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Matter is anything that has volume and mass.
All matter is made of substances called elements.
An element is a substance that cannot be
broken down into simpler substances by
physical or chemical means.
Elements are made up of atoms.
All atoms consist of even smaller particles—protons,
neutrons, and electrons.
The center of an atom is called the nucleus, which is
made up of protons and neutrons.
A proton is a tiny particle that has mass and a positive
electric charge.
A neutron is a tiny particle with approximately the
same mass as a proton, but it has no electrical charge
Surrounding the nucleus of an atom are electrons, smaller
particles that are in constant motion.
An electron has little mass, but it has a negative
electric charge that is exactly the same magnitude as
the positive charge of a proton.
In this
representation of
an atom, the
fuzzy area
surrounding the
nucleus is
referred to as an
electron cloud.
All atoms of an element have the same number
of protons. However, the number of neutrons of
an element’s atoms can vary.
Atoms of the same element that have different
mass numbers are called isotopes.
The atomic mass of an element is the average of
the mass numbers of the isotopes of an element.
Radioactive decay is the spontaneous
process through which unstable nuclei
emit radiation.
In the process of radioactive decay, a
nucleus can lose protons and neutrons,
change a proton to a neutron, or change a
neutron to a proton.
Because the number of protons in a
nucleus identifies an element, decay can
change the identity of an element.
An atom that gains or loses one or more
electrons from its outermost energy level has a
net electric charge and is called an ion.
•Positive ions – atoms that lose electrons
•Negative ions – atoms that gain electrons
The two most abundant elements in the
universe are hydrogen and helium. However,
the two most abundant elements in Earth’s
crust are oxygen and silicon.
A compound is a substance that is composed of
atoms of two or more different elements that are
chemically combined.
Compounds have different properties from the
elements of which they are composed.
Compounds are represented by chemical formulas
that include the symbol for each element followed
by a subscript number showing the number of atoms
of that element in the compound. (ex. MgCl2)
Making Compounds
A state of stability is achieved by some elements
by forming chemical bonds. A chemical bond is
the force that holds together the elements in a
Three types of bonds
covalent – share electrons
ionic – transfer electrons
metallic – sea of electrons
The change of one or more compounds into
other compounds is called a chemical reaction.
Chemical reactions are described by chemical
Water is formed by the chemical reaction between
hydrogen gas (H2) and oxygen gas (O2).
2H2  O2  2H2O
A mixture is a combination of two or more
components that retain their identities.
When a mixture’s components are easily recognizable,
it is called a heterogeneous mixture.
In a homogeneous mixture, also called a solution, the
component particles cannot be distinguished, even
though they still retain their original properties. A
solution can be liquid, gaseous, or solid
Solids are substances with densely packed
particles, which can be ions, atoms, or molecules.
Most solids are crystalline structures because the
particles of a solid are arranged in regular geometric
patterns, giving solids definite shape and volume.
An increase in temperature increases the
thermal vibrations of atoms in a solid.
When thermal vibrations become vigorous
enough to break the forces holding the solid
together, the particles can slide past each
other, and the substance becomes liquid.
Liquids take the shape of the container they
are placed in, but they do have a definite
Some vibrating particles can gain sufficient
thermal energy to escape a liquid.
This process of change from a liquid to a
gas at temperatures below the boiling point
is called evaporation.
Gases, like liquids, have no definite shape.
Gases also have no definite volume unless
they are restrained by a container or a
force such as gravity.
The periodic table of the elements is arranged so
that a great deal of information about all of the
known elements is provided in a small space.
Generally, each element is identified by a one-,
two-, or three-letter abbreviation known as a
chemical symbol.
All elements are classified and arranged
according to their chemical properties in the
periodic table of the elements.
The number of protons in an atom’s nucleus is
its atomic number.
The sum of the protons and the neutrons in an
atom’s nucleus is its mass number.
This diagram of the element chlorine explains
how atomic numbers and atomic mass are listed
in the periodic table of the elements.