Chemical element

An element is the simple chemical substance that consists of atoms of only one type and cannot be split by chemical means into a simpler substance. For example, gold, oxygen, and carbon are all elements. The number of protons in the nucleus of an atom determines the identity of an element. For example, atoms that have one proton are hydrogen, and atoms that have eight protons are oxygen.

Atoms are the smallest parts of a chemical element that still have the properties of that element. Elements are therefore identified by the properties of their atoms and in particular, the number of protons in the nucleus. All hydrogen atoms have one proton, all carbon atoms have six, all oxygen atoms have eight, etc.

Common elements
Carbon, hydrogen, nitrogen, and oxygen are frequently found in nature. Carbon is essential to all living organisms and is additionally found in many elements of nature as well as in man-made products. Hydrogen is a gaseous element found in great abundance throughout the universe. The most common form of hydrogen on earth is in its compound with oxygen, water. It is also used by man for many metallic and weapon-oriented purposes. Nitrogen makes up the majority of Earth’s air, but it is also used by man to manufacture fertilizers, poisons, and explosives. Oxygen is an especially essential element on earth as it makes up the ozone. The ozone protects earth and its inhabitants from the ultraviolet rays of the sun. These four elements abundant in nature work together to form a great number of organic and inorganic compounds.

Periodic table

 * Main Article: Periodic table

Elements are arranged into a table (called the Periodic Table of Elements) based on properties that different element share in common - properties that were recognized to repeat periodically when elements of increasing atomic masses were compared.

Each cell of the table contains a different element. The cells of most periodic tables contain the name and symbol of each element, the atomic number (the number of protons), and the atomic mass (average number of protons + neutrons). Some periodic tables will contain other information, such as the electron shell configuration of each element, or their state at room temperature (liquid, gas, or solid).

The electron configurations of atoms primarily determines their chemical reactivity, and particularly the the outer-shell (or "valence") electrons. The elements in the periodic table are arranged into rows (known as groups or families), and columns (known as periods) based on how many shells of electrons an atom possesses and the number of electrons in the outer-shell respectively.



Chemical symbol
Every element has its own chemical symbol or atomic symbol to identify it. Today most chemical symbols derive from the modern English names of the elements they represent. However, the metals of antiquity retain the symbols that derived from their original Latin or Greek names, e.g., natrium (sodium), kalium (potassium), hydrargyrum (mercury), etc.

Atomic number
The number of protons that an element has is called its atomic number. All atoms of the same element have the same atomic number. The atomic number of the various elements can typically be found above the element's chemical symbol in the Periodic Table of Elements.

Atomic mass
The total number of protons and neutrons that any given atom has is its atomic mass number. However, most entries in the periodic table identifies the atomic mass of each element. This number is the weighted average of the atomic mass numbers of the various forms (isotopes) of that atom that are found in nature.

Isotopes

 * Main Article: Isotope

Atoms of the same element that have differing numbers of neutrons are called isotopes. Every element that is found in nature (i.e., is not a synthetic element) normally exists as multiple isotopes, in varying proportions. Thus to compute the atomic mass, one computes a weighted average of the atomic mass numbers of all the isotopes, using as weights the proportions of the isotopes that typically exist in nature.

Two exceptions to this rule exist:

1. A synthetic isotope never has a part in computing the weighted-average atomic mass.

2. The atomic mass of a totally synthetic element, such as technetium, is the atomic mass number of the most commonly synthesized isotope, or the first such isotope reported.

Since isotopes have only different number of neutrons, they have the same electrical (bonding) properties, but frequently have different physical properties such as half-life. Many dating techniques involve measuring isotope quantities in a sample. (See radiometric dating) Furthermore, one can separate isotopes of the same element by using a gas centrifuge. This technique produces blends of isotopes that are significantly different from the natural blend. The most common application of this technique is the enrichment of a radioactive element, such as uranium, to produce fuel for a nuclear reactor, or to build a nuclear weapon.

Synthetic elements
A synthetic element is any chemical element that is not found in nature and exists only in laboratories. Synthetic elements come from controlled fusion of lighter atoms, using any of various particle accelerator instruments (linac, cyclotron, etc.) to accelerate atoms and cause them to collide with one another. The synthesis of new elements is done for two reasons:

1. To show that certain elements that are "missing" from the periodic table do in fact have the properties that the periodicity of all chemical elements would predict that they would have.

2. To produce new elements in order to study the type, quality, and quantity of particles that they might emit.

The list of synthetic elements includes technetium, promethium, plutonium, and every element having an atomic number higher than that of plutonium.

All synthetic elements, without exception, are radioactive. Most of them have very short half-lives.

Reference
화학 원소 Elemento químico