Dalton's Atomic Model

Dalton's atomic model sets up the building blocks for others to improve on. Though some of his conclusions were incorrect, his contributions were vital. He defined an atom as the smallest indivisible particle.

John Dalton

Though we know today that they can be further divided into protons, neutrons, and electrons, his explanation was revolutionary for that period of time. Here's how he defined the atom:

"Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. I have chosen the word “atom” to signify these ultimate particles." -John Dalton

Let's review the three basic laws before we get into Dalton's theory.

1. Law of conservation of mass

The law of conservation of mass states that the net change in mass of the reactants and products before and after a chemical reaction is zero. This means mass can neither be created nor destroyed. In other words, the total mass in a chemical reaction remains constant.

This law was formulated by Antoine Lavoisier in 1789. It was later found to be slightly inaccurate, as in the course of chemical reactions mass can interconvert with heat and bond energy. However, these losses are very small, several orders of magnitude smaller than the mass of the reactants, so that this law is an excellent approximation.

Does the following chemical reaction obey the law of conservation of mass?

$$\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}$$

The mass of $\ce{Ca}$, $\ce O$, $\ce H,$ and $\ce C$ are 40u, 16u, 1u, and 12u, respectively.

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Yes, they obey the law of conservation of mass. Let's verify it. The molecular mass of

$$\begin{aligned} \ce{Ca(OH)2}&= 40+32+2\\&=74 \\ \\ \ce{CO2}&=12+32\\&=44 \\ \\ \ce{CaCO3}&=40+12+48\\&=100\\ \\ \ce{H2O}&=2+16\\&=18. \end{aligned}$$

Substituting these values in the equation,

$$\begin{aligned}74+44 & = 100+18\\118 & =118.\ _\square \end{aligned}$$

2. Law of constant proportions

The law of constant proportions states that when a compound is broken, the masses of the constituent elements remain in the same proportion. Or, in a chemical compound, the elements are always present in definite proportions by mass.

It means each compound has the same elements in the same proportions, irrespective of where the compound was obtained, who prepared the compound, or the mass of the compound.

This law was formulated and proven by Joseph Louis Proust in 1799.

A person living in Australia sent a $100\text{ ml}$ sample of $\ce{CaCO3}$(calcium carbonate) to a person living in India. The person living in India made his own sample of $200\text{ ml}$ and compared it to his friend's. Which of the two compounds has a greater ratio of $\ce{Ca}:\ce C?$

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Both contain equal ratio of $\ce{Ca}$ and $\ce C$. This is guaranteed by the law of constant proportions. $_\square$

3. Law of multiple proportions

The law of multiple proportions states that when two elements form two or more compounds between them, the ratio of the masses of the second element in each compound can be expressed in the form of small whole numbers.

This law was proposed by John Dalton, and it is a combination of the previous laws.

Carbon combines with oxygen to form two different compounds (under different circumstances); one is the most common gas $\ce{CO2}$ and the other is $\ce{CO}$. Do they obey the law of multiple proportions?

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Yes, they do obey the law of multiple proportions. Let's verify it.

We know that the mass of carbon is $12\text{ u}$ and that of oxygen is $16\text{ u}$.
So, we can say that $12\text{ g}$ of carbon combines with $32\text{ g}$ of oxygen to form $\ce{CO2}$.
Similarly, $12\text{ g}$ of carbon combines with $16\text{ g}$ of oxygen to form $\ce{CO}$.

So, the ratio of oxygen in the first and second compound is $\frac{32}{16}=\frac21=2,$ which is a whole number. $_\square$

There is one other law which was proposed to find the relation between two different compounds.

4. Law of reciprocal proportions

The law of reciprocal proportions states that when two different elements combine with the same quantity of a third element, the ratio in which they do so will be the same or a multiple of the proportion in which they combine with each other.

This law was proposed by Jeremias Ritcher in 1792.

Dalton picked up the idea of divisibility of matter to explain the nature of atoms. He studied the laws of chemical combinations (the laws we discussed in the previous section) carefully and came to a conclusion about the characteristics of atoms.

His statements were based on the three laws we'd discussed earlier. He stated the following postulates (not all of them are true) about his atomic theory.

1. Matter is made of very tiny particles called atoms.
2. Atoms are indivisible structures, which can neither be created nor destroyed during a chemical reaction (based on the law of conservation of mass).
3. All atoms of a particular element are similar in all respects, be it their physical or chemical properties.
4. Inversely, atoms of different elements show different properties, and they have different masses and different chemical properties.
5. Atoms combine in the ratio of small whole numbers to form stable compounds, which is how they exist in nature.
6. The relative number and the kinds of atoms in a given compound are always in a fixed ratio (based on the law of constant proportions).

As said earlier, all the postulates weren't correct. Let us discuss the drawbacks of Dalton's atomic theory.

Drawbacks

• The first part of the second postulate was not accepted. Bohr's model proposed that the atoms could be further divided into protons, neutrons, and electrons.
• The third postulate was also proven to be wrong because of the existence of isotopes, which are atoms of the same element but of different masses.
• The fourth postulate was also proven to be wrong because of the existence of isobars, which are atoms of different elements but of the same mass.

Nonetheless, to propose the idea of an atom (considering the time period) is a great achievement, and we must appreciate Dalton's work.

Based on all his observations, Dalton proposed his model of an atom. It is often referred to as the billiard ball model. He defined an atom to be a ball-like structure, as the concepts of atomic nucleus and electrons were unknown at the time. If you asked Dalton to draw the diagram of an atom, he would've simply drawn a circle!

Later, he tried to symbolize atoms, and he became one of the first scientists to assign such symbols. He gave a specific symbol to each atom (see below).

It was only after J. J. Thompson proposed his model that the true concepts had come into existence. Later, Rutherford worked on Dalton's and Thompson's models and brought out a roughly correct shape of the concept. Finally, Bohr's model and the quantum mechanical model gave a complete model which we know of today.

• Atoms, Molecules, Elements, Compounds

• Bohr's Model