NATURAL SCIENCE
GRADE 9
ELEMENTS AND
COMPOUNDS
AT THE END OF THE LESSON LEARNERS SHOULD BE ABLE TO:
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Explain what is meant by an element
•
Identify elements by symbols
•
Explain the basic properties of elements
•
Demonstrate an understanding of the organization of elements in the periodic table
•
Explain the difference between an element and a compound
•
Recognize how compounds form
•
Explain the basic properties of compounds
•
Understand how are compounds and molecules built from elements
•
Identify the formulae for common compounds
WHAT IS AN ELEMENT?
An element is a pure substance that is made from a single type of atom. Elements are the building blocks for all
the rest of the matter in the world. The atoms of each element are chemically distinct and different from those of
any other element. There are currently 118 known elements. Of these, only 94 are thought to naturally exist on
Earth. Examples of elements include, oxygen, chlorine, lithium, hydrogen and more.
Elements are arranged in the Periodic Table according to their atomic numbers, which are the number of protons
in the nucleus.
The first element is hydrogen because a hydrogen atom has one proton in its nucleus. The second element is
helium because a helium atom has two protons in its nucleus.
We can use this system in listing of elements.
Each element has a name and a symbol, which consist of one or two letters.
For example:
O=oxygen
Li=lithium
Na=sodium
CHEMICAL
SYMBOLS
The element symbol is always almost accompanied by other information such
as atomic number and atomic weight. Atomic number describes the number
of protons in one atom of that element. For example, an atom of oxygen
contains 8 protons.
CHEMICAL SYMBOLS
Some chemical symbols are more difficult to understand. Na, for example, is the symbol for sodium. The Na comes from
the Latin name for sodium, which is natrium. These symbols were chosen very long ago, when many subjects were
studied in Latin.
Some simple rules to remember when using chemical symbols:
Every element has its own, unique symbol.
The symbol is usually (but not always) the first one or two letters of the name of the element.
The first letter of the symbol is always a capital letter.
If the symbol has two letters, the second letter is always a small letter.
Some elements have symbols that come from their Latin names.
COMMON ELEMENTS AND SYMBOLS
Element
Symbol
Element
Symbol
Aluminium
Al
Magnesium
Mg
Bromine
Br
Nitrogen
N
Calcium
Ca
Oxygen
O
Carbon
C
Phosphorus
P
Chlorine
Cl
Potassium
K
Copper
Cu
Silicon
Si
Gold
Au
Silver
Ag
Hydrogen
H
Sodium
Na
Iodine
I
Sulfur
S
Iron
Fe
Tin
Sn
Lead
Pb
Zinc
Zn
PROPERTIES OF ELEMENTS
Elements can be metals, non-meals, or metalloids
A metal is an element which is typically hard, shiny, fusible, malleable, and ductile, with good electrical and thermal
conductivity.
A nonmetal is an element which is a poor conductor of electricity, dull, non-malleable and non-ductile, they tend to be
gases or brittle solids.
A metalloid is an element having intermediate properties of both metals and nonmetals.
THE PERIODIC TABLE
Elements are organized based on their physical and chemical properties.
The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are
arranged by atomic number, electron configuration, and recurring chemical properties
•
The chemical elements are arranged in order of increasing atomic number.
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The horizontal rows are called periods and the vertical columns are called groups.
•
Elements in the same group have similar chemical properties. This is because they have the same number of outer
electrons and the same valency.
•
The periodic table arranges elements according to their atomic size and other properties.
•
The periodic table is organized into periods and chemical families.
CHEMICAL FAMILIES
Alkali metals- The elements in group 1 are known as alkali metals.
Halogens- The elements in group 7 are known as halogens.
Noble gases- The elements in group 0 are known as noble gases. They are very unreactive and exist as individual
atoms (monatomic).
Transition metals- The transition metals are elements which are found between groups 2 and 3.
Alkiline Earth Metals- The elements in family 2 which are less reactive to alkali metals.
THE PERIODIC TABLE
LET US WORK OUT THE FOLLOWING
What is the element symbol for oxygen?
Al
O
Pb
Sn
CONTINUING…..
An element is a substance that…..
exists naturally as a solid, liquid, and gas.
cannot be broken down into simpler substances.
is above 86 on the periodic table.
can be broken down only by chemical means.
CONTINUING
Which term refers to the number of protons in an atom and helps to place elements on the Periodic Table?
atomic number
atomic mass
mass number
Isotope
STATE IF THE FOLLOWING STATEMENTS ARE TRUE OR FALSE
1. Hydrogen is known as an alkali metal.
2. Some element symbols begin with lower case letters.
3. Sulfur, being a non-metal tends to be a poor conduct of electricity
4. The element symbol for fluorine is Fl.
5. Ne is the symbol for neon.
WHAT IS A COMPOUND?
When two or more atoms join, we call it a molecule. When two or more atoms of different elements join together, we call
it a compound. All compounds are molecules, but not all molecules are compounds. That is because a molecule can be
made up of two atoms of the same kind, as when two oxygen atoms bind together to make an oxygen molecule.
However, all compounds are made up of two or more different types of atoms.
Elements are rarely found in their pure state; compounds are much more common. There are just over 100 different
kinds of atoms, but there are millions of different kinds of substances made up of different types of molecules. Probably
everything you see around you is some type of compound. When atoms of different kinds combine to form a compound,
a new substance is created. New compounds do not have the same physical or chemical traits of the original elements.
They have a new life of their own.
CONTINUING…..
Compounds are written with formulas showing which
elements from the periodic table are combined. One
very familiar compound is water. When two
hydrogen atoms (H2) combine with one oxygen
atom (O), it makes the compound H2O, which we
know as water. All water molecules have this same
combination of atoms.
Water is not hydrogen or oxygen. You couldn't pour
oxygen and hydrogen atoms on a fire and expect to
put it out. But when they are bonded together as
water molecules, they behave like water.
DIFFERENCE BETWEEN ELEMENTS AND COMPOUNDS
Elements are substances that are made from one type of atom. An element cannot be broken down into any
other substance. There are 94 naturally occurring elements and everything in the universe is made from these
basic building blocks. Common examples include carbon, sulfur, oxygen, iron, copper, aluminum. Elements are
represented by symbols.
Compounds are substances made from atoms of different elements joined by chemical bonds. They can only be
separated by a chemical reaction. Common examples are water (H2O), salt (sodium chloride, NaCl), methane
(CH4). The symbols indicate which elements the compounds contain, and the number tells you the ratio in which
the atoms of the elements combine.
HOW COMPOUNDS FORM?
Compounds are formed when elements are joined and held together by strong forces called chemical bonds.
These bonds involve the electrons that orbit the nucleus of the atom. Electrons are in energy levels that occur at
certain distances from the nucleus, called shells.
These shells can each carry a certain number of electrons (for example, 2 in the first shell, 8 in the second, and so
on.) Atoms want to have their shells full of as many electrons as they can carry, and when their outermost shell
isn't full, atoms try to bond with other atoms by giving up or gaining electrons.
Atoms with an almost-empty outer shell will want to give up electrons, while atoms with an almost-full outer shell
will want to gain electrons in order to fill it up.
CONTINUING….
There are two main types of bonds that hold most
compounds together.
Ionic bonds form when one atom gives up, or
donates, an electron to another in order that both
will have a full outer shell. In doing so, the atoms are
bonded and create a compound.
Covalent bonds share electrons between atoms in
order to fill their electron shells. In the compound,
molecules are held together by the attraction
between the nucleus and the shared electrons.
HOW ARE COMPOUNDS AND MOLECULES BUILT FROM ELEMENTS?
The following video show how are compounds and molecules built from elements :
https://youtu.be/gS3kmdp4t_0
PROPERTIES OF COMPOUNDS
Physical changes do not break down compounds. Physical changes affect the size, shape, or state of the
substance, but not the chemical properties. You can change the state of matter, but the compound does not
change. If you leave an ice cube out in the sun it will melt into liquid water, but in either state it is still made of
water molecules. You can apply a physical force to a solid glass and break it, but the molecules that make up
will remain.
Chemical changes in compounds happen when chemical bonds are created or destroyed. Then the molecular
structure changes; new molecules form and a new substance is created. Often heat is used to begin a chemical
change, as when baking a cake. Another example of a chemical reaction is the rusting of a metal trash can. The
rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the air. Chemical bonds are
created and destroyed to eventually make iron oxide (Fe2O3), which we call rust.
FINDING THE FORMULA OF A COMPOUND
In a compound, the element that is on the left of the Periodic Table, is used first when naming the compound. In
the example of NaCl, sodium is a group 1 element on the left-hand side of the table, while chlorine is in group 17
on the right of the table. Sodium therefore comes first in the compound name. The same is true for FeS and KBr.
To find the formula of an ionic compound, first identify the cation and write down its symbol and charge. Then,
identify the anion and write down its symbol and charge. Finally, combine the two ions to form an electrically
neutral compound.
A compound may contain ions (an ion is an atom that has lost or gained electrons). These ions can either be
simple (consist of only one element) or compound (consist of several elements). Some of the more common ions
and their formulae are given in the table below. You should know all these ions.
EXAMPLES
What is the chemical formula for magnesium chloride?
Step one; list the ions involved
Mg2+ and Cl−
Step two: find the right combination
Magnesium has a charge of +2 and would need two chlorides to balance the charge. They will combine in a 1:2 ratio.
There is an easy way to find this ratio:
Draw a cross as above, and then you can see that Mg2+ →1 and Cl− →2.
Step three; write down the formula
MgCl2
EXAMPLES
Write the chemical formula for magnesium oxide.
Step one; list the ions involved
Mg2+ and O2−
Step two: find the right combination
Mg2+:2 and O2−:2
If you use the cross method, you will get a ratio of 2:2. This ratio must always be in simplest form, i.e. 1:1
Step three: write down the formula
MgO (not Mg2O2)
GIVE THE CHEMICAL FORMULA FOR EACH OF THE FOLLOWING COMPOUNDS.
potassium nitrate
sodium oxide
barium sulphate
aluminium chloride
magnesium phosphate
tin(II) bromide
manganese(II) phosphide
ANSWERS
KNO3
Na2O
BaSO4
AlCl3
Mg3(PO4)2
SnBr2
Mn3P2
THE END. THANK YOU