lesson 21 development of the periodic table Flashcards
why is it called the periodic table
because the word periodic means occurring at regular intervals
and in the periodic table we can see that elements with similar properties occur at regular intervals
all of the elements in a group have what
similar chemical properties
(the react in similar ways)
all of the elements in a group have the same
number of electrons in their outer energy levels
the scientist Johann Döbereiner noticed what
that elements in similar chemical properties often occurred in threes
called triads
after 40 years later Johann Döbereiner discovered triad, what occurred
the scientist John Newlands noticed something different
what did the scientist John Newlands did what
ordered the elements in increasing atomic weight
what did John Newland also see with every eighth element
every 8th element reacts in a similar way
What law did Newland discover
law of octaves however it wasn’t really taken seriously by other scientists
what did Dmitri Mendeleev discover
the first modern periodic table
Mendeleev started by ordering all the elements in order of what
in order of increasing atomic weight
if Mendeleev needed to, he’d do what
switch the position of all the elements
why did Mendeleev switch the position of all the elements if needed
so they fitted the patterns of other elements in the same group
Mendeleev realised to elements had not been discovered so he
left gaps in his periodic table where he thought an element was missing
Mendeleev was so confident that his table was correct, so he did what
he predicted the properties of the undiscovered elements based on other elements in the same groups
several years later the undiscovered properties were discovered and they
matched Mendeleev’s predictions
as Mendeleev’s predictions matched the undiscovered properties, what occurred
other scientists accepted that Mendeleev’s table was correct
the modern periodic table almost the exact same as Mendeleev’s apart from which important differences
in the modern periodic table the elements are arranged in order of atomic weight (number of protons)
when we order by atomic number, every element is what
in its correct group
when Mendeleev had discovered his table, what had not been discovered
protons so he ordered the elements by atomic weight
the problem with ordering the elements by atomic weight is that
elements can appear in the wrong order due to the presence of isotopes
the modern periodic table has group 0(noble gases) however these were not
fully discovered when Mendeleev published his table
