Download Atoms PPT

P. Sci.
Unit 8
Chapter 4
Atoms
Atomic Structure – timeline
• Ancient Greece - Democritus
proposed the atom – a tiny solid
particle that could not be
subdivided.
• 1904 – J.J. Thomson –
discovered that atoms contained
small, negatively charged
particles called electrons.
• 1911 – Ernest Rutherford –
proposed that the atom had two
parts – the nucleus in the center
(most of the mass) surrounded
by the electrons.
Dalton’s Atomic Theory
• Every element is made of tiny,
unique, particles called atoms that
cannot be subdivided.
• Atoms of the same element are
exactly alike.
• Atoms of different elements can
join to form molecules or
compounds
• 1913 – Niels Bohr –
hypothesized that
electrons traveled in fixed
orbits around the atom’s nucleus.
• 1913 – James Chadwick –
concluded that the nucleus
contained positive protons and
neutral neutrons.
• 1926 – Erwin Schrodinger –
developed the quantum
mechanical model – which is
based on the wavelike properties
of the electron. (not a particle –
leads to quantum physics)
• 1927 – Werner Heisenberg
– (the Heisenberg
Uncertainty Principle)
described that it is
impossible to know
precisely both an
electron’s position and
path at a given time. Led to
the Electron Cloud Theory.
electron
neutron
proton
Atoms
• The smallest particle that has the
properties of an element.
Parts of an Atom
• Nucleus – small, dense center of
an atom made up of 2
subatomic particles that are
identical in size and mass.
–Protons – have a
positive (+) charge
–Neutrons – have no charge
Parts of an Atom cont.
• Electrons – tiny subatomic
particles that have very little mass
that move around the outside of
the nucleus. These particles are
negatively charged and form a
“cloud” around
the nucleus.
• The number of protons and
electrons an atom has is unique
for each element.
Atomic Charge
• Atoms have no overall charge
because the protons (+) cancel
out the electrons (-).
Helium
2 protons - +2
2 neutrons 0
2 electrons - -2
total charge 0
Protons
• positive (+) charge
• Found in the nucleus
• # of protons = atomic #
• The number of protons identify
the element (atomic #)
Neutrons
• no charge
• Found in the nucleus
• Along with protons, makes up
atomic mass
• Atomic Mass – atomic number = # of neutrons
(rounded to whole #)
Electrons
• negative (–) charge
• travel in orbitals (or energy levels)
around the nucleus (electron cloud).
• Equals atomic number in neutral
atoms
• valence electrons - the # of
electrons in the outer shell and
relates to the oxidation #
• Unit of measure for atomic
particles is Atomic mass unit
(amu) protons and neutrons =
about 1 amu (electrons are
about 1/2000 of the size of
protons and neutrons))
Chemical symbols
• The one or two letter abbreviation
of the element name.
• Some are based on Latin name
• ALL 1st letter is upper case
• ALL 2nd letter is lower case
Mass Number or Atomic Mass
• the sum of the number of protons
and the number of neutrons in the
nucleus of an atom.
• # of neutrons = mass # - atomic #
Atom Summary
• Atomic Number = protons = electrons
• Atomic Mass = Protons + Neutrons
• Neutrons = atomic mass – atomic number
• Atomic symbols
– First letter is ALWAYS upper case
– Second letter is ALWAYS lower case
• Example: Identify the Number of Protons,
Neutrons and Electrons in Oxygen
Oxygen element 8 with mass 16
P=8
N = 16-8 = 8
E=8
Atom Summary
• Example 2: Identify the Number of
Protons, Neutrons and Electrons in Sulfur
and Sodium
Isotopes
• Atoms of the same element that
have a different # of neutrons and a
different atomic mass. (identified by
the element name followed by the
mass # )
• ex. C-12, C-14, B-10, B-11)
Carbon 14 = 8 neutrons 6 electrons
6 protons 6 protons
6 electrons Carbon 12 = 6 neutrons
Isotope Practice - Find Protons, Neutrons, and
Electrons for following elements:
• Silicon
– Protons = 14
– Electrons = 14
– Neutrons = 28-14=14
• Si-30 (Silicon-30)
– Protons = 14
– Electrons = 14
– Neutrons = 30-14=16
• Uranium
– Protons = 92
– Electrons = 92
– Neutrons = 238-92=146
• U-235 (Uranium-235)
– Protons = 92
– Electrons = 92
– Neutrons = 235-92=143
Remember protons ALWAYS equals the atomic number. In
neutral atoms electrons equals protons. But neutrons is
atomic mass minus atomic number. For isotopes you MUST
use the mass given in the name NOT the mass on the periodic
table
Average atomic mass
• the weighted - average mass of the
mixture of all an atoms isotopes.
The average atomic mass is close
to the mass of its most abundant
isotope.
• This is the number found on the
periodic table
Using Bohr’s Atomic Model
• Bohr was the 1st person to propose the concept of
electrons having specific energy levels
• This explained how electrons could give off light ( by
gaining or losing energy)
• Bohr’s Model can be used to show how the
electrons will arrange themselves around a nucleus.
•
BOHR
MODEL
Electrons are placed in energy levels surrounding
•
•
•
•
•
the nucleus
You fill the lower energy levels first
1st energy level can hold max of 2 electrons
2nd energy level can hold max of 8 electrons
3rd energy level can hold max of 18 electrons
4th energy level can hold max of 32
18e8eNucleus
(p+ & n0)
2e-
Example
Chlorine
17 protons
17 electrons
18 neutrons
Valence Electrons: Electrons found in outer
energy level.
Chlorine has 7 electrons in outer energy
level (ring 3)