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Atomic Structure Defining the Atom Structure of the Nuclear Atom • I CAN identify and describe the particles that form an atom. Defining the Atom EARLY MODELS OF THE ATOM • An ATOM is the SMALLEST PARTICLE of an ELEMENT that RETAINS its identity in a CHEMICAL REACTION. – The Greek philosopher DEMOCRITUS was one of the first people to suggest the existence of the ATOM. – Democritus described atoms as INDIVISIBLE and INDESTRUCTIBLE. • Early ideas of the atom DID NOT CATCH ON because the philosophers COULD NOT TEST their ideas. • More than 2000 YEARS passed before the idea of the atom became an accepted reality. – JOHN DALTON, an English teacher/chemist, used EXPERIMENTAL methods to test DEMOCRITUS’ ideas about the atom. Dalton’s Atomic Theory • Dalton formulated FOUR STATEMENTS that formed the basis of the modern idea of the atom: – All ELEMENTS are composed of ATOMS. – ATOMS of the SAME element are IDENTICAL. – ATOMS can COMBINE CHEMCIALLY to form COMPOUNDS. – ATOMS keep their IDENTITY during CHEMICAL REACTIONS. Sizing Up the Atom • Atoms are EXTREMELY SMALL! – A COPPER PENNY contains approximately 2.4 x 1022 atoms of copper. – The population of the Earth is approximately 7.0 x 109 (7 billion) people. – THEREFORE there is about 4,000,000,000,000 [4 TRILLION] times more ATOMS IN A COPPER PENNY than there are people on the Earth! STRUCTURE OF THE NUCLEAR ATOM SUBATOMIC PARTICLES (NOTE- While there are 17 Fundamental Particles identified – 6 Quarks, 6 Leptons, 4 Gauge Bosons and the Higgs Boson - and over 200 combination particles, we will focus on only three basic subatomic particles, since we can explain about 99% of all basic chemistry using these three particles) • Atoms are composed of THREE BASIC PARTICLES known as SUBATOMIC PARTICLES. • ELECTRONS were first discovered in 1897. – Electrons have a NEGATIVE electrical charge. – Electrons have a VERY SMALL MASS. It takes 1837 electrons to weigh as much as ONE PROTON. – The symbol for an electron is e- [note it’s lower case!] • PROTONS were first discovered in 1886. – Protons have a POSITIVE electrical charge. – Protons have a mass about 1837 times larger than ELECTRONS. – The symbol for a proton is upper case!] P+ [note the P is • NEUTRONS were discovered in 1932. • Neutrons have NO ELECTRICAL CHARGE (they are NEUTRAL)! • Neutrons have a mass ONLY SLIGHTLY GREATER THAN a proton. 0 • The symbol for neutron is N [note it is upper case!] THE NUCLEUS AND ELECTRON CLOUD • PROTONS and NEUTORNS are found in the NUCLEUS of the atom. – The nucleus is the SMALL, DENSE and SOLID central core of the atom. – More than 99.9% of the mass of an atom is located in the NUCLEUS. • The atom’s ELECTRONS are found in an area SURROUNDING THE NUCLEUS known as the ELECTRON CLOUD. – The electron cloud can be compared to BEES SWARMING AROUND A HIVE and it mostly just an area of EMPTY SPACE where as the nucleus is a SOLID structure. ATOMIC NUMBER • Protons play an extremely important role in the atom. • The NUMBER OF P+ determines the atom’s elemental identity. – Each different element on the PT has a unique number of P+ in its nucleus. • The NUMBER OF P+ in the atom’s nucleus is known as the ATOMIC NUMBER. • In a NEUTRAL ATOM (one that has not reacted with other atoms) the ATOMIC NUMBER is also equal to the NUMBER OF ELECTRONS the atom has. • ATOMIC NUMBER is abbreviated simply as “AN”. • ANs are WHOLE, SEQUENTIAL numbers found at the top of each element’s box on most periodic tables. ATOMIC MASS NUMBER • Along with the atomic number, the subatomic particles of an atom contribute to other numbers. • One important one is the ATOMIC MASS NUMBER, abbreviated as “AMN”. • As it’s name implies, it represents the MASS OF ONE ATOM. The unit for AMN is a derived unit known as the ATOMIC MASS UNIT, or simply “amu” or more simply “u”. • The AMN is calculated by adding the total number of PROTONS and NEUTRONS in the atom together. • On the Periodic Table, the AMN is generally on the top or bottom of each element’s box (depends on the PT you use) and, with only a few exceptions, is a number WITH A DECIMAL.