# Periodic Table of Elements

An element is a substance consisting of atoms that can no longer broken down into other substances, which all have the same number of protons, which translates into its atomic number. Since there are more than 100 of these elements, chemists and other scientists needed a method of classifying them. This lead to the original periodic table which was first proposed by Russian chemist Dmitri Mendeleev in 1869. Today it is called the Periodic Table of Elements.

The current Periodic Table is a method for listing about 115 different elements. The elements are listed by the structure of each element. Atoms are made up of protons, neutrons, and electrons. The Periodic Table indicates the number of protons and electrons each atom has in its outer shell, located outside the nucleus.

On the Periodic Table of Elements, the atoms are listed from left to right and top to bottom. All of the elements are listed in the order of their atomic number, which corresponds to the number of protons in each atom's nucleus.

The elements are lined up in cycles or periods, which is why it is called a 'periodic' table. They are first lined up in rows based on their atomic numbers, but then some columns are skipped so elements with the same number of electrons line up on the same column. Elements in the same columns will then have the same properties.

The seven or eight horizontal rows of the Periodic Table are called periods. The first period is the shortest and has only two elements, hydrogen and helium. The sixth horizontal row or period contains 32 elements. The left most element in a period, or row, has just one electron in its outer shell, and the right most element has a full shell.

The eighteen vertical columns are different groups and each have different properties. An example of a group is the gases called noble or inert. They are lined up in the final (18th) column or group of the Periodic Table. Each of these elements have a full outer shell of electrons, which mean they are very stable. When an element is stable they usually do not react to other elements. They don't mix well or easily.

A second example are the metals called alkali. They align in the first column, or group, and are all very similar having only one electron in its outer shell. These elements are very reactive, meaning they easily mix with the other elements.

The classification of grouping of these elements help chemists and other scientists understand, foresee, and predict how the different elements will react with each other during experiments or in other situations.

Finally, each element has a name and a one or two-letter abbreviation to make it easier for scientists to use the table. Some of the single-letter abbreviations are easy to remember, like H for hydrogen, O for oxygen, and C for carbon. Some may be a bit more difficult because the name of the element comes from a different language. For example, AU is the abbreviation for gold because gold comes from the Latin word aurum.

In summary, the Periodic Table of Elements is a helpful and useful tool for chemists and scientists. It is used as a quick method to discover how the different elements will react to each other.

A: Proton
B: Electron
C: Neutron
D: Element

A: 1869
B: 1968
C: 1698
D: 1815

A: 18
B: 32
C: 115
D: Infinite number

A: 1st
B: 6th
C: 18th
D: 24th

A: 1st
B: 6th
C: 18th
D: 24th

A: Elements in the 1st column are less reactive than elements in the last column
B: Elements in the last column are less reactive than elements in the 1st column
C: Elements in the 1st column are gases and elements in the last column are solids
D: Elements in the last column are gases and elements in the 1st column are solids

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