Atomic Structure
Atom – smallest unit of matter
Protons – positively charged; in nucleus
Neutrons – neutrally charged (no charge); in nucleus
Electrons – negatively charged; in energy levels around nucleus
These particles are themselves made up of different combinations of the quark, an even
smaller particle.
Four Forces
The nucleus (the positively-charged center of the atom) is held together by a strong
nuclear force, which “glues” protons together. The electrons are attracted to the nucleus
by an electromagnetic force. This holds the atom together. Gravity plays a part, too, but is
the weakest of the forces in nature. There is also the weak force, which is responsible for
radioactive decay.
Atomic



Number = # of protons
The atomic number identifies the element
If the atomic number changes, the element changes
The number of protons will equal the number of electrons in a neutral atom; this is
an atom without a charge
Atomic Mass Unit -- very small unit of mass used to measure subatomic particles
Mass Number = # of protons + # of neutrons
 Elements can have different numbers of neutrons in their nuclei. These are called
isotopes.
Shorthand Notation: 14 C
Hyphen Notation: Carbon – 14
Determine the numbers of protons, neutrons, and electrons:
14
1.)
C
2.)
Magnesium – 25
Write the isotope in shorthand notation:
1.)
7 protons, 8 neutrons, 7 electrons
2.)
21 neutrons, 20 electrons, 20 protons
Bohr Model

Electrons reside in specific energy levels around the nucleus
o 1st level: maximum of 2 electrons
o 2nd level: maximum of 8 electrons
o 3rd level: maximum of 18 electrons
Draw a Bohr model that has 8 protons, 7 neutrons, and 8 electrons.
Ions


Are formed when protons and electrons are not equal
Atom acquires a charge
o the charge is positive if there are more protons than electrons
o the charge is negative if there are more electrons than protons
Write in shorthand notation:
1.)
12 protons, 10 electrons, 13 neutrons
2.)
10 electrons, 9 protons, 9 neutrons
Determine numbers of protons, neutrons, electrons:
42
1.)
Sr +2
2.)
14
N -3