Download 4.1Atoms and Isotopes

Atoms, Ions, and Isotopes
“Up and at them”
Atoms
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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.
Sub-Atomic Particles
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Protons
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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).
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Electrons
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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
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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
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Equal to the number of protons in the atom.
Therefore, each atomic number is unique and defines each atom
Eg: Oxygen has an atomic number of 8 because it has 8 protons
Easily recognized on the periodic table for each element (see
next slide)
In chemical symbol notation, the atomic number is written on the
bottom left
12
e.g.
C
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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
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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
35Cl
E.g.
17
Mass Number
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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 Electrons
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In any neutral (uncharged) atom the
number of protons is always equal to
the number of electrons.
The number of electrons, therefore, is
also equal to the atomic number.
If there are 8 protons, there will be 8
electrons.
Number of Neutrons
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Can be determined with the following
formula:
Atomic Mass = Protons + Neutrons
Calculating Protons, Neutrons and
Electrons
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Find number of protons, neutrons, and
electrons and write chemical symbol
notation for each of the following
Atomic Mass
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Carbon-13
Sodium-23
Uranium-235
Ions
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Atoms which have either gained or lost
electrons by a chemical reaction of some
kind.
Ions which have lost electrons carry a
positive charge and are called CATIONS
Ions which have gained electrons carry
a negative charge and are called
ANIONS
In chemical symbol notation, the ionic
charge is written in the top right if there
is a charge
32S2e.g. 16
How many protons, electrons, and
neutrons?
16 protons, 16 neutrons, 18 electrons
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1.
2.
3.
Write chemical symbol notation for an
element with 19 protons, 21 neutrons, and
18 electrons
Write chemical symbol notation for an
element with 15 protons, 17 neutrons, and
18 electrons
How many protons, electrons, and neutrons?
206Pb4+
82
Answers:
1.
2.
3.
40K1+
16
32P315
82 protons, 78 electrons, and 124
neutrons.
Isotopes
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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) 21H
Radioactive hydrogen (tritium)1 3H
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Carbon has three isotopes: C-12 (most
abundant), C-13 (used in medical imagingMRI), and C-14 (used for dating fossils)
Tin (Sn) has the most isotopes of any element
at 10
Many isotopes are radioactive (unstable
nucleus that will eventually break apart and
release energy in sometimes harmful forms –
eg. Gamma rays)
Any isotope with an atomic number greater
than 82 is radioactive
Why Decimal Mass Numbers?
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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).
Examples
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How many protons, neutrons, and
electrons in C-12, C-13, C-14, Th-196?
C-12 = 6 P, 6 N, 6 E
C-13 = 6 P, 7 N, 6 E
C-14 = 6 P, 8 N, 6 E
Th-196 = 90 P, 106 N, 90 E