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1. Chapter 2: Structure of Matter
Atoms, Molecules and Ions
Chemistry
Grade 8
Unit 2
Done by Ms. Sara Chehab

2. The Atom
• Theatomis thebasicbuilding
blockfor all matter in theuniverse.
Forexample,air is madeupof atoms.
Thecomputerismadeupofatoms.
Ourhandsaremadeupof atoms.
Everythingis madeupof atoms.

3. Atomsareextremelysmallandaremadeupofa
fewevensmaller particles called subatomic
particles:electrons, protons,andneutrons.

4. An atom is neutral (has no charge). The negative
charge of the electrons is enough to neutralize the
positive charge of the protons.
An atom contains equal number of protons and
electrons.

5. Each element has a unique atom.
For example, the hydrogen atom is
different than the oxygen atom.
And the oxygen atom is different than
the carbon atom.

6. The number of protons in
an atom determines the
identity of the element. This
number is called the atomic
number of the element and
is denoted by the letter Z.
See the following examples:
Helium atom has 2
protons, so it has
atomic number Z=2
Nitrogen atom has 7
protons, so its
atomic number Z = 7
Sodium atom
has 11 protons,
so its atomic
number Z = 11

7. The Molecule
A molecule is a neutral group of atoms
joined together by a chemical bond.
Hydrogen molecule is
made up of 2
hydrogen atoms
bonded together.
Water molecule H2O is
made up of 2 Hydrogen
atoms and one oxygen
atom bonded together
Carbon dioxide molecule is
made up of one carbon
atom and two oxygen
atoms bonded together.
Examples:

8. Diatomic Molecules:
Diatomic molecules are moleculescomposed
ofonlytwoatoms,of the sameor different
chemical elements.
Examples:

9. Molecular Elements:
Molecular elements are molecules that
have two or more atoms of the same
elementbonded together.
Examples:

10. Molecular Compounds:
Molecular compounds are
molecules made up of two or
more different elements.
Examples:

11. The Ion:
The ion is anatomor groupof atomsthat
carries a positive or negative charge as a
result of having lost or gained oneor more
electrons.
Examples:

12. • A positively charged ion is
called a “cation”.
• A negatively charged ion is
called an “anion”.

13. Cations are formed
when atoms lose
electrons.
Examples:

14. Anions are formed
when atoms gain
electrons.
Examples:

15. Let us discuss some examples:
A magnesium atom has atomic number Z = 12.
We know that in an atom the number of electrons is equal to
that of protons.
So, number of electrons = 12.
If a Magnesium atom loses 2 electrons it will then have 10
electrons and 12 protons.
The charge of magnesium ion is : +12-10 = +2

16. A chlorine atom has atomic number Z = 17.
So the number of electrons = number of protons =
17.
If a chlorine atom gains one electron, then the
number of electrons becomes 18.
So, the charge of a chloride ion = +17-18 = -1

17. Monoatomic & Polyatomic Ions:
Note: Ions that contain only two atoms are called diatomic ions.