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Modern View of Atomic Structure Atoms consist of a tiny, positively charged nucleus (containing protons and neutrons) surrounded by a “cloud” of negatively charged electrons Relative charge of electron: −1 Relative charge of proton: +1 Neutrons carry no charge (electrically neutral) Atoms contain an equal number of electrons and protons, so atoms have no net electrical charge The nucleus is extremely small in comparison to the volume of the atom (Rutherford!) The majority of the atom’s volume is the space in which the electrons reside The electrons are attracted to the protons in the nucleus (electrostatic attraction between opposite charges) Atoms have very small masses - the heaviest known atom is about 4 x 10−22 g Therefore, atomic mass unit (amu) is used to describe the mass of an individual atom The proton and neutron have nearly identical mass, and are much heavier than the electron (1 proton has same mass as 1836 electrons) Atomic Number, Mass Number, and Isotopes The atoms of each element have a characteristic number of protons This is known as the atomic number For neutral atoms, the number of electrons must equal the number of protons Every carbon (C) atom contains 6 protons and 6 electrons Every neon (Ne) atom contains 10 protons and 10 electrons Atoms of a given element can differ in the number of neutrons they contain and consequently in mass Most C atoms have 6 neutrons, although some have more and some have less The mass number is the total number of protons and neutrons in the nucleus Mass number Atomic number 12 6 C Symbol of element The atomic number is often omitted when chemical symbols are used. Carbon-12 can be represented simply as 12 C Atoms that contain 6 protons and 8 neutrons would have the following symbol 14 C Atoms with identical atomic numbers but different mass numbers are called isotopes of one another Various carbon isotopes include 11 C 12 C 13 C 14 C Identify the subatomic particles in the following: 121 Sb 192 Os 163 Dy 94 Mo