CHEMISTRY NOTES:
• Atom: the smallest unit of matter that retains the
identity of the substance
• Atoms are composed of 2 regions:
• Nucleus: the center of the atom that contains the
mass of the atom. Protons and neutrons are located
here.
• Electron cloud: region that surrounds the nucleus
that contains most of the space in the atom
• The nucleus contains:
• Protons: positively charged particles. The number of
protons determines the identity of the atom
• Neutrons: neutrally charged particles
• Protons and neutrons make up almost all the mass of
the atom
• The electrons reside outside of the nucleus in the electron
cloud
• Electron: the particle with a negative charge and
almost no mass (its mass is about 1/2000th that of a
proton or neutron)
• In a neutral atom:
• The number of protons = the number of electrons
• An atom with an unequal number of protons and
electrons is called an ion. Ions have an overall charge.
• The charge represents the imbalance between
protons and neutrons. For example if an atom gains 2
electrons, its charge is -2 (the negatives outnumber
the positives by 2. If an atoms loses three electrons,
its charge is +3 (the positives outnumber the
negatives by 3)
• Isotopes are atoms of the same type that have different
numbers of neutrons. Because they are the same type of
atom, isotopes have mainly the same properties – the
difference is the mass is greater or less due to different
number of neutrons
• Example: Carbon atoms could be:
• “Carbon-12”: 6 protons, 6 electrons, and 6 neutrons,
OR
• “Carbon-13”: 6 protons, 6 electrons, 7 neutrons
• Atomic Notation: A standard way that chemists “address”
or identify a specific atom. Ex: see below
Mass number = number of protons plus number of neutrons in
that specific atom. (Sometimes atoms are written with their
name, then a dash, then their mass number. For example
carbon-13, means a carbon atom with a mass number of 13)
Atomic number= the number of protons. (This is what makes
one type of atom different from other types)
Symbol: The capital letter OR capital letter and lowercase letter
that stands for a certain element. (Ex. C is for Carbon and Ca is
for calcium)
Charge: Here the charge is +1. This means that the positively
charged protons outnumber the electrons by 1. We know there
are 11 protons, so this atom must have 10 electrons
Note: You only need to know how many electrons can
maximally fit in the first 3 shells (energy levels): 2, 8,
18.
Valence electrons: those electrons found in the
outermost shell (energy level) of an atom.
Bohr model: Example above. (It shows the nucleus
and then each energy level with the electrons which
reside there)
Lewis structure: consist of the elements symbol and a
dot for each of the valence electrons. Ex:
Organization of the P. T.
-Elements are arranged in order of
increasing atomic number
-Columns of elements are called groups or
families. (Going left to right can you name the
families?)
-Elements in each family have similar but
not identical properties.
-All elements in a family have the same
number of valence electrons.
For example:
Alkali Metals: Sodium, Lithium etc. have one valence electron
Alkali Earth metals: Beryllium, Magnesium, etc. have 2 valence electrons
Boron Family: Boron, Aluminum etc. have 3 valence electrons
Carbon Family: carbon, Silicon etc. has 4 valence electrons
Nitrogen Family: Nitrogen, Phosphorus etc. have 5 valence electrons
Oxygen Family: Oxygen, Sulfur etc. have 6 valence electrons
Halide or halogen family: Have 7 valence electrons
Noble gases: Helium, Neon, Argon, etc. have a complete outer shell. Helium
has 2 and all other noble gases have 8 valence electrons.
Horizontal Rows on the Periodic Table are
called periods. Members of a period do not
have similar characteristics. Periods are
numbered. The first period contains
hydrogen and helium. The second period
contains Li, B, B, C, N, O, F, and Ne.
The third period contains Na, Mg, Al, etc all
the way until Ar.