ATOMS AND MOLECULES excellup（原子和分子excellup）.pdfVIP

Finish Line Beyond th Science Class 9 ATOMS AND MOLECULES Symbols of Atoms of Different Elements Atomic Mass Molecules Ions Mole Concept ©2009 send your queries to enquiry@ Finish Line Beyond Atom An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged. The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons. A proton is positively charged. The number of protons in the nucleus of an atom is the atomic number for the chemical element. A proton has a rest mass, denoted m , of approximately p -27 1.673 x 10 kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted m , of n approximately 1.675 x 10-27 kg. The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator. The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. Atoms having the same number of protons, but different numbers of neutrons, represent the same element, but are known as different isotopes of that element. The isotope for an element is specified by the sum of the number of protons and neutrons. Examples of different isotopes of an element are carbon 12(the most common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon)