What is the atomic number? electron proton neutron

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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


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


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.


Columns of elements

What are columns of elements called?

groups 1 2 4 3 5 6 0 7


Rows of elements

periods What are rows of elements called?

1

2

3

4

5

6

7


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.


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.


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



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



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


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?


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.


What’s the electron arrangement?


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.


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+


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.

What is the atomic number? electron proton neutron

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