Chapter 4 The Periodic table

Elements An element is any substance which cannot be split into similar substances by chemical means.

History of the Periodic Table. Dobereiner’s Triads.—Group of 3 elements with similar chemical propertiesin which the atomic weight of the middle element is approximately equal to the average of the other two. Cl Ca Li Br St Na I Ba K Earliest attempt to trace a relationship between properties of elements and their atomic weights.

Newland’s octaves Newland’s Octaves.-Group of elements arranged in order of increasing atomic weight in which the 1st and 8th element of each group have similar chemical properties. Discrepancies: O and S in same group as Fe. N and P in same group as Mg Newland’s did not realise that some elements had still to be discovered.

Mendeleef Mendeleef’s Periodic table Arranged elements in order of increasing atomic weight-each 8th element had similar chemical properties. Periodic Law-when elements are arranged in order of increasing atomic weight the properties of elements vary periodically. 1. Left gaps for undiscovered elements. 2. In a few cases, he had to reverse the order to make them fall into groups of elements of similar chemical properties. e.g. I and Te

Henry Mosley Mosley The modern periodic table is an arrangement of elements of increasing atomic number.

Isotopes Isotopes-Atoms of the same atomic number but different mass number due to different numbers of neutrons in the nucleus. Eg. 6 6 10 10 1 1 1 C C Ne Ne H H H 12 14 20 22 1 2 3 (See Page 31 BOOK)

Mass Spectrometer Mass Spectrometer-may be used to identify the presence of isotopes, measure their relative atomic masses and measure the relative abundance of elements. (See BOOK Page 32)

An outline of what happens in a mass spectrometer Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't. The sequence is : Stage 1: The sample is vaporised. Stage 2: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions. Stage 3: Acceleration The ions are accelerated so that they all have the same kinetic energy.

Stage 4: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. Stage 5: Detection The beam of ions passing through the machine is detected electrically.

Mass spectrometer

Relative Atomic Mass Relative atomic mass- mass of the atom compared to 1/12th of the mass of the carbon 12 isotope.

Electronic Configurations Writing electronic configuration of atoms and ions (see book pages 35-38 ). Aufbau principle- Electrons occupy the lowest available energy level. Hund’s Rule of Max multiplicity-When two or more orbitals of equal energy are available to the electrons, they occupy them singly before occupying them in pairs. Pauli’s exclusion principal- No more than two electrons can occupy an orbital and they must have the opposite spin s-orbital-------2 electrons p-orbital-------6 electrons Examples Pages 39 and 40 BOOK d-orbital-------10 electrons Exam on configerations