Transcript Average Atomic Mass

Unit 3
Particles with
Internal Structure
The Elements
 Remember, elements are combined to form molecules the way
letters are combined to form words.
 Presently there are about 115 known elements.
 Only 88 occur naturally, the rest are made in laboratories.
 Only 9 elements account for most of the compounds found in
the Earth’s crust.
Table 3.1
3-3
All living things
are made up,
mainly, of oxygen,
carbon, hydrogen
and nitrogen.
3-4
Symbols For The Elements
Just as each state has a two-letter
abbreviation, each element has a one- or
two-letter symbol to make life simple for
chemists.
Some elements found in the human
body are: As, Cr, Co, Cu, F, I, Mn, Mo, Ni,
Se, Si, & V.
Notice the first letter is
ALWAYS capitalized and the
second letter, if present, is Not
capitalized.
Symbols For The Elements
 Some symbols make sense like O for oxygen
and H for hydrogen or Ni for nickel.
 Others, like Pb for lead or Fe for iron, don’t
automatically make sense; they originated
from the Greek or Latin names of plumbum
(Pb) and ferrum (Fe).
Dalton’s Atomic Theory
 Scientists studying matter in the eighteenth
century made the following observations:
Most natural materials are mixtures of pure
substances.
Pure substances are either elements or
combinations of elements called
compounds.
A given compound always contains the
same proportions (by mass) of the
elements.
John Dalton attempted to explain these
observations in 1808.
Dalton’s Atomic Theory
1.
2.
3.
4.
5.
Elements are made of tiny particles called atoms.
All atoms of a given element are identical.
The atoms of a given element are different from
those of any other element.
Atoms of one element can combine with atoms of
other elements to form compounds. A given
compound always has the same relative numbers
and types of atoms.
Atoms are indivisible in chemical processes. That is,
atoms are not created or destroyed in chemical
reactions. A chemical reaction simply changes the
way atoms are grouped together.
Formulas of Compounds
 The types of atoms and the number of each type
in each unit (molecule) of a given compound are
conveniently expressed by a chemical formula.
 The atoms are indicated by their symbols and the
number of each type is indicated by a subscript
(unless there is only one).
Ex) C6H12O6 or H3PO4
Practice

Write the formula for each of the following
compounds, listing the elements in the order
given:
a. A molecule contains four phosphorous atoms
and ten oxygen atoms.
b.
A molecule contains one uranium atom and
six fluorine atoms.
c.
A molecule contains one aluminum atom and
three chlorine atoms.
What is inside an
atom?
Build an Atom Activity
3-12
Modern Atomic Structure
 Every atom is composed of the three basic
subatomic particles. (Protons, electrons, neutrons)
 Different elements have different numbers of each
of these subatomic particles.
 The reason one element behaves differently than
another lies in the number and arrangement of
their electrons.
 When atoms get close to each other their electron
“clouds” can overlap and interact.
Figure 3.9:
A nuclear atom
viewed in cross
section.
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 In any box on the Periodic Table, what information can you find?
6
C
12.01
Average Atomic Mass = Protons
+ Neutrons
the weighted average of all the mass
numbers for each isotope of the
element
Atomic number = number of
protons, unique for every element, no
2 elements have the same atomic #
Element symbol = can be 1,2 or 3
letters, first letter is always
capitalized, and succeeding letters
are always lower case
Isotopes
Two atoms of the same element (same number of
protons) with different numbers of neutrons.
 Figure 3.10: Two isotopes of sodium.
Isotopes
There are two important numbers associated with
any given element:
1. Atomic Number – The number of protons in a
nucleus.
2. Mass Number – The SUM of the number of
protons AND neutrons (a.k.a. nucleons) in a
nucleus (NOT the sum of their masses).
 We should note that two different isotopes will
have the same atomic number, but different mass
numbers.

Isotopes
The two previous examples of isotopes of
sodium would be:
23
11
Na
24
11
Na
•The example on the left would contain 11
protons and 12 neutrons (23-11=12).
•The example on the right would contain 11 protons
and 13 neutrons (24-11=13).
Practice Problems

Write the symbol for each of the following atoms, and
list the number of protons, neutrons, and electrons for
each.
1) The cesium atom with a mass number of 132.
2) The iron atom with a mass number of 56.
3) The krypton atom that has 48 neutrons.
4) The nitrogen atom that has 6 neutrons.
M&M Isotope Lab
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Weighted Average Atomic Mass
Remember elements can have different isotopes which means that
they vary in their number of neutrons.
 If you have 3 different isotopes of the same element:
 15 atoms have a mass of 21
 8 atoms have a mass of 23
 2 atoms have a mass of 19
We can calculate the weighted average by multiplying the number of
atoms by their mass:
(15) (21) = 315
537 = 21.48
(8) (23) = 184
25
(2) (19) =+ 38
average atomic mass
537

Using % to find Average Atomic Mass
Usually we only know the percents of various isotopes
that make up different elements, we can use this to
calculate the average atomic mass.
 If we have 100% chlorine:
75.77% of mass is 35  .7577x35 = 26.52
24.23% of mass is 37.2423x37= 8.96
Add the 2 together to get the atomic mass:
26.52+ 8.96 = 35.48

Practice
 Oxygen has 3 isotopes 16O, 17O, 18O
99.76% of mass is 16O
0.04% of mass is 17O
0.20% of mass is 18O
What is the average atomic mass?
 Find the atomic mass if 99.64% of mass is 14N and 0.36% is
15N.
A Simple Version of the Periodic Table
Periodic Table
 When looking at periodic table elements are arranged in
horizontal rows by increasing atomic number.
 Horizontal rows are called “Periods”. Periods go left to right
Periodic Table
 The vertical columns are called “Groups” or “Families”
 Elements in families share similar properties
Metals,
Non-Metals
All
elements onSemimetals,
the periodic table are grouped
as metals,
semimetals or metalloids, or non-metals. Due to the
arrangement of the periodic table, it is easy to identify
each type of element.
Figure 3.12: Elements classified
as metals and nonmetals
Metals: Fall to left and under the stairs
Properties of Metals:
Efficient conduction of heat and electricity
Malleability
Ductility
A lustrous appearance
Positively charged ions
Non-Metals: Right and above the stairs
• Dull, Brittle
• Negatively charged ions
• Nonconductors
-insulators
Semimetals or Metalloids: Makeup the stairs
• Properties of both metals and nonmetals
• Semiconductors
Lanthanide and Actinide
Series
 Mostly human made elements
 Radioactive elements
Group 1A – Alkali Metals
Group 2A – Alkaline Earth Metals
Group 7A or 17: Halogen Family
Group 8A or 18: Noble Gases
 Do not react easily with anything, due
stable electron configuration
 All other elements strive to reach
noble gas configuration for maximum
stability by reacting with other
elements.
Transition Metals: Group 3B-12B
Who is a solid, liquid or gas?
When we look at the elements on the periodic
table, who is a solid, liquid or gas in their natural
state?
 Most elements are not found in their elemental
state, most elements are found in compounds with
other elements.
 Most elements on the periodic table are solids, so
we will point out those who are gas or liquid.
Liquids
Only 2 elements in their elemental
form are a liquid at 25 degrees Celsius:
Mercury and Bromine
Gases
 More elements exist in their
elemental form as a gas, but there
are some important distinctions to
make about these gases.
 The noble gases are a gas, called
monatomic gases. The prefix
mono- means one. And
monatomic gases exist as
individual atoms.
Figure 3.13: A collection of argon
atoms.
Gases
 There is another group of gases called
diatomic gases. The prefix di- means
two. These elements travel in pairs as
molecules.
Figure 3.14: Nitrogen gas
contains NXN molecules.
Figure 3.14: Oxygen gas
contains OXO molecules.
Diatomic molecules
There are 7 elements that exist as diatomic molecules, you will simply
need to find a way to memorize these.
If you notice, all of the halogens fall in this category, and then hydrogen,
nitrogen, and oxygen.
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What is an ion?
 When we discussed atoms before, we were always looking at a
neutral atom.
Neutral atoms always have equal numbers of protons and
electrons.
protons = +1 charge
electrons = -1 charge
 When atoms have unequal numbers of protons and electrons,
then the atom is a charged particle called an ion.
Ion Facts
 Ions are atoms, or groups of atoms, with a charge.
 The charge is created by different numbers of
protons and electrons.
 In an atom ONLY electrons can move.
 Atoms gain or lose electrons to become ions.
Cations and Anions
 There are 2 types of ions: cations and anions.
 Cations are ions with a (+) positive charge. To form a cation, an atom has
lost electrons.
Example: Na loses an electron and becomes Na+
 Anions are ions with a negative charge. To form an anion, an atom has
gained electrons.
Example: Cl gains an electron and becomes Cl-
Basic Names for Ions
 Cations do not change names from their neutral atoms.
Example: Magnesium loses 2 electrons and becomes Mg2+
which is named magnesium ion.
 Anions change the end of their name to –ide.
Example: Chlorine gains an electron and becomes Cl-. We
would change the name from chlorine to chloride.
Some Common Anion Names
 What would the names of the following ions be?
 Chlorine =
 Fluorine =
 Bromine =
 Iodine =
 Oxygen =
 Sulfur =
How to Determine the Charge
 When determining the charge for an atom we can use the
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periodic table to help.
The number of valence electrons determines the charge.
All atoms want 8 valence electrons.
If an atom has 1-3 valence electrons the atom will lose them
to become positive.
If an atom has 6-8 valence electrons the atom will gain
electrons to become negative.
We can determine the charge by looking at the periodic
table.
Figure 3.19: The ions formed by selected members of groups
1, 2, 3, 6, and 7.
Practice
 Determine the name and charge of the following ions:
Potassium
Bromine
Calcium
Sulfur
Aluminum
Strontium
Cesium