Mass number of an atom= number ofprotonsPLUSnumber ofneutronsfound in the nucleus of
anatom• The unit for atomic mass =AtomicMassUnits (amu
mass number
elements are made of “uncuttable” spheres called atoms)
400 B.C.Democritus
The weighted average of the masses of all the naturally occurring isotopes of an element • The
average considers the percent abundance of ea
Isotopes & AtomicMass
An atom is made of two main parts: – 1. The nucleus – 2. The electron cloud
what are the two main parts of an atom
Type of particle found in the nucleus • Protons have a positive one (+1) electrical charge • The
number of protons determines theelement • T
PROTONS
all the mass of an atom is in a positively charged nucleus while electrons orbit around
1911 – Rutherford’s Model
Appeared around 1926 and is the accepted model today • Electron cloud = an area around the
nucleus where the electrons are most likely to be
The electron cloud model
the solid sphere model
1800’s – John Dalton
Scientists hypothesize that electrons are one of the smallest particles that exist BUT they
believe that protons and neutrons are made of s
QUARKS
electrons are evenly embedded in a positively chargedsphere (plum pudding/cookie dough
model)
1904 – Thomson’s Model
Location • EXTREMELY SMALL (about 10-15meters) • The NUCLEUS of an ATOM is POSITIVELY
charged • Madeof two particles: – 1.PROTONS – 2. NEUT
nucleus
articles found in the nucleus of an atom • Have no electrical charge – are NEUTRAL (0 charge) •
Contribute to atomic mass
NEUTRONS
Anything that has mass and takes up space (volume) – Examples: • A brick hasmassandtakes up
space • A desk hasmassandtakes up space • A penc
what is matter
The smallest piece of matter that still has characteristics of the element Atoms make up
ELEMENTS! – Example: The element Silver (Au) is onl
what are atoms
Recall: Elements are different because of the number of PROTONS they have. • The number of
protons = the kind of atom(the fingerprint) – Exa
Protons identify the element…
nucleus is found in the center of the atom and is made of protons and neutrons; electrons travel
around nucleus in orbits
1913 – Bohr’s (and Chadwick’s) Model
Found in an ELECTRON CLOUD around thenucleus • There are orbits/levels within the electron
cloud – Each orbit/level can only hold a maximum
ELECTRONS
Thenucleusof an atomcontains most of the mass of the atom •The mass of a proton and a neutron is
about 1, 836 times greater than that of an
Atomic Mass
Chemical symbols are used to write the names ofelements • Each element has a corresponding
symbol – Examples: C, Al, Ne, Ag, N, Li, B, Be •
chemical symbols
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