Term
| Using the Bohr Theory to Describe Atoms and Ions |
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Definition
Recall that Bohr’s theory of the atom places protons and neutrons in the
nucleus of the atom, and electrons around the nucleus in specific shells.
There is a maximum number of electrons that can occupy each shell.
There is a maximum number of electrons that can occupy each shell. |
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Term
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Definition
You can draw models called Bohr diagrams to help you to visualize the
arrangement of electrons in different atoms.
Bohr diagrams help to explain the mechanism by which elements combine to form compounds.
Bohr
diagrams are a series of concentric rings (the shells) around a nucleus, with electrons shown as dots on the rings.
Figure 1 shows the Bohr diagram for
phosphorous. Inside the nucleus is the chemical symbol for phosphorous, P.
The number of protons and neutrons are often written in the centre, but this
is not strictly necessary for a Bohr diagram. |
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Term
| The Bohr Theory and Reactivity |
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Definition
The two most reactive chemical groups are Group 1 (1st column of the
Periodic Table) and Group 17 (17th column).
The least reactive chemical
group is Group 18.
The Bohr diagrams for these groups show that the Group 1 metals have only a single outer electron.
The Group 1 elements are the most reactive metals because their single outer electron is easily given up.
the Group 17 non-metals have only a single vacancy in their outer shells. The Group 17 elements are the most reactive non-metals because they easily acquire one more electron to fill their outer shells completely.
the Group 18 non-metals have a full outer shell.
The Group 18 elements are non-reactive because their outer shells are full. |
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Term
| Using the Bohr Theory to Predict the Composition of Compounds |
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Definition
In the Try This activity, you saw that the outer shells of all the elements in a chemical group are similar.
For example, lithium and sodium have a different
number of electrons and a different number of shells. Both, however, have a
single electron in the outer shell of their atoms.
For the first 20 elements in the
Periodic Table, the number of electrons in the outer shell of an atom
determines in which column (and group) the element appears in. |
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Term
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Definition
When a metal atom reacts with a non-metal atom, they form an ionic compound
The bonded lithium and fluorine ions form the
compound lithium fluoride (LiF). Figure 8 illustrates the process of electron transfer between the metal and non-metal.
Consider the collision of an atom of fluorine with an atom of lithium. Lithium has a single electron in its outer shell. Fluorine has a single vacancy in its outer shell.
During the collision, the lithium atom gives up its outer electron (to empty its outer shell), and the fluorine atom captures this electron (to fill its outer shell). Lithium, having lost an electron, becomes a positive ion, and fluorine, having gained an electron, becomes a negative ion.
The pair of oppositely charged ions are held together by the electrostatic
force that exists between charged objects.
The bonded lithium and fluorine ions form the compound lithium fluoride (LiF). |
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Term
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Definition
| When a metal atom reacts with a non-metal atom, they form an ionic compound.This process is called electron transfer. |
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Term
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Definition
The process of electron transfer between metals and non-metals is what creates an ionic bond—metal ions
joined to non-metal ions. |
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Term
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Definition
Molecular compounds are composed of molecules; two or more atoms joined together to form the smallest particle that has the same properties as the compound.
Molecular compounds are usually formed
from a collection of non-metal atoms.
The bond between atoms in molecules is not due to electron transfer, but to the sharing of electrons in the outer shells of the atoms. This is called a covalent bond. |
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Term
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Definition
Strictly speaking, ionic compounds do not have discrete molecules.
The
description of electron transfer above is a simplified version of the process
that leads to the formation of positive and negative ions.
In a real chemical reaction, an extremely large number of both positive and negative ions are
formed at once. The ionic compound formed is, in reality, a collection of
positive and negative ions that occupy the same space.
When in solid form,
Be
4 p
5 n
Mg
12 p
12 n
Ca
20 p
20 n
Figure 8 The formation of the ions
of lithium and fluorine, and the
compound LiF
Ions and Atoms
To perform this investigation,
turn to page 226.
In this investigation, you will
explore the differences in the
properties of an atom and its ion.
7B Investigation
7B Investigation
Figure 7 The Bohr diagrams for three of the Group 2 elements—(a) beryllium (Be), (b) magnesium
(Mg), and (c) calcium (Ca). Note that there are two outer electrons for each atom.
Li
3 p
4 n
F
9 p
10 n
Step 2
outer electron of lithium is
transferred to fluorine
Li
3 p
4 n
F
9 p
10 n
Step 3
positive L ion is held to
negative F ion by electric force
Li
3 p
4 n
F
9 p
10 n
Step 1
lithium atom—
one outer electron
fluorine atom—
seven outer electrons
(a) (b) (c)
LEARNING TIP
Try to work out your own
explanation of how lithium fluoride
is formed. Read the paragraph a few
sentences at a time, translate the
meaning of the sentences into your
own words, and create a picture in
your head.
222 Unit B Atoms, Elements, and Compounds NEL
the negative and positive ions collect and arrange themselves in a specific
structure, forming a crystal.
When dissolved in a solution, or in the liquid or gaseous state, the individual ions move about freely, independent of other ions. |
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Term
| The Ionic Bond and Ion Charge |
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Definition
As you have seen, for elements to form an ionic compound, they must first
form ions. Recall that the ion charge is the number of electrons that must
be given up or acquired to form an ion. |
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