The Structure of the Atom Chapter 2 – VCE Chemistry

Cathode Rays Scientists such as Michael Faraday, Heinrich Geissler and William Crookes (See picture) used electric current in a tube to produce a mysterious glow. The tube is called a Crookes tube (See diagram) The tube has a cathode (negative electrode) and an anode (positive electrode) This glow was a result of cathode rays. Light was travelling from the negative to the positive terminal. Experiments showed that the particles travelled in straight lines unless subjected to electrical or magnetic forces. Faraday Crookes

The Discovery of the Electron Joseph John Thomson investigated cathode rays from 1884 to 1919. Experiment 1 ‘Can the negative charge be separated from the cathode rays by means of magnetism?’ - Thomson concluded that the negative charge was inseparable from the rays. Experiment 2 ‘Can the rays be deflected by an electric field?’ - Thomson found that the rays did bend under the influence of an electric field. Experiment 3 ‘Can charge-to-mass ratio of the cathode rays measure how much they were being deflected by a magnetic field and how much energy they carried?’ - He found that the charge to mass ratio was over a thousand times higher than that of a proton, suggesting either that the particles were very light or very highly charged. 1 Thomson 2 3

Dalton was wrong! Thomson deduced that the particles that made up the cathode rays were negatively charged. Thomson concluded that atoms consist of a heavy part with a positive charge and a negative ‘corpuscles’ (Later called electrons) part held together by electrostatic attraction. This is known as the ‘plum pudding’ model Atoms therefore must have a neutral charge Plum Pudding Model

The Alpha-particle Scattering Experiment Ernest Rutherford and Hans Geiger investigated radiation travelling through or being blocked by varying thickness of materials. The radiation consisted of alpha-particles – atoms of Helium that have lost their electrons and hence are positively charged. The experiment surprisingly showed that some of the radiation reflected back through a thin sheet of gold. Rutherford concluded that a concentrated point in the atom, called the nucleus, must have caused the reflection. They also concluded that most of the atom is made up of empty space.

Rutherford’s Experiment http://micro.magnet.fsu.edu/electromag/java/rutherford/

Rutherford’s Nuclear Model Experimentally, Rutherford deduced that the positive charge had to be concentrated in a sphere less than 10-14m in diameter. Rutherford also concluded that the electrons move around the nucleus in random orbits. His nuclear model is shown on the right.

Questions 2.1 Complete questions 1-2 on P. 23 & 24 of Nelson VCE Chemistry.

Problems with Rutherford’s Model In Physics, experiments showed that particles which are accelerating lose energy in the form of radiation. If this was the case, the electrons would lose energy and be drawn into the nucleus in a fraction of a second. The radiated colours given off by moving particles can be seen using a spectroscope. Niels Bohr’s experiments, using the emission spectrum, concluded that electrons must not be giving off energy continuously as the spectrum was not continuous.

The Bohr Model of the Atom Bohr agreed with Rutherford, but differed in the idea of electrons. Bohr concluded that electron position effected their amounts of energy. The further an electron from the nucleus the greater the amounts of energy they posses.

Electron Shells - Bohr Bohr called the different orbits ‘shells’. The shells are called K,L,M,N etc. Electrons fill the lower shells and progress further from the nucleus. The number of electrons held by each shell is equal to 2n2 where n=number of shells. Under normal laboratory conditions, the electrons are in their ground state.

Explaining Spectra using Bohr’s Model Bohr linked the spectra of elements and the concept of level jumping. When electrons are heated they are able to jump to a higher level. They then return quickly to their ground state which releases photons (light energy). Flame tests can be used to view the release of photons.

Forming an Emission Spectrum

Electron Configuration The arrangement of electrons in their shells is termed the electron configuration.

Spectral Lines Example – An electron moves from level 5 to 2. There are 6 possible moves for the electron.

Questions 2.2 Complete questions 1-7 on P. 30 of Nelson VCE Chemistry.

Atomic Number The number of protons is unique to each element. The atomic number therefore is the number of protons present. The atomic number has the symbol Z. Elements are arranged on the periodic table in increasing atomic size. An element has equal number of protons and electrons – therefore being neutrally charged. Mass number (A) will be studied next chapter.

Problems with Bohr’s Model As technology improved, the spectral analysis of Bohr’s model was questioned. Problems such as; atoms don’t have colour, electrons aren’t visible and move at the speed of light, atoms can’t be drawn to scale, the nucleus contains protons and neutrons, the atom is spherical.

s < p < d < f < g The Scrödinger Model Spectral analysis of elements suggested that electron shells don’t have the same amounts of energy. Scrödinger called for sub-shells. Each shell is divided into subshells (also called sub-energy levels) - regions within a shell that have a similar amount of energy These have characteristic shapes and different amounts of energy s < p < d < f < g n = number of subshells Sharp (s) =2 Principal (p) = 6 Diffuse (d) = 10 Fine (f) = 14 Worked Examples Sodium – 2,8,1 1s22s22p63s1 Potassium – 2,8,8,1 1s22s22p63s23p64s1

The Subshells

Atomic Orbitals Electrons are not confined to circular orbits. Instead they move in regions of space called atomic orbitals. Each subshell is made up of orbitals. An orbital is the region of space in which the electron travels within the subshell. Orbitals are assigned a letter according to the subshell it is in (s, p, d, f, g) There can be: 1 s orbital, 3 p orbitals, 5 d orbitals, 7 f orbitals, 9 g orbitals.

The Energies of Subshells

Quantum Mechanical Model

Questions 2.3 Complete questions 1-6 on P. 38 of Nelson VCE Chemistry.

Review Questions Complete questions 1-6 on P. 40 - 41 of Nelson VCE Chemistry.