Hey guys!
Welcome to this Mometrix video over the periodic
table.
The periodic table of elements is, hands down, one of the most important tools you will use in chemistry.
Through its innovative organizational layout,
the table gives us concise and fundamental information;
not just about every single element, but also about trends that occur across all the elements.
Let's look at a short history of the Periodic
Table.
In 1789, the French chemist Antoine Laurent Lavoisier defined an element as a fundamental substance
that could not be broken down by
any chemical means then known.
He then proceeded to compile a list of 33 elements, which just so happen to not actually be elements;
but he did spur this idea of
a systematizing and understanding elements.
In 1803 English chemist, John Dalton, used
the general scientific recognition, that elements
combined with each other according to different
ratios by weight, to create an atomic theory
that claimed all elements were built out of
variable numbers of hydrogen atoms.
Because of this theory, Dalton created a scale
of atomic weight based on the hydrogen atom.
In 1869, Russian chemist Dmitry Mendeleyev
systematically arranged elements in a table
according to their atomic weights.
Mendeleev noticed something that no one else
had.
He realized that there was a number of unknown
elements with atomic weights between elements
that had already been discovered.
By simply leaving openings for elements that
he believed were undiscovered, he developed
an organizational scheme that vertically grouped
elements with alike properties.
He found that elements with low atomic weights
had similar chemical characteristics that
recurred every seven elements.
Among heavier elements, he found that characteristics
recurred every seventeen elements.
The exciting discovery that physical and chemical
characteristics of the elements are periodic
functions of their atomic weight became known
as the periodic law.
In 1879 Mendeleev's periodic table became
widely accepted, when it predicted the existence
of gallium, germanium, and scandium.
Since, the table has seen small changes, and
many, many elements have been added.
Now, let's look at how to read the periodic
table.
The vertical columns, the ones that run up
and down, are called groups.
The horizontal rows, the ones that run side
to side, are called periods.
There are 18 groups and 7 periods.
Now, let's look at all the stuff inside the
individual boxes.
The number at the top of the box represents
the atomic number.
The atomic number for hydrogen is 1, and the
atomic number for Uranium is 92.
The Letter in the middle is the atomic symbol.
So for Hydrogen it is H and for Uranium it
is U.
It is important to know the atomic symbol,
because not all tables have the names listed below;
plus that is how scientists notate
the elements in their writing.
Generally the atomic symbol correlates to the name, as C relates to Carbon; but sometimes they do not.
For example, the symbol for mercury is Hg.
The number at the bottom represents the atomic
mass.
Hydrogen has an atomic mass of 1.0079 and
Uranium has an atomic mass of 238.029.
When you move sequentially from top left to
the the bottom right across the periodic table,
the atomic number increases.
The atomic number represents exactly where
in the periodic table an element stands.
But, most importantly, the atomic number is
the same as the number of protons in the nucleus
of an atom of an element, as well as the same
amount of electrons that surround the nucleus,
when it is in a neutral state.
For example Hydrogen has one proton and one
electron and Uranium has 92 protons and 92 electrons.
An atom also contains neutrons, along with
protons within its nucleus.
The atomic mass, or atomic weight, of an element
is the combined number of protons and neutrons in the nucleus.
It's important to note that atoms of certain
elements have different versions, which just
means that elements have atoms with a different
number of protons within the nucleus.
These are called isotopes.
The atomic weight, in this case, is the average
of the atomic mass numbers of the different isotopes.
The elements can be organized by group or
period, but they can also be sorted into three
main categories: Metals, Nonmetals, and semimetals.
Metals are generally lustrous solids (though
mercury is a liquid at room temp), and often malleable.
More than half of the nonmetals are gaseous
at normal temperatures.
Semimetals, or metalloids, as the name suggests,
exist in between the distinctions of the metals and nonmetals.
What I have described are the more general
trends.
There are a number of further periodic trends
such as electron affinity, electronegativity,
atomic size, and ionization energy.
I hope this video can help you to better navigate
your way around the periodic table.
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See you next time!
