Periodic Table of Elements

The periodic table is the most important reference a chemist has, because it puts all the known elements into a meaningful pattern.

Elements are arranged from left to right and top to bottom, in the order of increasing atomic number. This order generally goes with increasing atomic mass.

The different rows of elements are called periods. The period number of an element signifies the highest energy level an electron in that element occupies (in the unexcited state).

The number of elements in a period increases as one moves down the periodic table because as the energy level of the atom increases, the number of energy sub-levels per energy level increases.

In 1869, the Russian chemist Dmitri Mendeleev noted that the repeating patterns of behaviour could be arranged in a sequence of elements. This led to the first periodic table of elements.Although Mendeleev is often considered as the 'father' of the periodic table', the work of many scientists contributed to its present form.

By 1869, a total of 63 elements had been discovered and as the number of known elements grew, scientists began to recognise patterns in properties and began to develop classification schemes.Law of Triads was the method used by Johann Dobereiner in 1817.

Later, an English chemist introduced the Law of Octave as another method of classifying the elements.

The last major changes to the periodic table resulted from Glenn Seaborg's work in the middle of the 20th Century. Starting with his discovery of Plutonium in 1940, he discovered all the transuranic elements (group of radioactive elements whose atoms are heavier than those of uranium) from 94 to 102.

He reconfigured (rearranged) the periodic table by placing the actinide series below the lanthanide series.

In 1951, Seaborg was awarded the Nobel Prize in Chemistry for his work. Element 106 has been named Seaborgium (Sg) in his honour.

Law of Triads

When elements are placed in the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean(average) of the atomic masses of the other two elements of the triad.

Law of Octaves

When elements are placed in increasing order of atomic masses, the properties of the eight elements are

 repeated.

Compiled by Janani Amarasekara

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Science vocabulary

Velocity - The speed of an object in a particular direction.

Microscope - An optical instrument that uses a combination of lenses to produce magnified images of very small objects.

Orbit - The path an object follows as it travels around another object.

Pole - The place on a magnet where the magnetic field is strongest.

Prototype - An original specimen that serves as a model for later examples.

Convection - The transportation of heat from one place to another by the movement of a liquid or gas. (A classroom is warmed by a hot air blower due to convection).

Cryogenics - The science of very low temperatures, far below the freezing point of water.

Theory - A general principle that explains or predicts facts or events.

Voltage - Electrical force or pressure (measured in volts).