1.2 Periodic table Flashcards
Mendeleev’s periodic table
In 1869 the Russian chemist Dmitri Mendeleev created his first draft of the periodic table.
He organised the elements into vertical columns based on their properties and the properties of their compounds.
He then started to arrange them horizontally in order of increasing atomic mass and as he worked, he found that a pattern began to appear in which chemically similar elements fell naturally into the same columns.
There were exceptions though as some elements didn’t fit the pattern when arranged by atomic mass.
Mendeleev worked to include all the elements, but he didn’t force an element to fit the pattern, rather he left gaps in the table that he thought would best be filled by elements that had not yet been discovered.
He also switched the order of the elements to maintain consistency down the columns.
Mendeleev’s predictions
Mendeleev quickly realised that elements with the same properties should be placed in the same column.
He realised that gaps in the table must correspond to elements that had not yet been discovered or isolated.
He used the properties and trends of other elements in the group with the gap to predict the properties of these undiscovered elements.
Problems with Mendeleev’s table
Once he was finished, Mendeleev thought he had organised the elements systematically but there were still some elements which didn’t quite fit in as neatly as he wanted.
This is because isotopes were not known in Mendeleev’s time, and he made no provisions for them in his table.
This meant that there was always going to be some level of inaccuracy in Mendeleev ́s work even though he did also consider the elements chemical properties as well as their atomic mass when sorting them.
The discovery of the proton lead to the determination of atomic number for each element.
This number is used to arrange the elements in the modern-day periodic table which fits with Mendeleev ‘s patterns.
Modern periodic table
There are over 100 chemical elements which have been isolated and identified.
Elements are arranged on the periodic table in order of increasing atomic number.
Each element has one proton more than the element preceding it.
This is done so that elements end up in columns with other elements which have similar properties.
The table is arranged in vertical columns called groups and in rows called periods.
Electronic configurations
We can represent the electronic structure of atoms using electron shell diagrams.
Electrons orbit the nucleus in shells and each shell has a different amount of energy associated with it.
The further away from the nucleus, the more energy a shell has.
Electrons first occupy the shell closest to the nucleus which can hold a maximum of 2 electrons.
When a shell becomes full of electrons, additional electrons have to be added to the next shell.
The second shell and third shell can hold 8 electrons each.
The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons.
In most atoms, the outermost shell is not full and therefore these atoms react with other atoms in order to achieve a full outer shell of electrons (which would make them more stable).
