The History of the Atom Flashcards
What did the Ancient Greeks believe about the atom?
In ancient Greece, most people thought that matter was made up of a single continuous substance that produced the four elements: earth, air, fire and water. However, some Greeks believed that objects could be cut until they were so small, they couldn’t be cut anymore. The ancient Greek philosopher Democritus thought that matter was made up of millions of tiny, uncuttable pieces of that same matter. His ideas drew on the teachings of Leucippus and Anaxagoras. The word atom comes from ‘atomos’, meaning uncuttable. Aristotle and Plato rejected these ideas, so they didn’t catch on because of their great influence.
What was the Solid Sphere Model?
Dalton described atoms as solid spheres and said that all elements are composed of indivisible atoms (whole number ratio experiments). It is impossible to create, divide or destroy an atom. All the atoms of a particular element had the same mass and atoms of different elements had different masses. He also formed the idea that atoms of different elements combine to form a compound. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.
What was the Plum Pudding Model?
To explain the two types of static electricity, Thomson suggested that the atom consisted of a positive ‘dough’ with negative electrons stuck in it.
He discovered the electron when he was experimenting with gas discharge tubes. This was consistent with the evidence available at the time. Solids can’t be squashed, so the atoms which make them up must be solid throughout. Rubbing two solids together often results in static charge so there must be negatively charged electrons on the outsides of atoms which can be transferred as atoms collide. This was the first sub-atomic particle to be discovered. He showed that electrons were negatively charged and electrons from all elements were the same.
As electrons had a negative charge, there had to be some source of positive charge inside the atom too. Also, as electrons were much lighter than the whole atoms, there had to be something to account for the rest of the mass of the atom. Thomson suggested that the electrons were located within the atom in circular arrays, like plums in a pudding of positive charge.
What was the Nuclear Model?
Rutherford and his students (Geiger and Marsden) conducted the gold foil experiment to test the plum pudding model. They fired positively charged alpha particles at a thin sheet of gold suspended in a vacuum. Most particles went straight through or were slightly deflected, but some reflected backwards. Rutherford proposed a new theory, where there’s a small, dense, positively charged nucleus at the centre, surrounded by a ‘cloud’ of negative electrons – most of the atom’s empty space – that were held in orbit by the positive nucleus, due to electrical forces.
What was the Planetary (Bohr) Model?
Electrons in a ‘cloud’ around the nucleus of an atom would be attracted to the nucleus, causing the atom to collapse. Bohr suggested that electrons can only exist in fixed shells and orbited the nucleus in different energy levels or at specific distances from the nucleus. He stated that each shell has a fixed energy, and that electrons furthest from the nucleus have higher energy. He identified that an electron can jump from a lower orbit to a higher orbit by absorbing energy (done in the forms of heat, light or electricity). These electrons then lose energy when they cool, returning to the lower orbit, giving them the emission spectra for all different elements. Bohr also suggested that reactivity depends on electrons, and that full shells are stable (hence inert elements).
What was the Electron Cloud Model?
Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. He discovered that electrons don’t move in orbits (or in a set path at all). He theorized electrons move in waves, and they have no exact location. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.
What is the Quantum Mechanical Model?
The proton had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass, by Henry Moseley. But there was still a difference between the atomic number and the atomic mass. Scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass. This was accepted and models included the neutron for a further 20 years. Chadwick used a version of Rutherford’s experiment, using a sheet of beryllium and a paraffin block. He proved that a proton-sized neutral particle existed. Quantum mechanics explains electrons as always moving. The more dense the dot, the more likely it is to be in that place.
What did Robert Boyle propose?
Robert Boyle proposed that there were some substances that could not be made simpler. These were the chemical elements, as we now know them.
What did Henri Becquerel discover?
Henri Becquerel discovered radioactivity. This showed that particles could come from inside the atom. Therefore the atom was not indivisible.
What ideas did Gilbert Lewis put forward?
- the inertness of the noble gases was related to their having full outer shells of electrons
- ions were formed by atoms losing or gaining electrons to attain full outer shells
- atoms could also bond by sharing electrons to form full outer shells
What can Dalton’s model be used to explain?
Dalton’s model can still be used to explain the geometries of crystals.
What can Bohr’s model be used for?
Bohr’s model can be used for a simple model of ionic and covalent bonding.
What can the charge cloud idea be used for?
The charge cloud idea is used for a more sophisticated explanation of bonding and the shapes of molecules.
What can the simple model of electrons orbiting in shells be used for?
The simple model of electrons orbiting in shells is useful for many purposes, particularly for working out bonding between atoms.
What does the solution to the Schrodinger equation give?
The solutions to this equation give the probability of finding an electron in a given volume of space called an atomic orbital.
