2.3 The Periodic Table
and Atomic Theory
Electrons, Bohr models
and Ions
By PresenterMedia.com
Atomic Structure Review
Subatomic particles – where are they
Protons
Neutrons
Electrons
Are in the nucleus, and are positively charged
Along with Protons, make up most of the mass of the atom
Are also in the nucleus
Are neutral – have NO charge
Surround the nucleus in different energy levels,
or “shells”. They have a NEGATIVE charge
Let’s remind ourselves of how small an atom is!
(Video link)
How many of each subatomic
particle in an atom?
Protons = Atomic number of the element
Neutrons = Mass number – Atomic number
(Rounded atomic weight – mass number)
Electrons = In an atom (electrically neutral), electrons are
equal in number to the protons.
In an Atom:
Protons = Atomic number = Electrons
Neutrons = Mass number – atomic number
Let’s look at Phosphorous
From the Periodic table:
Atomic number = 15 therefore,
# of Protons = 15 and
# of Electrons = 15
Mass number = 31
Mass number – Atomic number = 31 – 15 = 16
# of Neutrons = 16
Where do electrons exist?
Where do they get “put”?
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Electrons exist in energy shells.
Each Energy level can hold a certain
number of electrons
The first level can hold up to 2 electrons, the
second level up to 8, the third up to 8, and
the 4th up to 18.
**this is not entirely accurate, but will work
for our needs now
We can actually really only guess at where an electron
exists (video link)
Electron configuration
Simplified for Science 9
We are only worried
about general patterns
in the periodic table,
and electron
configuration for the first
20 elements!
So for now, electrons fill
up the shells like in the
diagram to the right.
Electron energy levels
are not circular, but we
use this to model the
configuration.
Bohr Models
Bohr models show electron arrangements in shells
For Phosphorus – there are 15 electrons. SO:
Bohr Models
Using Bohr models we can see more trends in the
Periodic Table.
What do you notice?
Bohr Models
Trends in the Periodic table
Each family has the same number of outer shell
electrons – also known as valence electrons
Each period
has the same
number of
“shells”
Stability of atoms
To be stable, atoms would “like” to have full outer
shells.
The Noble Gases are very unreactive, and rarely form
compounds. This is because these elements have full
outer shells of electrons. Their valence shells are full.
Valence electrons (electrons in the outer shells) have
the strongest influence on the ability of elements to
form bonds.
Filled valence shells mean more stability
Forming Ions
Most other atoms gain or lose electrons in order to achieve the
stability of noble gases.
Gaining or losing electrons makes atoms into ions.
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Metals lose electrons to form positive ions
This seems counter intuitive (losing makes a positive?)
One way to look at it is that giving is a positive trait, and
metals are “givers” so form positive ions
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Non-metals gain electrons to form negative ions
Non-metals are “takers” so are negative
Forming ions
Ions have a similar electron arrangement to the nearest noble
gas
Example: Sodium ion (Na+) has 11 protons (11+) and
10 electrons (10-) for a total charge of 1+
Forming Ions
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Examples