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.
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 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.
Reading 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 the top left to 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 neutrons 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.
Metals, Nonemetals, and Semimetals
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! See you next time!