Structure of the Atom

Structure of an atomAn atom consists of a central positively charged nucleus containing protons and neutrons (nucleons)Diameter approx. 10-15 m (1 femtometre)Electrons surround the nucleusAtomic diameter approx. 10-10 m roughly 100 000 x nucleus diameter

Properties of sub-atomic particlesNote: e = charge of an electron = - 1.6 x 10 - 19 C+ 1.6 x 10 -19- 1.6 x 10 -19000.00051.67 x 10 -271.67 x 10 -279.11 x 10 -31+ 111- 1

chargemass in coulombsrelative to a protonin kilogramsrelative to a protonprotonneutronelectron

IsotopesThese are atoms that have the same number of protons but different numbers of neutrons

Isotopes have the same proton number and so are all of the same element

Isotope notationcarbon 14C-14

Answers:NU235Complete:14723869220238119926929926927117

symbolAZnumber of protonsnumber of neutrons147772099112389292146116652359292143

Specific chargespecific charge = charge of particle mass of particle

unit: coulombs per kilogram (C kg-1)

QuestionCalculate the specific charge of a nucleus of helium 4

helium 4 contains 2 protons and 2 neutronscharge = 2 x (+ 1.6 x 10-19 C) = + 3.2 x 10-19 Cmass = 4 x 1.67 x 10-27 kg = 6.68 x 10-27 kg

specific charge = 4.79 x 107 Ckg-1

CompoundsYou need to learn these two laws.

Prousts Law of constant composition The ratio of masses of elements in a compound is the same, however much of it there is. e.g. water 16/2 (8/1)

Daltons Law of multiple proportions When two elements combine to produce different compounds the mass of one element that combines with a fixed mass of another element is a simple ratio,,, 1:1, 1:2, 1:3

You need to know the timeline (well the experiments) for the discovery of the atomic structure that we know to date.

Diffusion & Brownian motionSee section in text book page 41

J.J. Thomson (1897)Thomson was determined to discover an explanation of the illuminating cathode ray tubes whilst working at the Cavendish Laboratory in Cambridge

Cathode RaysThomson suggested the rays were a stream of particles called corpuscles (we now call them electrons)Previously it was thought that the atom was indivisibleLeap in our understanding of matterThough the actual figures were not known, the mass to charge ratio (m/e) was shown to be 1000 times that of hydrogen

The Experiment

The electron was accepted as a sub atomic particle and therefore a model of the atom was created. The plum pudding model

Rutherford (1911)The electron was accepted as a subatomic particle The model of how electron were arranged in an atom was called the plum pudding modelIt was a student of J.J. Thomson, Ernest Rutherford who was to change the understanding of the structure of the atom.

Geiger & Marsden ExperimentAlpha particles were fired into a thin piece of gold leafA screen surrounded the gold foil to detect were the alpha particle were being deflected to.

The Results

Plum Pudding Atoms Radioactivity Alpha Scattering Geiger and Marsden "Shells off Tissue Paper" The Nucleus The Bohr Atom Scattering Experiment

Rutherford modelIt was thought that the alpha particles would deviant by a small amountHowever, many particle went straight through and some completed rebounded!

Rutherford split the atom to discover the proton

J. Chadwick (1932)He was a student of Rutherford and whilst working at Cambridge discovered that the charge on the nucleus was always less than the mass of it.He thought that another particle was responsible for the extra mass.He suggested that it was a neutron.

Discovery of the Neutron

********