Atoms and Periodic Table of elements 2025

The basics of atoms and periodic table

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Atoms And The Periodic Table
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Atomic Structure
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Element – matter that is
composed of one type of atom
Elements are abbreviated in
scientific shorthand – either
a letter or a pair of letters
called chemical symbol. Ex:
Aluminum – Al Copper –
Cu
Atom – smallest piece of
matter that still has the
properties of the element.
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Protons – positive
charge (1
+
)

Neutrons – neutral or
no charge

Electrons – negative
charge (1
-
)

Nucleus – located in
center of atom consists
of protons and
neutrons; electrons
surround the nucleus
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Electron Cloud – (current) model which shows electrons
travelling in specific energy levels around a nucleus;
electrons closest to nucleus have low energy, electrons
farther away have high energy.
* Last energy
level can only
hold max of 8
electrons – H
and He are
exceptions
Electrons in Energy Levels
Energy Level in AtomEnergy Level in Atom
1 2
2 8
3 18
4 32
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Masses of Atoms
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Smaller
particles of
matter called
quarks exist
but scientist
still consider
atoms the
basic
building
blocks of
matter



Quark – smaller particles that
make up protons and neutrons –
six quarks are to exist
Atomic mass – composed mostly
of protons and neutrons in the
nucleus – also called mass
number

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the number of
protons in an
atom; number of
protons also
identifies the
element

* If you needed to find # of neutrons of an
atom:
# of Neutrons = mass number – atomic number
Atomic
number
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Isotopes
–atoms of the same
element that have
different number of
neutrons.
B
10
(Boron – 10) or B
11
(Boron – 11)
Different isotopes have different properties.
Average atomic mass
weighted average mass of
an element’s isotopes.
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Common Mass Numbers
ElementSymbol
Atomic
No.
ProtonsNeutrons
Mass
No.
Ave.
Atomic
Mass
BORON B 5 5 6 11
10.81
amu
CARBO
N
C 6 6 6 12 12.01
amu
OXYGEN O 8 8 8 16 16 amu
SODIUM
Na 11 11 12 23 22.99
amu
COPPER
Cu 29 29 34 63 63.55
amu
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The Periodic Table
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In the late 1800’s, Dmitri Mendeleev devised first periodic table
based on atomic mass – however, some elements were out of
order. In 1913, Henry G. J. Moseley arranged elements by
atomic number and is what we use today.
Periodic table – table where elements are organized
by increasing atomic number (number of protons)
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The periodic table is arranged by groups and periods
Groups – vertical columns of elements with similar
properties - groups are numbered 1-18.
elements in the same group have the same number of
electrons in their outer energy level








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•Horizontal rows of elements that contain increasing
numbers of protons and electrons. Period are
numbered 1-7
each row in the periodic table ends when an outer
energy level is filled
Periods
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15. Each of the seven energy levels can hold a maximum
number of electrons.
Level 1 – 2 electrons Level 2 – 8 electrons

* outer level can hold a max of 8 electrons – except
Hydrogen & Helium (only max 2)
Electron Dot Diagram
use the element symbol and dots to represent
outer energy level electrons.
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Metals, Nonmetals &
Metalloids
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Metals
–good conductor of heat and
electricity, all but mercury are
solid at room temperature
* Metals are located to the left of
stair step

Alkali Metals
–(Group 1) are the most reactive
of all metals; don’t occur in
nature in their element form

Alkaline Earth Metals
–(Group 2) shiny, ductile and
malleable; combine readily with
other elements
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Transition Elements
–(Group 3 – 12) most familiar metals
because they often occur in nature
uncombined

Inner Transition Metals
(listed below table)
Lanthanide Series – elements with atomic # 58 –
71
Actinide Series – elements with atomic # 90 –103

Nonmetals
–elements that are usually gases or brittle
solids at room temperature; most can
form ionic and covalent compounds –
located to the right of the stair step.
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Noble gases – (Group 18) exist as isolated atoms. They are all
stable because outer energy level is filled
Metalloids – elements that make up stair step; have metallic
and non-metallic properties
Metalloids are part of the mixed groups – which contain metals, non-metals
and metalloids (mixed groups are Group 13, 14, 15, 16, 17)
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