular model to justify the law of conservation of mass.

When a magnesium ribbon is burned in air, the oxide formed weighs more than the original ribbon.

1. a) Explain this gain in mass using the atomic-molecular model.
2. b) Predict what would happen to the total mass if we could also weigh all the air that took part in the reaction.

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.

1. a) Count the atoms of each element before and after.
2. 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.
3. c) Is mass conserved in this decomposition? Justify your answer.

The industrial synthesis of ammonia is written as: $N_2 + 3\ H_2 \rightarrow 2\ NH_3$.

1. a) Design a model using balls (choose a color for each element) to represent this reaction, before and after.
2. b) Verify, by counting atoms in your model, that conservation of mass is respected.
3. c) How many ammonia molecules are formed for each nitrogen molecule?

A student writes the combustion of methane in their notebook as follows: $CH_4 + O_2 \rightarrow CO_2 + H_2O$.

1. a) Count the atoms of each element on both sides of the arrow.
2. b) Does their diagram respect the law of conservation of mass?
3. c) Correct the equation so that the atoms balance and explain what you changed.

The formation of iron oxide can be written as: $4\ Fe + 3\ O_2 \rightarrow 2\ Fe_2O_3$. The diagram represents the atoms before and after.

1. a) Count the iron and oxygen atoms before and after.
2. b) An old, heavily rusted iron gate weighs more than when it was new: explain this using the atomic-molecular model.
3. c) Where have the atoms that increase its mass come from?

In a closed container there are $10$ molecules of $H_2$ and $10$ molecules of $O_2$, and the reaction $2\ H_2 + O_2 \rightarrow 2\ H_2O$ takes place. The figure shows the container before the reaction.

1. a) How many water molecules are formed?
2. b) Are there any molecules of gas left over? How many and of which gas?
3. c) Relate the result to the law of definite proportions.

The atomic-molecular model makes it possible to explain the two laws of chemical reactions you have studied.

1. a) Explain the law of conservation of mass using the model: why does mass not change if atoms do not disappear and each type of atom always has the same mass?
2. b) Explain the law of definite proportions using the model: why does a compound always have the same proportion by mass if its formula is always the same?