History of Atomic Theory

History of Atomic Theory How has the structure of matter been understood throughout history? Everything is made of matter. Matter is anything with mass and volume.   But what is matter? Many people have had ideas about this. Physicists are still trying to figure out what gives things mass. But let us start at the beginning….

The Greeks 450 BC Democritus and Leucippus – matter was made of tiny, indivisible particles. Democritus called these particles atomos, after the Greek word for indivisible.

“The Man” did not agree with them.. Aristotle believed that matter could be subdivided without limits. He took a dim view of the atomists. Since many people looked up to Aristotle, his views prevailed for the next 2000 years!

Alchemy and the Alchemists Over time, the idea of a pure substance, different from mixtures and other pure substances, came to be accepted. The modern notions of elements and compounds were not firmly established until the late 18th century. Alchemists claimed to be able to “transmute” one element into another (usually more valuable) element

Cutting Edge 18th Century Technology: The Pneumatic Trough Used for collecting gases (elements and compounds) Around 97% pure (water vapor is the rest)

The Beginnings of Atomic Theory Isolating pure substances became more routine in the 18th century Antoine Lavoisier (1770) was the most detail-oriented of the early chemists He developed a system of chemical nomenclature that is still used today He studied chemical reactions using precise, home built instruments

Lavoisier’s Law Through constant experimentation, Lavoisier showed that the total mass in all chemical reactions does not change In short, “matter is neither created nor destroyed” Today this is called the law of conservation of matter (1st Law of thermodynamics) Superhero movies complete ignore this law in their action scenes Einstein extended the law to energy with his famous equation E=mc2

Scientific Laws A scientific law is a set of observations , by multiple observers, that is repeatable under a variety of conditions. Scientific laws do not offer an explanation for a particular phenomena – they are effects linked to causes.

Another example of a scientific law Car tires deflate slightly in the dead of winter, even if they have no leaks. With warmer weather, they return to normal. Charles’ Law: increasing temperature of gas causes an increase in volume of that gas, and visa versa Note that there is no explanation for “why” here!

Scientific Laws and Theories Scientific laws are a body of evidence concerning a phenomenon A scientific theory is an explanation for a phenomenon that can be described by one or more scientific laws. Initial attempts at explaining phenomena often have to be modified in light of new evidence. For matter, atomic theory was modified numerous times because of new discoveries that could not be explained by the model of the time.

Law of Definite Proportions Joseph Proust (1799) studied simple compounds (pure substances made of two elements). Proust showed that the percentage by mass of each element in a compound is always the same. For example, In Water (l) > Oxygen (g) + Hydrogen (g) The ratio of the masses of oxygen to hydrogen is always 8 : 1

Law of Multiple Proportions John Dalton, a really smart English schoolteacher, did some chemistry on the side Dalton studied combinations of elements that made more than a single compound. Dalton found that a fixed mass of one element led to simple whole number ratios for the other.

What does that mean? If we take two compounds of hydrogen and oxygen, water and hydrogen peroxide: For water, for 1 g of hydrogen produced when water is decomposed, 8 g of oxygen are produced. For hydrogen peroxide, for 1 g of hydrogen produced by decomposition, 16 g of oxygen are produced So the ratio of oxygen in the two compounds is 16/8 = 2: a whole number!

The Law of Multiple Proportions When two elements combine to form more than one compound, the masses of one element that combine with a given mass of the other element are in simple whole number ratios.

Scientific Theories A theory is a explanation of the events described by one or more scientific laws. John Dalton took three scientific laws: The law of conservation of matter The law of definite proportions The law of multiple proportions He proposed the first atomic theory – an idea based on actual evidence!

Dalton’s Theory All elements are composed of atoms, which are indestructible and indivisible. All atoms of the same element are exactly alike – they have the same mass. Atoms of different elements are different – they have different masses. Compounds are formed by the joining of atoms of two or more elements. In any compound, the atoms of the elements are joined in a definite whole number ratio (1:1, 1:2, 3:2, etc)

How Does Dalton’s Theory Hold Up? Dalton’s Atomic Theory has some limitations. Here are some things he didn’t know: atoms have a complex structure, which can be altered temporarily during chemical change. (late 19th century) Atoms can be changed from one element to another, but not be chemical reactions. (late 19th century) Atoms of the same element are not necessarily exactly the same. Their chemical properties are alike, but they may have different masses. (mid-20th century)

How can all of those laws be true if atoms of an element can have a different mass? Every natural sample of an element contains the same proportions of the elements with different masses!   The elements always have the same average mass.