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• CHEMICAL ELEMENTS -
– pure substances that cannot be decomposed by ordinary means to other substances.
Sodium
Aluminum
Bromine

• The elements, their names, and symbols are given on the
PERIODIC
TABLE
• How many elements are there?

Dmitri Mendeleev (1834 - 1907)

Glenn Seaborg
(1912-1999 )
• Discovered 8 new elements.
• Only living person for whom an element was named.

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.

• 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 (n o )
– no electrical charge
– mass = 1.009 amu but we can round to 1

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

• 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
Z
10
B
5

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)

– 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
H
2
O C
8
H
10
N
4
O
2
- 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

• Atoms of the same element (same Z) but different mass number (A).
• Boron-10 ( 10 B) has 5 p and 5 n
• Boron-11 ( 11 B) has 5 p and 6 n
11 B
10 B

Figure 3.10: Two isotopes of sodium.

Bone scans with radioactive technetium-99.

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.

 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 atomic number
23 Na
11

Which of the following represent isotopes of the same element?
Which element?
234
X
92
234
X
93
235
X
92
238
X
92

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
Atomic Number
Charge
(if ion)

Hydrogen
1
1 H
Protons: 1
Neutrons: 0
Electrons: 1

Sodium
23
11
Protons: 11
Neutrons: 12
Na Electrons: 11

Rhenium
75
186
Re
Protons: 75
Neutrons: 111
Electrons: 75

Rhenium isotope
187
Re
Protons: 75
Neutrons: 112
Electrons: 75
75

EXAMPLE
How many protons, neutrons and electrons are found in an atom of
133
55
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, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12 C
6
13 C
6
14
6
C
#p + _______ _______ _______
#n o _______ _______ _______
#e _______ _______ _______

12 C
6
#p + 6
#n o 6
#e 6
13 C
6
6
7
6
14 C
6
6
8
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) 34 X
16
2) 34 X 3) 36 X
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

A CATION forms when an atom loses one or more electrons.
An ANION forms when an atom gains one or more electrons
Mg --> Mg 2+ + 2 e-
F + e- --> F -

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
19
K + 16 O -2
8
41
20
Ca +2
#p + ______
#n o ______
#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 -
___________
___________

-3 -2 -1
+1
+2
By losing or gaining e-, atom has same number of e’s as nearest Group 8A atom.

10 B
11 B
• Because of the existence of isotopes, the mass of a collection of atoms has an average value.
• Boron is 20% 10 B and 80% 11 B. That is, 11 B 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.
• 6 Li = 7.5% abundant and 7 Li = 92.5%
– Avg. Atomic mass of Li = ______________
• 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10%
– Avg. Atomic mass of Si = ______________

The Periodic Table

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

Reaction of potassium + H
2
O
Cutting sodium metal

Group 2A: Alkaline Earth Metals
Magnesium
Magnesium oxide

• Lighter than air balloons
• “Neon” signs
• Very Unreactive because they have full electron levels
XeOF
4

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 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.

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
has lost an electron and therefore has a positive charge
An
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?
+1
Na
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
(1871-
1937).

Results of foil experiment if
Thomson’s
Plum
Pudding model had been correct
.

What Actually Happened