Transcript Atoms, Elements, and Ions

Elements,
Atoms, and
Ions
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The Language of Chemistry
• CHEMICAL ELEMENTS
-
– pure substances that cannot be decomposed by
ordinary means to other substances.
Aluminum
Sodium
Bromine
The Language of Chemistry
• The elements,
their names, and
symbols are given
on the
PERIODIC
TABLE
• How many
elements are
there?
The Periodic Table
Dmitri Mendeleev (1834 - 1907)
Glenn Seaborg
(1912-1999 )
• Discovered 8
new elements.
• Only living
person for
whom an
element was
named.
The Atom
An atom consists of a
• nucleus
–(of protons and neutrons)
• electrons in space around the
nucleus.
Electron cloud
Nucleus
• An _____ is the smallest particle of
an element that has the chemical
properties of the element.
Copper
atoms on
silica
surface.
Distance across = 1.8 nanometer (1.8 x 10-9 m)
Subatomic Particles
• Quarks
– component of
protons &
neutrons
– 6 types
– 3 quarks =
1 proton or
1 neutron
He
ATOM
COMPOSITION
The atom is mostly
empty space
•protons and neutrons in
the nucleus.
•the number of electrons is equal to the
number of protons.
•electrons in space around the nucleus.
•extremely small. One teaspoon of water has
3 times as many atoms as the Atlantic Ocean
has teaspoons of water.
ATOMIC COMPOSITION
• Protons (p+)
– + electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic
mass units (amu) but we can round to 1
• Electrons (e-)
–
negative electrical charge
– relative mass = 0.0005 amu
but we can round to 0
• Neutrons (no)
no electrical charge
– mass = 1.009 amu but we can round to 1
–
Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
Atomic number
Atom symbol
AVERAGE Atomic Mass
Mass Number, A
• C atom with 6 protons and 6 neutrons
is the mass standard
• = 12 atomic mass units
• Mass Number (A)
= # protons + # neutrons
• NOT on the periodic table…(it is the
AVERAGE atomic mass on the table)
• A boron atom can have
A = 5 p + 5 n = 10 amu
A
10
Z
5
B
CHEMICAL COMPOUNDS are
composed of atoms and so can be
decomposed to those atoms.
The red compound is
composed of
• nickel (Ni) (silver)
• carbon (C) (black)
• hydrogen (H) (white)
• oxygen (O) (red)
• nitrogen (N) (blue)
Compounds
– composed of 2 or
more elements in a
fixed ratio
– properties differ
from those of
individual
elements
– EX: table salt
(NaCl)
A MOLECULE is the smallest unit of a
compound that retains the chemical
characteristics of the compound.
Composition of molecules is given by a
MOLECULAR FORMULA
H2O
C8H10N4O2 - caffeine
ELEMENTS THAT EXIST AS
DIATOMIC MOLECULES
Remember:
HONClBrIF
or
BrINClHOF
These elements only
exist as PAIRS. Note
that when they
combine to make
compounds, they are
no longer elements so
they are no longer in
pairs!
Dalton’s Atomic Theory
John Dalton (1766-1844) proposed an atomic
theory
While this theory was not completely correct, it
revolutionized how chemists looked at
matter and brought about chemistry as we
know it today instead of alchemy
Thus, it’s an important landmark in the history
of science.
Dalton’s Atomic Theory - Summary
1. matter is composed, indivisible particles
(atoms)
2. all atoms of a particular element are
identical
3. different elements have different atoms
4. atoms combine in certain whole-number
ratios
5. In a chemical reaction, atoms are merely
rearranged to form new compounds; they
are not created, destroyed, or changed into
atoms of any other elements.
Problems with Dalton’s Atomic Theory?
1. matter is composed, indivisible particles
Atoms Can Be Divided, but only in a nuclear
reaction
2. all atoms of a particular element are identical
Does Not Account for Isotopes (atoms of the same
element but a different mass due to a different
number of neutrons)!
3. different elements have different atoms
YES!
4. atoms combine in certain whole-number ratios
YES! Called the Law of Definite Proportions
5. In a chemical reaction, atoms are merely rearranged
to form new compounds; they are not created,
destroyed, or changed into atoms of any other
elements.
Yes, except for nuclear reactions that can change
atoms of one element to a different element
Isotopes
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n
• Boron-11 (11B) has 5 p and 6 n
11B
10B
Figure 3.10: Two isotopes of
sodium.
Isotopes &
Their Uses
Bone scans with
radioactive
technetium-99.
Isotopes & Their Uses
The tritium content of ground water is
used to discover the source of the water,
for example, in municipal water or the
source of the steam from a volcano.
Atomic Symbols
 Show the name of the element, a hyphen, and
the mass number in hyphen notation
sodium-23
 Show the mass number and atomic number
in nuclear symbol form
mass number
23 Na
atomic number
11
Isotopes?
Which of the following represent
isotopes of the same element?
Which element?
234
92
X
234
93
X
235
92
X
238
92
X
Counting Protons, Neutrons,
and Electrons
• Protons: Atomic Number (from periodic table)
• Neutrons: Mass Number minus the number of protons
(mass number is protons and neutrons because the
mass of electrons is negligible)
• Electrons:
– If it’s an atom, the protons and electrons must be
the SAME so that it is has a net charge of zero
(equal numbers of + and -)
– If it does NOT have an equal number of electrons, it
is not an atom, it is an ION. For each negative
charge, add an extra electron. For each positive
charge, subtract an electron (Don’t add a proton!!!
That changes the element!)
MODERN ATOMIC THEORY
–Chemical Symbols
»There are about a dozen common elements
that have a single capitalized letter for their
symbol
»The rest, that have permanent names have
two letters.
• the first is capitalized and the second is
lower case.
»Some elements have symbols from their Latin
names.
»Ten of the elements have symbols from their
Latin or German names.
–Symbols and Atomic Structure
»A molecule is a particle that is composed of
two or more atoms held together by a
chemical bond.
»Isotopes are atoms of an element with
identical chemical properties, but different
masses due to a difference in the number of
neutrons.
»The atomic mass of an element is the average
of all the atomic masses of the isotopes.
• an isotopes contribution is determined by
its relative abundance.
»The mass of an element is the mass of the
element compared to an isotope of carbon
Carbon 12.
• Carbon 12 is assigned an atomic mass of
12.00 g.
• 12.00 is one atomic mass unit
»The number of protons and neutrons in an
atom is its mass number.
»Atomic numbers are whole numbers
»Mass numbers are whole numbers
»The atomic mass is not a whole number.
Atomic Mass
Symbol
Atomic Number
Charge
(if ion)
Hydrogen
1
1
H
Protons: 1
Neutrons: 0
Electrons: 1
Sodium
23
11
Protons: 11
Neutrons: 12
Electrons: 11
Na
Rhenium
186
75
Re
Protons: 75
Neutrons: 111
Electrons: 75
Rhenium isotope
187
75
Re
Protons: 75
Neutrons: 112
Electrons: 75
EXAMPLE
How many protons, neutrons and electrons are found in an
atom of
133
55
Cs
Atomic number = protons and electrons
There are 55 protons and 55 electrons
Mass number = sum of protons and neutrons
133 – 55 = 78
There are 78 neutrons
Learning Check – Counting
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
6
13C
6
14C
6
#p+ _______
_______
_______
#no _______
_______
_______
#e- _______
_______
_______
Answers
12C
13C
14C
6
6
#p+ 6
6
6
#no 6
7
8
#e- 6
6
6
6
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
14
IONS
• IONS are atoms or groups of atoms with a
positive or negative charge.
• Taking away an electron from an atom gives a
CATION with a positive charge
• Adding an electron to an atom gives an
ANION with a negative charge.
• To tell the difference between an atom and an
ion, look to see if there is a charge in the
superscript! Examples: Na+ Ca+2 I- O-2
Na
Ca
I
O
Forming Cations & Anions
A CATION forms
when an atom
loses one or
more electrons.
Mg -->
Mg2+
+ 2 e-
An ANION forms
when an atom
gains one or
more electrons
F + e- --> F-
PREDICTING ION CHARGES
In general
• metals (Mg) lose electrons ---> cations
• nonmetals (F) gain electrons ---> anions
Learning Check – Counting
State the number of protons, neutrons, and
electrons in each of these ions.
39
K+
19
16O -2
41Ca +2
8
20
#p+ ______
______
_______
#no ______
______
_______
#e- ______
______
_______
One Last Learning Check
Write the nuclear symbol form for the
following atoms or ions:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 46 e-
___________
Charges on Common Ions
-3 -2 -1
+1
+2
By losing or gaining e-, atom has same
number of e-’s as nearest Group 8A atom.
AVERAGE
ATOMIC
MASS
11B
10B
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• Boron is 20% 10B and 80% 11B. That is, 11B is
80 percent abundant on earth.
• For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu
Isotopes & Average Atomic Mass
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• 6Li = 7.5% abundant and 7Li = 92.5%
–Avg. Atomic mass of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
–Avg. Atomic mass of Si = ______________
The Periodic Table
Periods in the Periodic
Table- periods are horizontal
rows
Groups in the Periodic Tablegroups are vertical columns
Elements in groups react in similar ways!
• Introduction
–The periodic table is made up of rows of
elements and columns.
–An element is identified by its chemical symbol.
–The number above the symbol is the atomic
number
–The number below the symbol is the rounded
atomic weight of the element.
–A row is called a period
–A column is called a group
Regions of the Periodic Table
Group 1A: Alkali Metals
Reaction of
potassium + H2O
Cutting sodium metal
Group 2A: Alkaline Earth Metals
Magnesium
Magnesium
oxide
Group 7A: The Halogens
(salt makers)
F, Cl, Br, I, At
Group 8A: The Noble
(Inert) Gases
He, Ne, Ar, Kr, Xe, Rn
• Lighter than air
balloons
• “Neon” signs
• Very Unreactive
because they
have full electron
levels
XeOF4
Transition Elements
Lanthanides and actinides
Iron in air gives
iron(III) oxide
Periodic Table
• This table is a remarkable way to show the
relationships between differing kinds of
elements
• The modern table was devised (made) in 1869
by Dimitri Mendeleyev
• He arranged the elements by weight and by
their chemical properties
• "...if all the elements be arranged in order of
their atomic weights a periodic repetition of
properties is obtained." - Mendeleyev
• The Modern Periodic Table
• Periodic Patterns
–The chemical behavior of elements is
determined by its electron configuration
–Energy levels are quantized so roughly
correspond to layers of electrons around
the nucleus.
–A shell is all the electrons with the same
value of n.
»n is a row in the periodic table.
–Each period begins with a new outer
electron shell
–Each period ends with a completely filled
outer shell that has the maximum number
of electrons for that shell.
–The number identifying the A families
identifies the number of electrons in the
outer shell, except helium
–The outer shell electrons are responsible
for chemical reactions.
–Group A elements are called
representative elements
–Group B elements are called transition
elements.
• Chemical “Families”
– IA are called alkali metals because the react with water
to from an alkaline solution
– Group IIA are called the alkali earth metals because they
are reactive, but not as reactive as Group IA.
» They are also soft metals like Earth.
– Group VIIA are the halogens
» These need only one electron to fill their outer shell
» They are very reactive.
– Group VIIIA are the noble gases as they have completely
filled outer shells
» They are almost non reactive.
• Four chemical families
of the periodic table:
the alkali metals (IA),
the alkaline earth
metals (IIA), halogens
(VII), and the noble
gases (VIIIA).
Metal: Elements that are usually solids at room temperature.
Most elements are metals.
Non-Metal: Elements in the upper right corner of the periodic
Table. Their chemical and physical properties are different
from metals.
Metalloid: Elements that lie on a diagonal line between the
Metals and non-metals. Their chemical and physical
properties are intermediate between the two.
–When an atom or molecule
gain or loses an electron it
becomes an ion.
»A cation has lost an electron
and therefore has a positive
charge
»An anion has gained an
electron and therefore has a
negative charge.
–Elements with 1, 2, or 3 electrons in their outer
shell tend to lose electrons to fill their outer
shell and become cations.
»These are the metals which always tend to
lose electrons.
–Elements with 5 to 7 electrons in their outer
shell tend to gain electrons to fill their outer
shell and become anions.
»These are the nonmetals which always tend
to gain electrons.
–Semiconductors (metalloids) occur at the
dividing line between metals and nonmetals.
EXAMPLE
What would the charge be on a sodium ion?
Since sodium in in Group IA it is a metal and so would
LOSE an electron
You can tell how many would be lost by the group number
Group 1A elements lose 1 electron
So the charge would be +1
Remember an electron is negatively charged. When you lose
them atom becomes positively charged…
when you gain them it becomes negatively charged
EXAMPLE
How would you right the symbol for the sodium CATION?
Na
+1
How many outer electrons does sodium have before it
loses one?
It has 1…remember the group number!
Rutherford’s experiment.
The modern view of the atom was
developed by Ernest Rutherford (18711937).
Results of
foil
experiment
if
Thomson’s
Plum
Pudding
model had
been
correct.
What Actually Happened