| Abstract from DBPedia | Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 picometers across. They are so small that accurately predicting their behavior using classical physics, as if they were tennis balls for example, is not possible due to quantum effects. More than 99.94% of an atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, then the atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively – such atoms are called ions. The electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by the nuclear force. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay. The number of protons in the nucleus is the atomic number and it defines to which chemical element the atom belongs. For example, any atom that contains 29 protons is copper. The number of neutrons defines the isotope of the element. For example, a copper atom with 34 neutrons is copper-63 (29+34), and with 36 neutrons is copper-65; natural copper is about 70% Cu-63 and the rest is Cu-65. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. For example, New York City's Statue of Liberty was originally made of pure copper, but over the years, the surface combined with oxygen, carbon and sulfur atoms to make a green patina on the copper. The ability of atoms to attach and detach is responsible for most of the physical changes observed in nature. Chemistry is the discipline that studies these changes.原子（げんし、（希: τὸ ἄτομον, ἡ ἄτομος、英: atom）という言葉には以下の3つの異なった意味がある。 1.
* 紀元前5～4世紀の古代ギリシアの哲学者レウキッポスとデモクリトスが提唱した、自然を構成する分割不可能な最小単位。哲学上の概念であり、経験的検証によって証明された科学上の対象ではない。詳細は「原子論」を参照。 2.
* 19世紀前半に提唱され、20世紀前半に確立された、元素の最小単位。その実態は原子核と電子の電磁相互作用による束縛状態である。物質のひとつの中間単位であり、内部構造を持つため、上述の概念「究極の分割不可能な単位」に該当するものではない。 3.
* 上述の原子の概念を拡張し、一般に複数の粒子の電磁相互作用による束縛状態を原子と定義した時、この意味における原子のうち、原子核と電子のみからなるもの以外を異種原子と言う。 (Source: http://dbpedia.org/resource/Atom) |
