Atomic Structure
Two regions of every atom:
Nucleus - is made of
protons and neutrons - is small and dense
Electron cloud -is a region where you
might find an electron -is made up of many “shells”
p np p n n
n
Atoms are mostly empty space
= the #
of
protons
in the
nucleus
Atomic Number (“Z”)
Subatomic particles;
Name Symbol Charge
Relative mass
Electron e- -1 0.0005
Proton p+ +1 1
Neutron n0 0 1
Electrons weigh almost nothing at all!
Opposite charges attract each other
( + and - )
Alike (similar) charges repel each other
( - and - )
= the #
of
protons
in the
nucleus
Atomic Number (“Z”)
Electrons can only be in certain, specific
energy
levels (“shells”)
(distances from the nucleus)
in the electron cloud
1st 2nd 3rd
Bohr Model Diagrams
electron
shell ( = “energy level” )
…show energy levels (shells)
and the electrons in them.
The nucleus
is not shown.
- Shells fill in order - Inner shells fill first.
1stshell gets 2 e-
2ndshell gets 8 e- - Each shell can hold a certain number of electrons:
3rdshell gets 8 e-
Valence Shell:
The outermost shell that contains electrons.
Valence Electron:
An electron in an atom’s valence shell
Negative electrons are found in
‘shells’ = ‘energy levels’
positive protons
& neutral neutrons
are found in
the dense core
(nucleus) of atoms.
Modern View of the atom:
• Two regions
• Nucleus- protons and neutrons
• Electron cloud- has many shells (energy levels)
Ions:
Mini-Lecture
Counting Charge: Ions
“Atomic Number” Z
is the number of protons.
determines what element an atom is.
( Z ) – (# of e-) = net charge of the atom
Ion: An atom with a net charge
Cation: positive ion
Anion: negative ion
Counting Charge: Ions
Fe
( Z ) – (# of e-) = net charge of the atom If you see an element symbol alone, assume it has an equal number of p+and e-, and therefore no net charge.
Ca S Mg
Ti
3+Cl
1-Fe
2+O
2-Cations and anions are indicated by writing the net charge of the ions in superscript to the right of the element symbol.
Symbols
Symbols
What is the What is the… …
– – Net charge Net charge
– – number of protons number of protons
– – number of electrons number of electrons
– – Atomic number Atomic number Br 1-
Symbols
Symbols
What is the… What is the …
– – Net charge Net charge
– – number of protons number of protons
– – number of electrons number of electrons
– – Atomic number Atomic number Ca 2+
Symbols
Symbols
What is the What is the… …
– – Net charge Net charge
– – number of protons number of protons
– – number of electrons number of electrons
– – Atomic number Atomic number O 2-
Bohr
Diagrams
Counting Nucleons; Isotopes
“Atomic Number” Z
is the number of protons.
determines what element an atom is.
if the atom is neutral, Z = # of e-
“Mass Number”
the number of protons + neutrons.
called the mass number, because p+and n0 are what make up the mass of atoms; the mass of electrons is insignificant.
Isotopes
Atoms of the same element can have differing masses due to differing numbers of neutrons.
Example: Carbon-12 ( 12C ) Carbon-13 ( 13C ) are isotopes of Carbon.
Isotopes:
Atoms that share the same atomic number (same element; same # of p+)
but have a different mass number.
(different isotopes of that element: different # of n0)
Two ways of writing isotopes:
Contain the symbol of the element, the mass number and the atomic number:
X
Mass number
Atomic
number
39 19 K
1.)
2.)
Put the mass number after the element’s name:carbon- 12 carbon –14 uranium-235
Symbols
• Find the
– number of protons – number of neutrons – number of electrons – Atomic number
– Mass Number
19 9 F
Symbols
Symbols
Find the Find the
– – number of protons number of protons
– – number of neutrons number of neutrons
– – number of electrons number of electrons
– – Atomic number Atomic number
– – Mass Number Mass Number
80 Br
35
Symbols
Symbols
if an element has an atomic if an element has an atomic
number of 34 and a mass number
number of 34 and a mass number
of 78 what is the
of 78 what is the
– – number of protons number of protons
– – number of neutrons number of neutrons
– – number of electrons number of electrons
– – Complete symbol Complete symbol
Symbols
Symbols
if an element has 91 protons and if an element has 91 protons and
140 neutrons what is the
140 neutrons what is the
– – Atomic number Atomic number
– – Mass number Mass number
– – number of electrons number of electrons
– – Complete symbol Complete symbol
Symbols
Symbols
if an element has 78 electrons and if an element has 78 electrons and
117 neutrons what is the
117 neutrons what is the
– – Atomic number Atomic number
– – Mass number Mass number
– – number of protons number of protons
– – Complete symbol Complete symbol
Study Guide
Measuring Atomic Mass
• When measuring the mass of atoms, we do not use grams because the numbers would be too small; instead we use the
1 amu
= 1/12 the mass of a carbon-12 atom.
( official definition)
Atomic Mass Unit (amu)
Why use amu?
Because an amuis basically defined as the mass of a proton or a neutron,
The mass number of an atom is
the mass of the atom in amu.
A Carbon-12 atom has a mass of 12 amu.
A Potassium-40 atom has a mass of 40 amu.
Atomic Mass
• How heavy is an atom of oxygen?
• It depends on number of protons and neutrons:
– Remember: the mass of an electron is negligible.
• There are different isotopes of oxygen.
• So, we take an average based on how common each isotope of oxygen is and calculate the…
Average Atomic Mass
Average Atomic Mass: A Summary
• Definition: the weighted average of all naturally occurring isotopes of an element.
• Units of atomic mass are the amu.
• Average atomic masses are not whole numbers because they are averages.
Sneak Preview:
The average atomic mass of an element (in amu) is also numerically equal to the mass of one ‘mole’ of that element in grams.
