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C
Atoms and ions
Atoms and ions
Electron arrangements
Electron arrangement
for carbon
Electron arrangement
for sulfur
Atoms and ions
An ion is an atom, or group of atoms, that has gained
or lost electrons.
Atoms are always neutral because their
number of protons (with a positive
charge) is equal to their number of
electrons (with a negative charge).
Neutral
Ions are charged particles because their numbers of
protons and electrons are not equal.
When an atom has lost electrons it
becomes a positively-charged ion.
When atoms lose electrons they
leave behind a positive charge.
A charge of 2+
When atoms gain electrons they
form negative ions.
When atoms gain electrons
they gain a negative charge.
A charge of 2–
A positive ion has more protons than electrons.
A negative ion has more electrons than protons.
Atoms and ions
An ion is an atom or group of atoms that has gained or
lost electrons.
Atoms have no charge because their
numbers of protons and electrons
are equal.
Positive ions have more protons than
electrons because they have lost
electrons.
Negative ions have more electrons
than protons because they have
gained electrons.
Neutral
A charge of 2+
A charge of 2–
Electron arrangements
Electrons are found in layers or shells outside the
nucleus.
 The first shell holds a maximum of 2 electrons.
 The second shell holds a maximum of 8 electrons.
 When the outer shell contains 8 electrons, a new
shell is begun.
Example: Electron arrangements
Magnesium has atomic number 12. What is its electron
arrangement?
Magnesium will have 12 protons and 12 electrons.
Magnesium’s electron arrangement is 2, 8, 2
(2 + 8 + 2 = 12)
Fluorine has atomic number 9. What is its electron
arrangement?
Fluorine will have 9 protons and 9 electrons.
Fluorine’s electron arrangement is 2, 7
(2 + 7 = 9)
The periodic table of elements
The periodic table of elements lists all the elements in
order of their atomic number.
The columns of the table (called groups) contain elements
with similar properties. They are numbered 1–18.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Non-metals
Metals
The metals are on the left of the pink line and the nonmetals are on the right.
2
O
We can read the electron arrangement of atoms
directly from the periodic table.
What is the electron arrangement of oxygen?
O is the 6th element in the second row.
Its electron arrangement is
2, 6
6
2
Al
What is the electron arrangement of aluminium?
Al is the 3rd element in the third row.
Its electron arrangement is
2, 8, 3
8
3
2
8
8
1
K
What is the electron arrangement of potassium?
K is the 1st element in the fourth row.
Its electron arrangement is
2, 8, 8, 1
Electron arrangement for carbon
12
6C
Carbon’s atomic number = 6
So
Number of protons = 6
Number of electrons = 6
Electrons in first shell = 2
Electrons in second shell = 4
The electron arrangement is 2, 4
Electron arrangement for sulfur
32
S
16
Sulfur’s atomic number = 16
So
Number of protons = 16
Number of electrons = 16
Electrons in first shell = 2
Electrons in second shell = 8
Electrons in third shell = 6
The electron arrangement is 2, 8, 6
Electron arrangements and the periodic table
An atom’s electrons are arranged in layers or shells.
The first shell can only hold 2 electrons, while the
others can hold 8. An atom with 12 electrons will thus
have the arrangement 2, 8, 2.
Elements are listed on the periodic table in order of
atomic (proton) number. Metals appear on the left of
the table with non-metals on the right.
The columns on the periodic table are called groups.
Elements in the same group have similar properties.
We can read an atom’s electron arrangement off the
periodic table. Oxygen is the 6th element in the second
row so its arrangement is 2, 6.
3C 1 Electron structure of the elements 3C 2 The internal structure of atoms
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