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The Bohr-Rutherford Model of the Atom
Bohr and Lewis
diagram drawing
review;
HW, read Russian
story.

The Bohr model is like a ‘planetary’ model: a core nucleus of
protons and neutrons is surrounded by electrons. The
electrons are in distinct energy levels (shells). The protons
and neutrons make up most of the mass of an atom. If the
atom is neutral, the protons = electrons. An ion gains or loses
electrons to get a full valence (outer) shell.
-
protons = 1+ charge
neutrons = 0 charge
electrons = 1- charge
 The shells hold the same number of electrons as there are
elements in each period (row):
2 – 8 – 8 – 18 electrons
Atomic Notation and Drawing Bohr and Lewis Diagrams:
Atomic Notation:
 The mass number of an element tells us the protons and
neutrons – these particles both weigh 1 atomic mass unit (1 amu).
We round the mass number to a whole number (it’s a decimal due
to isotopes – tomorrow…)
 The masses are scaled from a standard – C-12
which has a mass of 12 amu (more later)
 The atomic number tells us the number of protons in the
nucleus. This is what defines an element
Bohr Diagram:
Lewis Dot Diagram:
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Going down a column (group), what happens to the size of the
atom? (eg. Li compared to Na compared to K)
Going across a period, what happens to the strength of the
nucleus’s positive charge?