Structure of an Atom PDF

## Science in Context ### The First Ideas About Atoms Democritus (about 460-370 BCE) thought about the structure of materials and what would happen if you cut something into smaller and smaller pieces until you could not cut it any smaller. He concluded that you would come to an indivisible partic...

## Science in Context ### The First Ideas About Atoms Democritus (about 460-370 BCE) thought about the structure of materials and what would happen if you cut something into smaller and smaller pieces until you could not cut it any smaller. He concluded that you would come to an indivisible particle, which he called an "atom." Building on this idea that everything was not made of water or air, but of atoms, he went on to say that atoms of different substances had different sizes and shapes, and that they were able to join together to make even more different substances. The other **philosophers** could not agree on Democritus' creative thought and later Aristotle put forward the idea that matter was made from water, air, fire, and earth. This idea was believed to be true until scientists began investigating chemical reactions. ### Investigating Chemical Reactions Antoine Lavoisier (1743-1794), a French chemist, investigated the changes that took place when two chemicals reacted and formed a new compound. He weighed the chemicals before the reaction and then weighed the compound that was formed. Lavoisier found that the total mass of the chemicals was the same as the mass of the compound that was produced. From this result and from the results of similar experiments, Lavoisier set out his law of conservation of mass, which stated that matter is neither created nor destroyed during a chemical reaction. Lavoisier was assisted in his work by his wife Marie-Anne, who made sketches of the new pieces of equipment that were devised for the investigations and translated the work of other scientists into French for Antoine to read. Joseph Proust (1754-1826), another French chemist, followed Lavoisier's example by carefully weighing the chemicals in his experiments. He discovered that when he broke up copper carbonate into its elements - copper, carbon and oxygen - and then weighed them, they always combined in the same proportions of five parts copper, four parts oxygen and one part carbon. He found that other substances were made from different proportions of elements and these proportions were always the same too, no matter how large or how small the numbers of elements that were used. From his work, Proust devised the law of definite proportions, which stated that the elements in a compound are always present in a certain definite proportion, no matter how the compound is made. We can see that Lavoisier and Proust were working in the same country, at about the same time, but a little later, another scientist in another country built up a theory based on their work. ### Dalton's Atomic Theory John Dalton (1766-1844) was an English chemist who studied gases, and from his investigations on the combining of carbon and oxygen he produced two gases. The first of them seemed to be made from one particle of carbon joining with one particle of oxygen, and in the second gas it seemed that one particle of carbon joined with two particles of oxygen. From his own observations, and from reading about the work of Lavoisier and Proust, Dalton put together his atomic theory. He suggested that: - All matter is composed of tiny particles called atoms. - Atoms cannot be divided up into smaller particles, and they cannot be destroyed. - Atoms of an element all have the same mass and properties. - The atoms of different elements have different masses and different properties. - Atoms combine in simple whole numbers when they form compounds. *** ## Science in Context ### The Plum Pudding Atom The scientists in these early chemical studies on atoms used a unit called "atomic weight" to compare the elements. Today, we use the term "relative atomic mass" or RAM. An English chemist, William Prout (1785-1850), studied the atomic weights of the different elements and thought he could use them to explain the structure of atoms. He knew that hydrogen had the lowest atomic weight, and that the atomic weights of all the other elements appeared to be multiples of the atomic weight of hydrogen. This suggested to him that all the other elements were made up from different numbers of hydrogen atoms. This idea was later shown to be completely wrong, but it did make scientists such as Joseph J Thomson (1856-1940) think that atoms might have a structure inside them. During the 19th century, great developments were made in the study of electricity and the development of electrical equipment for use in investigations. One of these pieces of equipment was the **cathode ray tube**, which produces rays when it is connected into an electrical circuit. The ray is produced from the material from which the cathode is made. Thomson investigated these rays and discovered that they were made of tiny particles, which had a mass over a thousand times smaller than a hydrogen atom. Thomson called the particles "corpuscles," but George Stoney (1826-1911), an Irish physicist, named them **electrons**. When he used different materials for the cathode, he always found that the electrons they produced were the same. In 1904, Thomson devised a model of the structure of the atom from his studies on electrons. He proposed that electrons were present in the atom. He knew that electrons were negatively charged and atoms were neutral, so the negatively charged electrons must be balanced by a positive substance in the atom. He described the atom as being like a plum pudding, with the negatively charged electrons being surrounded by a positively charged "pudding." *** ## Ernest Rutherford and the Atom Ernest Rutherford (1871-1937) was born and raised in New Zealand. After his successful studies on electricity and magnetism at the University of New Zealand in Wellington, he moved to Cambridge University in England to work with JJ Thomson. He spent time studying radioactive materials and the radiation that they produced with Paul Vilard (1860-1934), a French scientist. Between them, they discovered that there are three types of radiation - **alpha particles, beta particles and gamma rays**. Rutherford had found that alpha particles were large, positively charged particles, much bigger than electrons, and he decided to use them to test Thomson's idea about the plum pudding structure of the atom. Rutherford's plan was to hang up a thin sheet of gold and surround it with a screen that could detect alpha particles, as shown. He would then fire alpha particles at the sheet and the alpha particles would eventually hit the screen and be detected. From the marks made by the alpha particles on the screen, he could work out the structure of the atom. Rutherford predicted that if Thomson's model was correct, all the alpha particles would pass straight through the gold atoms and make a mark directly behind the gold sheet. The experiment: **Diagram:** > A few alpha particles bounce back from the gold. > > A beam of alpha particles. > > A very thin gold sheet. > > A source of alpha particles. > > A screen. > > Most alpha particles pass straight through, undetected. > > Some alpha particles are deflected. **Explanation:** > Alpha particles fired at the gold sheet. > > Gold atom. > > Nucleus. > > Gold sheet. The experiment was set up and the alpha particles were fired at the gold sheet. Most of alpha particles made a mark directly behind the metal sheet, but some made marks all round the screen. This did not fit in with the prediction, and suggested that the atoms had a structure that was not like a plum pudding. Rutherford reasoned that as some alpha particles appeared all over the screen, they must be hitting and "bouncing off" something inside the atoms that repelled them, but, as most passed through, there must be a large amount of empty space in an atom to let the alpha particles through. *** ## Electrons, Protons and Neutrons After further thought, Rutherford concluded that an atom did not have a positively charged "pudding" around the outside, but instead had a positively charged centre or **nucleus**, which was surrounded by negatively charged electrons. He used his thoughts to construct a new model of the atom, as shown. In this model, he showed that he believed that the atom had a highly charged centre and was surrounded by a large amount of empty space, in which the electrons were spread out in an orderly way. Rutherford's later work identified particles in the nucleus which he named **protons** after Joseph Proust, who first suggested that atoms might have smaller particles inside them. When Rutherford looked at all the evidence that had been collected about atomic structure, he found that there seemed to be something missing to explain all the results. He thought that there must be other particles present in the nucleus which were similar to protons but which did not have an electrical charge. He thought that for a structure to have no overall charge, it must be made of a positively charged proton and a negatively charged electron together, and he called the particle a "neutral doublet." This critical look at the data led other scientists to evaluate the methods used for investigating atomic structure, refining them for further investigations. In 1932, an English scientist, James Chadwick (1891-1974), fired alpha particles at beryllium atoms and knocked out particles that had a similar mass to protons but no electrical charge. He had discovered the neutral doublet predicted by Rutherford, and he called it the **neutron**. Further work on this particle by others showed that it was not composed of a proton and an electron - rather, it was a particle with a similar structure to a proton but with no electrical charge. - What scientific knowledge and understanding did Rutherford use to describe the structure of the neutral doublet? - Assess the accuracy of Rutherford's prediction about a neutral particle. ### The Electrostatic Charge Inside the Atom We have seen in the models of Thomson and Rutherford that the nucleus had a positive electric charge and the electrons had negative electric charges. This difference in electrical charge creates an **electrostatic force** between the nucleus and the electrons in the atom. It is this electrostatic force, created by the attraction between positive protons and negative electrons, that keeps each atom together. *** ## Process Wear the woollen jumper and hold out the balloon from its thread. The balloon should hang down. Rub the balloon on a sleeve. This will transfer electrons from the atoms in the wool to atoms in the rubber of the balloon, as shown. *** ## Viewing Atoms Atoms are too small to be seen by microscopes which use light, like the ones you may use in the laboratory. Scientists and engineers have developed microscopes which fire electrons at materials to make images of the atoms inside them. In the images, the detail of the atoms cannot be seen, but their position in the material can. *** ## Summary - **Science in context:** People have wondered about the structure of materials for a long time, and the term "atom" was used by Democritus in ancient Greece to describe them. - **Science in context:** Antoine Lavoisier investigated chemical reactions and established the law of the conservation of mass, a discovery built on by Joseph Proust. - **Science in context:** John Dalton's atomic theory outlined the composition of matter and atoms, the properties of atoms of an element, and how they combine in reactions. - **Science in context:** Joseph J Thomson devised the plum pudding model of atoms, with negatively charged electrons surrounded by a positively charged pudding. - **Ernest Rutherford's experimentation with atomic structure led to his model of the structure of the atom.** - **Electrons have negative charge, protons have positive charge and neutrons have no charge.** - **The chemical properties of elements are linked to their atomic structure.** - **Electrostatic force is the force created by the attraction between positive protons and negative electrons that keeps the atom together.**