Chapter 2: A Particle View of Matter Week 1

Because atoms are too small to see, even with the most powerful microscopes, chemists used models to represent them. These models have changed with time as more is discovered about atoms. In 1897, Thomson took the latest experimental evidence for the existence of positive matter and negative particles and proposed an atomic model.

Thomson’s Model He imagined that the atom consisted of a sphere of positive matter in which negative particles were randomly embedded, like plums in a pudding…thus becoming the “Plum Pudding” model.

Ernest Rutherford Rutherford completed experiments that produced results that could not be explained by the “Plum Pudding” model. His work bought us even closer to the atomic model that we use today. He proposed the most of the mass of an atom and all of the positive charge was located in the centre of the atom, the nucleus.

Rutherford continued… He also said the most of the volume of an atom is empty space occupied by the electrons. And, the force of attraction between the positive nucleus and negative electrons is electrostatic. - +

James Chadwick’s Discovery In 1932, James Chadwick discovered an uncharged particle that had a mass a little greater than the proton. He called this particle a neutron and reasoned that they were held in the nucleus with the protons. With this discovery, scientists had discovered three subatomic particles, making it possible to identify individual atoms.

Outcomes of Chadwick’s Discovery… Chadwick’s discovery explained a problem that Rutherford had faced earlier. When he was working with Thorium, Rutherford identified a radioactive material chemically similar to Thorium. He called this Thorium-X. Other chemists also found similar substances for the elements in the periodic table between Pb and U.

Explaining Isotopes British scientist Frederick Soddy decided to place similar substances in the same position in the periodic table. He suggested that varieties of particular elements existed and called them isotopes. Isotopes = Greek for same place. With the discovery of neutrons, Chadwick could now explain isotopes. They had the same number of protons and electrons but a varying number of neutrons.

Rutherford’s model proposed that the electrons move in circular orbits around the nucleus with an electrostatic force of attraction operating between positive and negative particles.

Conflicting Ideas… While Rutherford’s model worked for many aspects of an atom, there were some limitations relating to the arrangement of electrons around the nucleus: – The laws of physics that had been used to explain the behaviour of large objects to indicate that electrons moving in a circular orbit should continually emit electromagnetic radiation (light). – As the radiation is emitted, electrons should lose energy and spiral into the nucleus. – The model did not explain why, when an element is heated, only light of certain specific energies is emitted, and not the light of every energy.

Refining Rutherford’s Model In 1913, Niels Bohr, suggested that the laws of physics that scientists used to explain the behaviour of large objects did not apply to the motion of small objects, such as electrons. He proposed the electrons: – Circle the nucleus without losing energy – Could move in certain fixed orbits of particular energies.

Energy and Heating The orbit that an electron moves depends on the energy of the electron. A low energy orbits closer to the nucleus while a high energy orbits further out. Heating an element can result in an electron absorbing energy and ‘jumping’ to a higher energy state. After this the electron emits this excess energy, as light and returns to its original state.

Extending Bohr’s Model Scientists quickly extended on Bohr’s ideas. Evidence for the existence of energy levels in other atoms were obtained from studies of successive ionisation energies of elements. For example, the ionisation energy for the removal of the first electron is fairly low. The next eight require more energy but are all removed at similar levels.

Extending Bohr’s Model cont… This conclusion of successive ionisation energies led scientists to conclude that electrons were grouped in different energy levels, called shells. Electrons in the same shell: – Are about the same distance from the nucleus – Have about the same energy. This means that a shell can be described based on the amount of energy needed to move an electron out of this region.

Extending Bohr’s Model cont… Each energy level or shell, can hold different numbers of electrons. Shell NumberMaximum Number of Electrons n2n 2

Electronic Configuration The way in which electrons are arranged is known as the electronic configuration of an atom. In a stable atom the electrons are as close to the nucleus as possible, meaning that an electron fills up from the inside out. After the second shell, the electron fills in stages. The outermost shell can never contain more than 8 electrons, regardless of its ability to hold more than that.

Electronic Configuration cont… For example, despite the fact the the third shell can hold 18 electron, it will not hold more than 8 unless the fourth shell has at least two electrons in it. The electrons in the outermost shell of an atom are known as the valence electrons. These are the ones that require the least amount of energy to be removed. It is these electrons that are involved in chemical reactions.

In 1926, Erwin Schrodinger proposed that electrons behaved as waves around the nucleus. His description is known as wave mechanics or quantum mechanics. Scientists now believe that small particles such as protons, electrons and neutrons behave according to the laws of quantum mechanics.

Quantum Mechanics The electron is now seen as a vague object that behaves like a cloud of negative charge. Unlike the Bohr model, there are no definite orbits for electrons. They are instead thought to move in regions of space, called orbitals.

Shells and Subshells Quantum mechanics shows the following: – Within an atom there are major energy level which are known as shells. These are numbered 1, 2, 3 etc. – Within these shells are energy levels of similar energy called subshells. These are lettered as s,p,d,f – The energy of the subshells increases in this order – Subshells are made up of orbitals.

Energy Levels within an Atom… Shell NumberNumber of Subshells Subshell in Shell Number of Orbitals in Subshell Maximum Number of Electrons 111s12 222s 2p s 3p 3d s 4p 4d 4f s 5p 5d 5f 5g

Energy Levels