Download Atomic number

Chapter 3, Section 3
Counting Atoms
All atoms contain the same particles… Yet all
atoms are not the same.
What makes them different??
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Think about those 3 basic components
of the atom:
Protons
Electrons
Neutrons
Protons – identify each element. Change this
number and you’ve changed elements.
Electrons – in a neutral atom equal the number of
protons. Change this number and you’ve changed the
charge of the atom. (This is another chapter!0
Neutrons – our focus today!
Protons identify an element.
All atoms of an element must have the same number
of protons, but the number of neutrons can be
different.
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The atomic number (Z) of an element is the
number of protons of each atom of that element.
Z = number of protons
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Find your periodic table…
An element’s atomic number is usually above its
symbol and the elements are placed in order of
increasing atomic number. (Thanks, Mr. Mendelyeev)
The atomic number identifies an
element.
So if the number of protons (Z) changes,
the identity of the element changes.
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The number of neutrons and the number of
electrons can vary and the atom will still be the
same element.
We said when electrons change, the charge of
the atom changes…
But what happens when the
number of neutrons changes?
Let’s see about that!
Remember that a neutral atom has the same
number of … (You tell me!)
The number of electrons & neutrons can vary
and the atom will still be the same element.
But if the number of protons changes, then
the atom becomes an atom of a different
element.
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Let’s look at an element on the periodic
table and sort out this information…
Pick an element from your periodic table and
draw it’s entry in your notes…
Include:
Atomic number
Symbol and name
Average atomic mass. **
We’ll talk about average atomic mass in a bit…
Atomic number
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Isotopes
Isotopes are atoms of the same element that have
different masses.
The isotopes of a particular element all have the
same number of protons and electrons but
different numbers of neutrons.
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Take hydrogen, for example
All hydrogen atoms have 1 proton, but they can have
different numbers of neutrons – or no neutrons at all!
Protium is the most common and most stable
(99.9885% of the H atoms found on Earth)
1 proton + 0 neutrons
=1
Deuterium also known as heavy hydrogen
(0.0115% found on Earth)
1 proton + 1 neutron
=2
Tritium is radioactive and not very common.
1 proton + 2 neutrons
=143
Most elements are a mixture of isotopes
Most elements consist of mixtures of isotopes.
Tin has 10! (That’s the most of any element)
The atoms in any sample of an element will most
likely be a mixture of several isotopes in various
proportions.
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Mass number
The mass number (A) of an element is the total
number of protons and neutrons that make up the
nucleus of an isotope.
We have to know the name or atomic number of
the element and the mass of the isotope to
identify the isotope. (These will be given.)
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Mass Number (A)
A = Z + n0
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Two isotopes of carbon
A nuclide or isotope symbol for carbon.
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Two isotopes of chlorine
Atomic number, Mass number and Isotopes
A
X
Z
Mass Number
Atomic Number
Element Symbol
Nuclide symbol
Atomic number (Z)
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C
6
14
C
6
number of protons in nucleus
Mass number (A) number of protons + number of neutrons
(or (Z) + number of neutrons)
Isotopes are atoms of the same element (X) with different numbers
of neutrons in their nuclei.
FYI: The Isotopes of Hydrogen
To find neutrons:
Mass number – atomic number = number of neutrons
235 (protons + neutrons) -92 protons = 143 neutrons
235
92
U
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There are 2 ways we can write isotopes –
actually 3…
1. Hyphenated notation
2. Nuclide (or nuclear symbol)
Designating isotopes and
finding neutrons **add in your notes
Hyphen notation = element (hyphen) mass number
For example: uranium-235
Nuclear symbol:
mass no. (A)
atomic no. (Z)
235
92
U
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Let’s practice this:
Write the nuclear symbol for oxygen-16
1) What 3 things do you need?
2) What is the order you write them?
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Let’s try the ones in your notes
Tin-121
Write the Nuclide symbol, Atomic No. and
Mass Number.
List the number of protons, neutrons & electrons
found in Xenon-113.
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121
50
Sn
Xenon-113
p+ = 54 e- = 54 n0 = 59
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Let’s work on some of these:
Subatomic particles
Sample problem A, pg. 75
Then do the practice questions, pg. 76: 1-3
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Answers:
1. 35 p, 35 e-, 45 neutrons
1. 13
6 C
3. phosphorus-30
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Let’s check it!
1. How many p+, e- and n0 are in the following:
Beryllium (Be) Atomic number 4
Mass number 9
2. Element X has two isotopes, X-23 and X-30.
Element X has 8 protons. Write the symbol for
each isotope using superscripts and subscripts
to represent the mass number and atomic
number.