The document summarizes key information about atomic structure: - The nucleus is positively charged and contains nearly all an atom's mass, while electrons are much smaller and negatively charged, orbiting in shells outside the nucleus. - Electrons are arranged in shells (also called energy levels) around the nucleus, with the first shell holding up to 2 electrons and subsequent shells holding up to 8 electrons each. - Atoms can be represented using Bohr models that show the nucleus and electrons arranged in shells, with the number of protons and neutrons indicated in the nucleus.

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The nucleus is:
 made up of protons and neutrons
 positively charged because of the
protons
 dense – it contains nearly all the
mass of the atom in a tiny space.
Electrons are:
 very small and light, and negatively charged
 able to be lost or gained in chemical reactions
 found thinly spread around the outside of the nucleus,
orbiting in layers called shells.
Protons, neutrons and electrons

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Protons, neutrons and electrons

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How are electrons arranged?
Electrons are not evenly spread, but exist in layers called
shells. (The shells can also be called energy levels).
The arrangement of electrons in these shells is often
called the electron configuration..
Note that this diagram is not drawn to scale – the atom is
mostly empty space. If the electron shells are the size
shown, the nucleus would be too small to see.
1st
shell
2nd
shell
3rd
shell

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How many electrons per shell?
Each shell has a maximum number of electrons that it can
hold. Electrons will fill the shells nearest the nucleus first.
1st
shell holds
a maximum of
2 electrons
2nd
shell holds
a maximum of
8 electrons
3rd
shell holds
a maximum of
8 electrons
This electron arrangement is written as 2,8,8.

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Bohr Models
• Used to represent a model of an atom.
• To draw a Bohr model follow these steps:
(We will use Helium as an example)

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Making a Bohr Model Using Helium
1. Look to the periodic table and determine
how many protons, neutrons and electrons
are in 1 atom of helium.
P=____ N=_____ E=_____
2. Draw a circle and label the # of P and N
in the inside of the circle
P= 2
N= 2

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Making a Bohr Model Using Helium
3. Draw your 1st
electron shell.
4. Draw up to 2 electrons in the 1st
shell.
5. If you need to add more electrons, you need to
add more electron shells! Remember…2, 8, 8!!!
P= 2
N= 2
P= 2
N= 2

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Calculate electron configurations

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Which element?

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Valence Electrons are:
 The electrons in the outermost shell
 Responsible for atomic bonding
 Equal to the last digit of the group number
 How many valence electrons in this atom? What
group would it be in?
Valence electrons

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LEWIS (DOT) SYMBOLS FOR THE ELEMENTS
A Lewis dot structure for an atom consists of
the symbol for the element and one dot for
each valence electron.

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How to Draw a Lewis Structure
1) Find your element on the periodic table.
2) Determine the number of valence
electrons by looking at the group
(column)
3) This is how many electrons you will draw.

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Lewis Structures
1) Write the element
symbol.
2) Carbon is in the 14th
group, so it has 4
valence electrons.
3) Starting at the right,
draw 4 electrons, or
dots, counter-
clockwise around the
element symbol.

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Lewis Structures
1) Check your work.
2) Using your periodic
table, check that
Carbon is in the 4th
group.
3) You should have 4
total electrons, or
dots, drawn in for
Carbon.

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Lewis Structures
What would the Lewis Dot
Structure for Phosphorus
look like?

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