Download 1a-11 - Writing Elemental Symbols

Writing Elemental Symbols
Distinguishing Among Atoms
Introduction
We all know that individual elements are different from
all other elements.
Oxygen is different from carbon.
Carbon is different from gold.
John Dalton’s Atomic Theory stated that:
“Atoms of the same element are identical and the
atoms of any one element are different from the atoms
of any other element.”
Atomic Number
We now know that elements are different from each other
because they contain different numbers of protons.
The number of protons, p+, in the nucleus of an atom is
called the atomic number.
Each element has its own atomic number:
hydrogen has an atomic number of 1
helium has an atomic number of 2
lithium has an atomic number of 3
uranium has an atomic number of 92
Mass Number
In addition to the atomic number, each atom of an
element has a mass number.
Most of the mass of an atom is in the nucleus.
The nucleus is composed of protons and neutrons, n0.
The mass number of an atom is the sum of the protons
and the neutrons in the nucleus.
hydrogen usually has 1 p+ and 0 n0:
➠ mass number = 1
carbon usually has 6 p+ and 6 n0:
➠ mass number = 12
Element Symbols
The symbols are a shorthand way of telling us what
element we are dealing with.
Most symbols are based on the common name of the
element:
H for hydrogen
He for helium
O for oxygen
C for carbon
N for nitrogen
Element Symbols
Some of the symbols are based on historical, usually
latin, names:
Pb for lead (from plumbum)
Fe for iron (from ferrum)
Au for gold (from aurum)
Na for sodium (from natrium)
K for potassium (from kalium)
W for tungsten (from wolfram [german])
Element Symbols
When we are using the symbols to talk about an element
in a chemical reaction, we most often use the symbol by
itself.
Sometimes, we want to know the atomic number and the
mass number of the element we are working with.
In that case, we use the symbol with the atomic number
and the mass number.
Element Symbols
First, we write the symbol.
Next, we write the atomic number in the lower left.
Next, we write the mass number in the upper left.
This is the complete element symbol
12
6
C
Isotopes
John Dalton felt that all atoms of a particular element
were “identical.”
He was wrong.
Individual atoms of an element all have the same atomic
number, the same number of protons.
Individual atoms of an element sometimes have different
mass numbers.
These are called isotopes.
Isotopes of an element have the same number of protons
but a different number of neutrons.
Isotopes
For example, most carbon atoms have 6 protons and 6
neutrons; ➠ mass number = 12.
12
6
C
Isotopes
For example, most carbon atoms have 6 protons and 6
neutrons; ➠ mass number = 12.
However, some carbon atoms have 6 protons and 7
neutrons; ➠ mass number = 13.
12
6
C
13
6
C
Isotopes
For example, most carbon atoms have 6 protons and 6
neutrons; ➠ mass number = 12.
However, some carbon atoms have 6 protons and 7
neutrons; ➠ mass number = 13.
And, other carbon atoms have 6 protons and 8 neutrons;
➠ mass number = 14.
12
6
C
13
6
C
14
6
C
Counting
When we use the elemental symbols, we can count the
number of protons, neutrons, and electrons in an atom.
The number of protons is the same as the atomic
number.
The number of neutrons is the same as the mass
number minus the atomic number.
The number of electrons is the same as the number
of protons in a neutral atom.
Ions have a different number of electrons than
protons and we will cover those later.
Counting
So, the number of p+ is the atomic number.
The mass number is the number of p+ plus the number of
n0.
The number of e- is the same as the number of p+.
# of
+
p
# of
plus # of
12
6
e
# of
0
n
+
p
C
Counting
Let’s do an example.
Find the number of p+, n0, and e- in phosphorus-31.
The number of p+ is the same as the atomic number.
The number of e- is the same as the atomic number.
The number of n0 is the mass number minus the atomic
number.
31
15
P
# p+ = 15
# e- = 15
# n0 = 31 - 15 = 16
Counting
Let’s do another example.
Find the number of p+, n0, and e- in platinum-195.
The number of p+ is the same as the atomic number.
The number of e- is the same as the atomic number.
The number of n0 is the mass number minus the atomic
number.
195
78
Pt
# p+ = 78
# e- = 78
# n0 = 195 - 78 = 117
Counting
Let’s do another example.
Find the number of p+, n0, and e- in uranium-238.
The number of p+ is the same as the atomic number.
The number of e- is the same as the atomic number.
The number of n0 is the mass number minus the atomic
number.
238
92
U
# p+ = 92
# e- = 92
# n0 = 238 - 92 = 146
Counting
Let’s do another example.
Find the number of p+, n0, and e- in potassium-40.
The number of p+ is the same as the atomic number.
The number of e- is the same as the atomic number.
The number of n0 is the mass number minus the atomic
number.
40
19
K
# p+ = 19
# e- = 19
# n0 = 40 - 19 = 21