The number of protons in an atom is called theatomic number. For example, all atoms with 6 protons are atoms of the chemical elementcarbon, and all atoms with 92 protons are atoms of the elementuranium. The number of neutrons in the nucleus does not have to be the same in every atom of an element. Atoms of the same element with different numbers of neutrons are calledisotopes. Saying that a substance "contains only one type of atom" really means that it contains only atoms that all have the same number of protons.
Elements are the basic building blocks for all types of substances. If a substance contains more than one type of atom, it is acompound or amixture. The smallest particle of a compound is amolecule.
118 different chemical elements are known to modernchemistry. 92 of these elements can be found innature,[1] and the others can only be made inlaboratories. The human body is made up of 26 elements.[1] The last natural element discovered was uranium, in 1789.[3][4] The first man-made element wastechnetium, in 1937.
Chemical elements are commonly arranged in theperiodic table. Where the elements are in the table tells us about their properties relative to the other elements.
Chemical elements are given a uniquechemical symbol. Chemical symbols are used all over the world. This means that, no matter which language is spoken, there is no confusion about what the symbol means.[5] Chemical symbols of elements almost always come from their English or Latin names. For example,carbon has the chemical symbol 'C', andsodium has chemical symbol 'Na', after the Latinnatrium.Tungsten is called 'W' after its German name,wolfram.[3][5] 'Au' is the symbol for gold and it comes from the Latin word for gold,aurum.[2][6] Another symbol which comes from Latin is 'Ag'. This is the elementsilver and it comes from the Latinargentum.[6]Lead's symbol, 'Pb', comes from the Latinplumbum[6] and the English wordplumber derives from this as pipes used to be made out of lead.[1] Some more recently discovered elements were named after famous people, likeeinsteinium, which was named afterAlbert Einstein.[3]
Elements can join (react) to form purecompounds (such aswater,salts,oxides, andorganic compounds). In many cases, these compounds have a fixed composition and their ownstructure andproperties. The properties of the compound may be very different from the elements it is made from.Sodium is ametal that burns when put into water andchlorine is a poisonous gas. When they react together they makesodium chloride (salt) which is generally harmless in small quantities and edible.
Some elements mix together in anyproportion to form new structures. Such new structures are not compounds. They are calledmixtures or, when the elements are metals,alloys.
Most elements in nature consist of atoms with different numbers of neutrons.[7] Anisotope is a form of an element with a certain number of neutrons. For example, carbon has two stable, naturally occurring isotopes:carbon-12 (6 neutrons) and carbon-13 (7 neutrons).Carbon-14 (8 neutrons) is a naturally occurring radioactive isotope of carbon. At least two isotopes of each element are known (except forOganesson, of which only a few atoms have been made).
