Atoms and Molecules
Atom is smallest particle that can
be identified as a particular
substance.
Molecule is two or more atoms
bonded to each other.
Elements
Around 110 different elements have been
identified. Of these around 80 are found in
nature on Earth.
Most common element on Earth is oxygen,
symbol O.
Most common element in universe is
hydrogen, symbol H.
Symbols
Symbols are shorthand for the name of the
element.
Usually first one or two letters of name.
Atoms
Atom is the basic unit of an element. Atoms
contain protons, neutrons, and electrons.
Protons – positive charge (+1)
Neutrons – neutral, no charge
Electrons – negative charge (-1)
Nucleus
Contains protons and neutrons.
All positive charge of atom is in nucleus.
Majority of mass is in nucleus.
Protons have mass of 1 amu.
Neutrons have mass of 1 amu.
Electrons
Mass is about 1/2000 of the mass of a
proton.
Electrons orbit the nucleus similarly to
planets orbiting the sun.
All negative charge of the atom is due to
electrons.
Charge of the atoms
Elements have no net charge.
The number of protons equals the number of
electrons.
The Subatomic Particles
Proton – the identity particle. The number of
protons in the nucleus determines which
element the atom is.
Neutrons – a neutral particle
Electrons – the personality particle. The
electrons give the atom its characteristic
behaviors.
The Symbol Box
The Atomic Number – usually at top of box.
Is the number of protons in the nucleus.
In an element, the atomic number is also the
number of electrons.
The Symbol – shorthand for name.
The Atomic Mass – usually at the bottom of
box. Is the sum of the protons and
neutrons.
Masses
AMU – atomic mass unit. 1.67 x 10-24 g
Proton and neutron 1 AMU
Election mass – 9.11 x 10-28 g
Carbon atom: mass = 12 amu
Gain/Loss of Electrons
Atoms can gain from other atoms or lose electrons
to other atoms.
An atom that loses an electron has a net positive
charge because there are more protons than
electrons. Remember the atom started with
equal numbers of protons and electrons.
An atom that gains an electron has a net negative
charge because there are more electrons than
protons.
Ions
When an atom has lost or gained an
electrons it is called an ion.
Ions with positive charge are known as
CATIONS
Ions with negative charge are known as
ANIONS
The Periodic Table
An organized table of all known elements.
To the left of the stairstep are metals
To the right of the stairstep are nonmetals
Along both sides of the stairstep are
metalloids.
Rows are known as periods
Columns are known as groups
Neutrons
Isotopes
To be a particular element, an atom must have a
specific number of protons, i.e. 8 protons are in
an oxygen atom.
Most commonly an oxygen atom has 8 neutrons.
Sometimes the nucleus contains has a different
number of neutrons. Isotope is the term for
atoms of a single element that have different
numbers of neutrons.
Specifying an Atom
See paper on symbols on
www.drbillboyle.com