The document discusses the arrangement of electrons in atoms. It states that electrons fill the atomic shells from closest to farthest from the nucleus, with the first shell holding up to 2 electrons and subsequent shells holding up to 8 or more electrons. It defines valence electrons as the electrons in the outermost shell and explains that elements with the same number of valence electrons have similar chemical properties.

1. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
How are electrons arranged in the
atom?
The first shell
• is closest to the nucleus,
• is always filled first,
• can hold a maximum of
2 electrons.

2. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
How are electrons arranged in the
atom?
2nd shell
• can hold up to 8 electrons.
3rd and subsequent shells
• are filled next.
• may contain a maximum of
8 or more electrons.

3. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Example: A magnesium atom
A magnesium atom (proton number =12) has 12 electrons.
The first shell has 2 electrons.
The second shell has 8 electrons.
The third shell has 2 electrons.
Thus, its electronic structure or electronic configuration
can also be represented as (2, 8, 2).

4. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
What are valence electrons?
The shell that is furthest from
the nucleus which contains
electrons is called the outer
shell or the valence shell.
The electrons in this shell are called
valence electrons (or valency electrons).

5. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
A diagram of an atom’s outer electronic structure
shows only the valence electrons in the valence shell.
Hence, even though
sodium’s electronic
configuration is 2,8,1, we
may show it this way
(if instructed).
12n
11p

6. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
The Importance of Valence Electrons
The chemical properties of an element
depend on the number of valence electrons.
For example, sodium (2, 8, 1) and potassium (2, 8, 8, 1)
have similar chemical properties
because in each case there is 1 electron in the outer shell.

7. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
The Periodic Table
In the Periodic Table, elements
are arranged in order of
increasing proton number.
1 2
3 4 5

8. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
The vertical columns of elements called groups.
The horizontal rows of elements called periods.

9. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
The Periodic Table
Elements in the same group have the same number
of valence electrons.
Hence, elements in the same group have
similar chemical properties.