Chemistry - The Periodic Table

Robert Boyle

the Irish scientist who gave the first accurate definition of an element

Element

a substance that cannot be split into simpler substances by chemical means

Humphry Davy

discovered many elements by passing electricity through their compounds

e.g. potassium, sodium, calcium, barium, strontium, magnesium

Dobereiner

scientist who thought that elements could be categorised in groups of threes

Triad

a group of three elements with similar chemical properties in which the atomic weight (relative atomic mass) of the middle element is approximately equal to the average of the other two

e.g. Chlorine, Bromine, Iodine

Newlands’ Octaves

arrangement of electrons in which the first and eighth element, counting from a particular element, have the same properties

Limitations of the Law of Octaves

• Newlands did not realise that some elements had not yet been discovered, and did not leave gaps to account for these elements

• does not work for the modern Periodic Table, as the noble gases have since been discovered

Mendeleev’s Periodic Law

when elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements recur periodically

Mendeleev’s Periodic Table

• placed all elements with similar properties in the same vertical column

• left gaps in his table

• predicted the properties of undiscovered elements

• reversed the order of some elements (Tellurium and Iodine)

Atomic Number

the number of protons in the nucleus of that atom

Moseley

• showed that if elements are arranged in order of increasing atomic number, it is not necessary to reverse the order of some elements

• x-ray studies showed that Mendeleev had been correct in placing tellurium above iodine, in spite of the higher atomic weight of tellurium

• elements fall naturally into the correct group when arranged by atomic number

Modern Periodic Table

an arrangement of elements in order of increasing atomic number

Modern Periodic Law

when elements are arranged in order of increasing atomic number, the properties of the elements recur periodically

Mass Number

the sum of the number of protons and neutrons in the nucleus of an atom of that element

Isotopes

atoms of the same element (i.e. they have the same atomic number) which have different mass numbers due to the different number of neutrons in the nucleus

Relative Atomic Mass

• the average of the mass numbers of the isotopes of the element

• as they occur naturally

• taking their abundances into account and

• expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units

Processes of the Mass Spectrometer

1. Vaporisation

2. Ionisation

3. Acceleration

4. Separation in a Magnetic Field

5. Detection

Principle of Mass Spectrometry

charged particles moving in a magnetic field are deflected to different extents according to their masses and are thus separated according to these masses

Uses of the Mass Spectrometer

• identify the presence of isotopes

• measures the relative abundances of isotopes

• measures the relative atomic masses and relative molecular masses

• identify unknown compounds

• used to test for drugs

Aufbau Principle

when building up the electron configuration of an atom in its ground state, the electrons occupy the lowest available energy levels

Hund’s Rule

states that when two or more orbitals of equal energy are available, the electrons occupy them singly before filling them in pairs

Pauli Exclusion Principle

no more than two electrons may occupy an orbital, and they must have opposite spin