what is the atomic number? electron proton neutron
TRANSCRIPT
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What is the atomic number?
Every element has a unique atomic number. This is the
number of protons in the nucleus of each atom.
What is the atomic number of
this helium atom?
A neutral atom must have equal numbers of protons and
electrons, so the atomic number of an element also gives
the number of electrons.
Helium has 2 protons, so its
atomic number is 2.
Atoms are neutrally charged,
so what links atomic number
and the number of electrons?
electron
proton
neutron
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The periodic table
Arranging all the elements by their atomic number and their
properties led to the creation of…
…the periodic table
Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg ? ? ? ? ? ? ?
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Na Mg Al Si P S Cl Ar
Li Be B C N O F Ne
H He
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Missing elements!
In this periodic table the symbols are replaced by atomic
numbers. Some of the numbers are missing – where?
87 88 89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
37 38 39 40 41 42 43 44 45 46 47 38 49 50 51 52 53 54
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
11 12 13 14 15 16 17 18
3 4 5 6 7 8 9 10
1 2
Two more rows of elements fit here.
They are called the lanthanides
and actinides.
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Columns of elements
What are columns of elements called?
groups 1 2 4 3 5 6 0 7
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Rows of elements
periods What are rows of elements called?
1
2
3
4
5
6
7
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Patterns, atomic number and electrons
What links atomic number and the properties of elements?
The periodic table shows that patterns in the properties of
elements are linked to atomic number.
atomic number = number of protons
atomic number = number of electrons
number of protons = number of electrons
Electrons!
As atomic number increases by one, the number of electrons
also increases by one.
This means that the elements in the periodic table are
also arranged in order of the number of electrons.
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How are electrons arranged?
Electrons are arranged in shells around an atom’s nucleus.
(The shells can also be called energy levels).
This electron arrangement is written as 2,8,8.
1st shell holds a maximum of
2 electrons
2nd shell holds
a maximum of
8 electrons
3rd shell holds
a maximum of
8 electrons
Each shell has a maximum number of electrons that it can
hold. Electrons will fill the shells nearest the nucleus first.
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Electrons in period 1
Elements in period 1 only have electrons in the first shell.
Why are there only two elements in period 1?
The first shell can only hold a maximum of two electrons,
so period 1 only includes the elements hydrogen and helium.
What is special about the outer shell of helium?
1 2
H He
1 2 4 3 5 6 0 7
1
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Electrons in period 2
Elements in period 2 all have a complete first shell.
The second shell is completed one electron at a time going
across the period from left to right.
What is special about the outer shell of neon?
2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
Li Be B C N O F Ne
1 2 4 3 5 6 0 7
What happens to electrons in the second shell in period 2?
2
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Electrons in period 3
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
Elements in period 3 have complete first and second shells.
The third shell is completed one electron at a time going
across the period from left to right.
What happens to electrons in the third shell in period 3?
Na Mg Al Si P S Cl Ar
1 2 4 3 5 6 0 7
What is special about the outer shell of argon?
3
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What are the patterns of electron arrangement?
Consider the electron arrangements of the first 20 elements
in the periodic table.
What is the pattern of outer shell electrons in a group?
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
1 2 4 3 5 6 0 7
1
2
3
4
2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
1 2
2,8,8,1 2,8,8,2
What is the pattern of outer shell electrons across a period?
What is the pattern of full electron shells in a group?
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Electron trends in the periodic table
Trends down a group:
The point at which a new period starts is the point at
which electrons begin to fill a new shell.
The number of a group is the same as the number of
electrons in the outer shell of elements in that group,
except for group 0.
the number of outer shell electrons is the same;
the number of complete electron shells increases by one.
the number of outer shell electrons increases by one;
Trends across a period:
the number of complete electron shells stays the same.
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What’s the electron arrangement?
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Glossary
atomic number – The number of protons in an atom. Sometimes called the proton number.
electron arrangement – A shorthand way of writing the number of electrons in an atom’s electron shells.
element – A substance made up of only one type of atom.
group – A column in the periodic table containing elements with the same number of outer shell electrons and similar chemical properties.
period – A row in the periodic table containing elements with the same number of full electron shells.
periodic table – The table that lists all the elements in order of increasing atomic number, arranged into groups and periods.
property – Any characteristic of an element.
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Cation Formation
11p
+
Na atom
1 valence
electron
Valence
e- lost in
ion
formatio
n
Effective nuclear
charge on
remaining
electrons
increases. Remaining e-
are pulled in
closer to the
nucleus. Ionic
size decreases. Result: a smaller
sodium cation,
Na+
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Anion Formation
17p
+
Chlorine
atom with
7 valence
e-
One e- is
added to the
outer shell. Effective nuclear
charge is reduced and
the e- cloud expands.
A chloride ion
is produced. It
is larger than
the original
atom.