Pure Substances and Mixtures
• Pure substances cannot be broken down by
physical means.
• Pure substances contain only one kind of
molecule.
– Molecules are small groups of atoms that make up
matter. Example: Water is a molecule of two
hydrogen atoms, and one oxygen atom
• Atoms are the smallest particles of elements
• Two types of pure substances:
• Elements – pure substance made of only one
kind of atom
• Compounds – pure substances made of two or
more different kinds of elements joined
together
– Compounds cannot be separated by physical
means
– Compounds are joined in definite proportions.
• Mixtures – combination of 2 or more different
kinds of substances ( molecules)
• In a mixture, each molecule keeps its own
identity.
• Mixtures can be separated by physical means.
• Two types of mixtures:
• Homogeneous mixture – combination of
substances in which the appearance is the
same throughout; also called a solution.
– Examples: salt water, sugar water
• Heterogeneous mixture – made of different
substances that can be seen – does not
appear the same throughout.
– Examples – salad, cereal, pizza
• Atoms and Elements
• Facts about atoms and elements:
– Atoms are the basic building blocks of all matter (the smallest
particle of matter)
– There are different kinds of atoms.
– Each kind of atom is an element.
– Elements are pure substances that cannot be broken down
into simpler substances by ordinary chemical means.
• Parts of an Atom:
– Nucleus – central dense core of the atom. It is made of:
» Protons – positively charged particles
» Neutrons – neutrally charged particles
– Electron cloud – surrounds the nucleus of the atom
» Electrons – negatively charged particles found in the
electron cloud
• Atomic models – used to represent the atom; the
following is needed to make an atomic model
– Atomic number – number of protons in the nucleus of an
atom
– Atomic mass – equal to the number of protons and the
number of neutrons in the nucleus of the atom
• Making atomic models – the following information is
needed for an atomic model
– # of protons = the atomic number
– # of neutrons = the atomic mass – atomic #
– # of electrons = 3 of protons
» Energy levels – electrons surround the nucleus in energy
levels
» Rules for energy levels:
• 1st energy level contains no more than 2 electrons
• 2nd energy level contains no more than 8 electrons
• 3rd energy level contains no more than 18 electrons
• Last energy level contains no more than 8 electrons
• Valence electrons is the name given to electrons
in the last energy level of the atom. There will
NEVER be more than 8 valence electrons
• Elements in columns 1A through VIIIA: the
number of valence electrons can be
determined by the column number; 1A has
1 valence electron,
IIA has 2 valence electrons, etc.
• Electron Dot Configurations
• Electron Dot configurations – are used to represent
atoms; 2 things are needed:
– Symbols of elements
– Valence electrons
• Draw the electron dot configuration of the
following elements:
• H
Mg
B
C
• N
O
F
He
Kr
• IV. Compounds – two or more elements held
together by a chemical bond.
• Chemical bond – the point at which elements
are joined in a compound; the bond is made
by either sharing or giving and taking valence
electrons.
• Chemical formula – the recipe for a compound
•
Bonds for compounds:
Sodium Chloride
Water