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A substance that cannot be split up into simpler substances by chemical means.


Scientist who discovered many elements by passing electricity through their compounds.


Scientist who thought that elements could be categorised in groups of threes. He called such a group a triad.


A group of three elements with similar chemical properties in which the atomic weight of the middle element is approximately equal to the average of the other two.


Scientist who arranged elements in groups of eight. He said that the first and eighth element of each group has similar properties. He summarised his ideas in his "Law of Octaves."


Groups of elements arranged in order of increasing atomic weight, in which the first and the eighth element of each group have similar properties.


Scientist who developed the basis for the modern Periodic Table. He arranged the elements in order of increasing atomic weight, but switched the order of some elements so that their properties would fit and left gaps for undiscovered elements.

Mendeleev's Periodic Law

When elements are arranged in order of increasing atomic weight the properties of the elements vary periodically.


Scientist who developed a method of determining the atomic number of an element using X rays.

Modern Periodic Law

When elements are arranged in order of increasing atomic number, the properties of the elements vary periodically.

Differences in the modern Periodic Table

1. Elements are arranged in order of increasing atomic number. 2. There are more elements e.g. the noble gases. 3. There are no gaps. 4. The transition elements are listed in a separate block

Atomic number

The number of protons in the nucleus of that atom.

Mass number

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


The letter used to indicate the atomic number.


The letter used to indicate the mass number.


Atoms of the same element that have the same atomic number but have different mass numbers due to the different number of neutrons in the nucleus.


Hydrogen 1 isotope


Hydrogen 2 isotope


Hydrogen 3 isotope

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.

Stages in mass spectrometry

Vaporisation, Ionisation, Acceleration, Separation and Detection

Mass spectrometer Uses

(i) Identify the presence of isotopes, (ii) measure the relative abundances of isotopes, (iii) measure relative atomic masses and relative molecular masses and (iv) identify unknown compounds.

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