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Atoms and Isotopes
“Up and at them”
Atoms An atom is composed of a central
nucleus which consists of protons and neutrons, along with orbiting electrons that exist within ‘clouds’ or orbitals.
These protons, neutrons, and electrons are commonly known as SUB-ATOMIC PARTICLES.
Subatomic Particle
Location Mass Charge
Proton Nucleus 1 amu +1
Neutron Nucleus 1 amu 0
Electron Orbitals (shells or electron cloud)
~ 0 (1/1800)
-1
Sub-Atomic Particles Protons
Found in nucleus Positive charge (+1) Mass 1 atomic mass unit (a.m.u.) If an atom gains or loses one or more protons,
it becomes an atom of a different element E.g. If N lost a proton, it would become C
(losing or gaining a proton takes a massive amount of energy – can really only occur in the laboratory if the proper equipment is available).
Electrons Exist outside the nucleus in orbitals Negative charge (-1) Very small mass compared to protons (Mass approx 1/1800
a.m.u.) If an atom gains or loses one or more electrons, it becomes an
ion (this occurs often in nature since the energy required is somewhat available).
Neutrons Found in nucleus of atom Neutral charge (0 charge) Made up of a proton fused with an electron Mass 1 a.m.u. (in fact, slightly more than 1 amu), but assumed to
be 1 amu. Each element has atoms with different amounts of neutrons –
the different atoms are called isotopes
Atomic Number Equal to the number of protons in the atom. Therefore, each atomic number is unique and
defines each atom Eg: Sodium has an atomic number of 11 because it has
11 protons Easily recognized on the periodic table for each element In chemical symbol notation, the atomic number is
written on the bottom left e.g. 12C
6
Periodic Table
Atomic Mass
Atomic Number
Chemical Symbol Notation
Atomic Mass
Atomic Number
The atomic number is found at the top of each square on the periodic table.
Atomic number
If you have a periodic table you can instantly find the number of protons that each atom possesses.
It should also be noted that any element existing in its neutral state will also have the same number of electrons as its Atomic #.
Atomic Mass Total number of all protons and
neutrons in an element (always a whole number) [units are in a.m.u.]
Note - electrons are too small to affect total mass
In chemical symbol (isotope) notation, the atomic mass is a whole number and is written on the top left
E.g. 35Cl17
Mass Number To find mass number from a periodic
table, look under the symbol (or name)
It will be explained later why decimal values are given on the periodic table
Mass number
Number of Neutrons Can be determined with the
following formula: Atomic Mass = Protons +
Neutrons
Number of Electrons Atoms have a neutral charge
(uncharged). Since protons have a positive
charge and electrons have a negative charge, the number of electrons must be equal to the number of protons in an atom.
Calculating Protons, Neutrons and Electrons Find number of protons, neutrons,
and electrons and write chemical symbol notation for each of the following
Carbon-13 Sodium-23 Uranium-235
Atomic Mass
Isotopes Isotopes are atoms of the same
element that have different atomic masses due to different amounts of neutrons
Hydrogen has three isotopes Ordinary hydrogen (protium) 1H Heavy hydrogen (deuterium) 2H Radioactive hydrogen (tritium) 3H
1
1
1
ExamplesName Protons Neutrons Electrons Mass
Carbon-13
15 17
92 238
Why Decimal Mass Numbers? The mass reported on the periodic table
is actually a weighted average of the masses of all the isotopes of an element.
Most hydrogen is Hydrogen – 1, therefore the average mass is very close to 1 (1.0079).
For most atoms, one isotope is much more common than the other(s).
Homework ATOMIC STRUCTURE WORKSHEET
ONLY PART I