Thanks to the advance of theories and technologies, we could develop a very sophisticated model of atoms which could be verified to some extend by various microscopic information.

However, that wasn't so easy for some pioneers in chemistry who lived in the 19th century, when they were trying to understand the nature of matters. But still, they gave us a picture of what matters are made of and what it looks like inside an atom, via their careful observations and scientifically sound postulates, which have become the laws in modern chemistry.

We will now see how scientists learn about matters and atoms without using modern microscopes to really zoom into matters and see what is making up matters. At the same time, we will see some of the important fundamental laws in chemistry.

## Law of definite proportions

Before people realized matters are made of atoms, Joseph Proust noticed that the mass percentage of a certain element in a compound is never changed, regardless of the size and source of the compound. So he correctly concluded that compounds always have the definite proportions of each element. This has later been accepted as the **law of definite proportions**.

## Law of multiple proportions

The law of definite proportions was further investigated by John Dalton who found that "*if two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers*".

Ths could be understood by taking the following analogy. Image we have three boxes which we can't see through. The first box contains 1 gram of apples and 3 grams of oranges. The second box contains 1 gram of apples with 4.5 grams of oranges. The third box contains 1 gram of apples and 6 grams of oranges. We can have other boxes with apples and oranges, but none of them will have 5 grams of oranges with 1 gram of apples. So we can say that the mass of oranges is always increase by 1.5 grams and never by 0.5 grams. That tells us each orange will have a mass of 1.5 grams. Since we do not have half an orange or 1/3 of an orange, the mass of oranges in the boxes can only increase by 1.5 grams at a time. If we have one orange we have 1.5 grams, two oranges will give us \( 2 \times 1.5 = 3\ grams \), and three oranges \( 3 \times 1.5 = 4.5\ grams \), etc. So the masses of oranges in different boxes will always be a whole number multiple of the smallest mass, which tells us the number of oranges in that box.

Therefore, if we know, for example, when carbon forms compounds with oxygen, 1 gram of carbon will combine with either 1.3 grams of oxygen or 2.6 grams of oxygen, but never 1.4 grams, we could say that oxygen will exist in portions where the smallest and indivisible portion is 1.3 grams if the smallest indivisible portion of carbon has a mass of 1 gram. That leads us to the idea of elements exist in certain small portions which can not be further divided (just like the way we imagine for an orange).

This is now known as the **law of multiple proportions**, which, together with the law of definite proportions, forms the basis of stoichiometry guiding us the calculations in chemical reactions. We'll learn more about *stoichiometry* in the future.

So now we could shed some light on what's making up matters, even without the help of modern technology.

## Dalton's atomic theory

Here, we're going to talk about another big contribution of John Dalton, namely **Dalton's atomic theory**. (Spoiler alert: we'll see a third important contribution by John Dalton in the future)

After Dalton discovered that elements in compounds always have proportions that are whole number multiple of a certain value, he correctly concluded that **elements exist as atoms which are the smallest indivisible units of the element carrying a certain mass**. This is now known as

*Dalton's atomic theory*.

According to Dalton's atomic theory, elements must exist in the form of individual atoms. Each atom carries a certain mass and cannot be broken up. When different atoms combine together in a certain ratio to form compounds, compounds will always have a whole number multiple of the mass of a single atom.

On the other hand, the ratio of number of different atoms in a particular compound is never changed. As a result, the mass ratio of these elements in that compound is also definite. This allows us to differentiate *mixtures* and *compounds*.

In short, matters are made of atoms and atoms will serve as the most basic building block of all matters.