NIS – PHYSICAL SCIENCE
Lecture 83 – Lecture 84
Stability in Bonding
Ozgur Unal
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Combined Elements
What is the Statue of Liberty made of?
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Combined Elements
Some of the matter around you is in the form of uncombined
elements, such as copper, oxygen etc.
However, when the conditions are right, elements can
combine to form compounds.
Example: Iron + Oxygen  rust
Example: Sodium + Chlorine  Table salt
When a compound forms, its characteristics become
different from the elements that make it up.
Example: Sodium  silvery solid metal
Chlorine  greenish yellow gas
Table salt  white crystalline solid
How about water?
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Formulas
Each element has a unique symbol.
How many compounds can form from the known elements
on Earth?
How do we distinguish these compounds?
Each compound is represented by a unique symbol.
A chemical formula tells what elements in a compound
contains and the exact number of atoms of each element in a
unit of that compound.
Example: H2O contains the elements Hydrogen and Oxygen.
The subscript 2 indicates that there are
2 Hydrogen atoms for 1 Oxygen atom.
How about H2SO4?
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Formulas
Check out Table 1!!
H2O and H2O2 have similar chemical formulas. Does this
mean that they are similar compounds? Explain.
H2O is water.
H2O2 is called hydrogen peroxide.
Although these two compounds have similar formulas, they
have very different chemical properties.
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Atomic Stability
Why do atoms form compounds?
The electric forces between electrons and protons hold
atoms and molecules together  compounds form
However, the six noble gases (Group 18 elements) are
unusally stable  hardly form compounds
Why?
Recall the electron dot diagrams..
The atoms with 8 electrons in their
outer energy levels are chemically
stable.
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Atomic Stability
When you look at the elements in Groups 1, 2 and 13
through 17, you see that none of the elements has stable
energy level.
These elements need to either lose or gain electrons to be
chemically more stable.
Example: Na and Cl
When Na (Group 1 element) loses 1 electron, it will have 8
electrons in its outer energy level.
When Cl (Group 17 element) gains 1 electron, it will have 8
electrons in its outer energy level.
Therefore, Na and Cl can form a compound by exchanging
electrons  NaCl
An atom that has lost or gained electrons is called an ion.
Example: Na+ and Cl-
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Atomic Stability
What is the chemical formula of the compound formed by Ca
and Cl?
Ca becomes the ion Ca+2 when it loses two electrons.
Cl becomes the ion Cl- when it gains one electron.
Two Cl atoms gains electrons from one Ca atom  CaCl2
The oppositely charged ions in the compound attract each
other by electric force.
The net charge of any compound is 0.
How about the compound H2O?
How does it form?
In H2O, the O and H atoms share
electrons to become chemically
stable.
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Atomic Stability
Similarly, in CO2, NH3, HCl etc, the atoms share electrons,
instead of exchanging electron.
There are NO ions in these type of compounds.
These types of compounds form between non-metal
elements.
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