A Brief History of Chemistry

A Brief History of Chemistry So when does chemistry begin? If we consider chemistry to be the controlled use of a chemical reaction, then we might suggest that it begins when we can make fire (somewhere around 750,000 years ago). Here are some highlights of the earliest examples of the use of chemistry (in chronological order): For a very long time people have been using pigments to make dyes (not really sure for how long, certainly for 10’s of thousands of years). Fire hardening of clay to make ceramics – Earlier than 8000 B. C.

Isolation of pure metals from ores (Cu, Au) – circa 4000 B.C. First alloy (bronze, from Cu and Sn) – circa 2000 B.C. Smelting of Fe (iron age) – about 1500 B.C. And about 500 years later … They added a little C to make steel. Egyptians used natural pigments and juices to preserve human bodies – about 900 B.C.

In the 7th century B.C. Thales of Miletus speculated about whether one substance could be transformed into another. This began the movement known as khemeia, which lasted for approximately 1000 years in Greece. Pre-Socratic philosophers developed the idea that matter was composed of four elements - Earth, Fire, air, water Leucippus and Democritus did not challenge this idea, but they added their own twist – atoms. Democritus 460 – 370 B.C.

They held that the nature of things consists of an infinite number of extremely small particles, which they called Atomos, the Greek word for indivisible. Democritus described these particles as indestructible and containing no empty space. He felt these ‘atoms’ moved through space and that if they collided they could interlock, forming substances. Unfortunately, Democritus ideas were not well received. The opposition included Plato and then, most notably, Aristotle.

Aristotle disregarded the atomistic ideas of Democritus for 3 reasons… First, he felt it was far more important to understand the form of something rather than its substance. Second, according to Aristotle, one of the reasons for discarding the concept of an infinite number of atoms moving in infinite space was that in infinite space no natural motion can take place, because in infinite space no place or point can be assigned unambiguously as the endpoint of any object's motion. Perhaps most importantly, he supported the school of thought that matter was infinitely divisible rather than made up of fundamental particles. So the idea of fundamental particles making up atoms was pretty much ignored for the next 2000 years !!

In fact, in the western world, the end of the Golden Age of Greece was the end of scientific thought for quite a long time. As one age was ending in the west, a new one was beginning in Egypt, Arabia, and China. This new movement, which began about the 2nd or 3rd century B.C. derives its name from the Arabic word for thekhemeia, Alchemy -- al-kimiya The goal of the alchemists in the east was immortality, a search for the elixir of life. While they never achieved this, obviously, some significant discoveries were made.

One of the most renowned alchemists of this period was an Arab named Jabir ibn – Hayyan who lived during the 8th century A.D. He described several chemicals, including some inorganic acids, acetic and nitric, for example, and developed several applied chemical techniques. His descriptions were so complicated and involved, a word was derived from his name - Gibberish

In the 9th century A.D., Another Islamic scientist, the Persian physician Al-Razi, invented the use of plaster of paris to immobilize broken bones. An early example of the use of chemistry for medical treatment. In China, about 1000 A.D., alchemists combined sulfur, salt peter, and honey to produced a recipe for eternal life. The mixture came with a warning: Use caution, or this may burn your house down. The discovery? Gunpowder !!!

Alchemy did not really develop in Europe until early in the 12th century A.D. Unlike their counterparts in the east, the goal of the European alchemists was wealth. They hoped to develop a way to transmute base elements, like Pb or Fe, into gold. Though never accomplishing this, by constantly messing around with metals, they improved element purification and developed even better alloys. While significant discoveries may have been made during this period, there was no real process or organization. For this reason, we would not call what was being done science. The first idea for any type method for scientific study comes in the 13th century.

Roger Bacon, a Franciscan friar, proposed that speculation, analogy and even logic are not sufficient. Bacon said that it was necessary to utilize observation, experimentation, and verification. Apparently, the alchemical world wasn’t quite ready for a methodology. It didn’t catch on for several centuries. In 1661, Robert Boyle, an Irish aristocrat, wrote a book called The Skyptical Chymist Among other things, the book argued that ideas about chem should be based on evidence.

Boyle also worked extensively with gases and one of his ideas was that gases were compressible because they were made up of small atoms, with empty space in between them. atoms! That’s right, At about the same time, in Germany, Hennig Brand becomes the first to isolate a non-metallic element, Phosphorus. Wait ‘til you hear the recipe! In the 18th century scientific study really takes off. A main area of interest is combustion. One of the first significant ideas about combustion was developed by Georg Stahl in the early part of the century.

Stahl’s idea was that things which could burn gave off a substance he called phlogiston. When the substance had released all its phlogiston, It stopped burning. Stahl’s idea was not refuted until the 1770’s. This was due primarily to the work of two scientists. Joseph Priestly isolated gases from several types of reactions. He is most well-known for his discovery of oxygen.

The second scientist, Antoine Lavoisier, Was a contemporary and it has been suggested, a competitor, of Priestly. He demonstrated that Priestly’s gas, which he named oxygen, was responsible for combustion. This revelation finally ended the phlogiston theory of Stahl. He also showed that if it took place in a sealed container, no mass was lost during combustion. This discovery, combined with work on about 100 other reactions, allowed Lavoisier to propose the law of conservation of matter.

He also combined oxygen with a gas that had been discovered by Henry Cavendish (more on him later) and produced water. This finally refuted the Greek’s 4-element theory for good. In 1778!! Lavoisier demonstrated the importance of carefully making and recording all measurements. He even wrote the first chemistry text. For all his contributions, Lavoisier is often referred to as the Father of Modern Chemistry. Unfortunately, his scientific career was… cut short Want to hear the story?

Near the end of the 18th century, Joseph Proust showed that a given compound always contains the same proportion of elements by mass. This is known as the law of definite proportions. In the early 1800’s an English school master used the work of Lavoisier, Proust, and his own research to develop the first modern atomic theory. His name was John Dalton. Part of Dalton’s work was developing the law of multiple proportions. According to this law, different com-pounds made of the same elements, have mass ratios related by small whole numbers.

For example, there are two compounds made exclusively of hydrogen and oxygen. In one, the ratio of O:H is 8 to 1. Its formula is In the other, the ratio of O:H is 16 to 1. Its formula is H2O H2O2 Dalton’s atomic theory was comprised of 4 statements: All elements are composed of tiny, indivisible particles called atoms. Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

Atoms of different elements can combine with one another in simple whole number ratios to form compounds. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are not changed into atoms of another, by a chemical reaction. Dalton’s theory was accepted, unchanged, for nearly a century, when the existence of the first subatomic particle is established. Who did exactly what is not always clear, but what follows is a reasonably accurate description.

J. J. Thomson is given credit for discovering the electron in 1897. While working at the Cavendish, Thomson was using a device called a Crook’s – or cathode-ray - tube. The Cavendish was a famous lab, at the University of Cambridge, named after Henry. More on it, later. Anyway, Thomson deflected the beam with both charged electrodes and a magnetic field (see demo). Thomson was able to determine that the particles in the beam were negatively charged. He was also able to calculate the charge to mass ratio. Thomson’s work was enhanced by Robert Millikan.

In 1911, Millikan devised one of the classic experiments in atomic physics and chemistry, the oil-drop experiment. He constructed a chamber with a graduated view lens. The chamber had oppositely-charged plates. He used an atomizer to spray tiny oil drops between the plates. By adjusting the charge on the plates until a drop was suspended, he determined the charge and mass of the electron (also thanks to Thomson’s results).

J. J. Thomson, who is now the director of the Cavendish, develops a model of the atom. It is called the plum-pudding model . The positively-charged, spherically-shaped, atom resembles the pudding part. And the electrons are spread throughout the atom, like the currants and raisins in the pudding. There would be enough electrons to offset the positive charge of the atom. In 1911, this model would be challenged by Rutherford, Geiger, and Marsden.

Ernest Rutherford was a New Zealander who studied under Thomson at the Cavendish laboratory. By 1911 he was at Victoria University, in Manchester. His former mentor suggested that E. Ruth test the plum-pudding model. So Ernie, along with his students Hans Geiger and Ernest Marsden, designed what has come to be known as … The Gold Foil Experiment!

The experiment had a radioactive source, encased in a lead box. The box had a tiny opening to allow a stream of radioactive alpha particles to be emitted. The small, dense, positively-charged alpha particles traveled thru a slit in circular, zinc sulfide-coated screen. When a particle hit the screen a flash of light was given off.

Since alpha particles are more dense than gold atoms they expected them to pass freely through the gold foil -- not exactly what happened. (let’s go back to the last slide) Rutherford described his surprise the following way: "It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you." So what caused the particles to be deflected? Rutherford explained it like this: An atom has a nucleus.

Rutherford explained that the mass and positive charge must be contained in a central dense core in the atom. He called this core – the nucleus. He said that the rest of the atom, outside the nucleus, was mainly empty space. Within this region of empty space, is where the electrons orbit around the nucleus. While this disproved the plum-pudding model, Rutherford still didn’t know what caused the + charge.

In 1919 (shortly before he returned to the Cavendish to become director), Rutherford fired alpha particles at nitrogen nuclei causing them to release protons. Hydrogen had previously been shown to have these positively-charged particles, but Rutherford showed that these protons were present in all nuclei. Rutherford even predicted that there must be a third type of sub-atomic particle, a neutral particle, that would stabilize the proton repulsion in the nucleus. This 3rd particle was finally discovered by JamesChadwick (at the Cavendish) in 1932. neutron. It was called the

So, here we are, 1932 and after all that history, this is our version of atomic structure: The atomic mass unit (amu) is a relative mass value. It is equal to 1/12 the mass of the carbon-12 nucleus The electron actually has a mass of 9.11 x 10-28 g. This mass is insignificant (1/1840) compared to that of the proton, or neutron, so it is given a value of 0 amu.

Isotopes • Atoms of the same element with different masses. • Same # of protons, different # of neutrons • Isotopic Symbol: X = element symbol X m m = mass number (sum of p+ & no) n n = atomic number (# of p+)

Examples 12C 13C 14C 6 6 6 #P _______ _______ _______ #N _______ _______ _______ #E _______ _______ _______

More Isotope Examples p+ = no = e- = F 19 9 W 184 p+ = no = e- = Write the symbol for the element with 53 p+ and 74 no.

Isotopes Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium?

Isotopes Ga-69 68.9 amu x 60.2 = 41.5 amu for 69Ga 100 Ga-71 70.9 amu x 39.8 = 28.2 amu for 71Ga 100 Atomic mass Ga = 69.7 amu

Lead has four different isotopes. 204Pb has a mass of 203.994 amu and an abundance of 1.32%. 206Pb has a mass of 205.993 and an abundance of 26.31%. 207Pb and 208Pb have masses of 206.991 and 207.899 amu and abundances of 20.78% and 51.59%, respectively. Calculate the relative (average) atomic mass of lead. 203.994 amu x .0132 = 2.69 amu 54.20 amu 205.993 amu x .2631 = 206.991 amu x .2078 = 43.01 amu 107.3 amu 207.899 amu x .5159 = __________ 207.2 amu

Isotopes Calculate the atomic mass of silicon if given the following information.

A Brief History of Chemistry