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


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


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


articles found in the nucleus of an atom • Have no electrical charge – are NEUTRAL (0 charge) • Contribute to atomic mass


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


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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