شرح دروس science للمراحل الابتدائيه والاعداديه
The atom: is the smallest unit of an element which undergoes a chemical reaction without being changed.
1. The structure of the atom:
The nucleus: it contains positively charged protons &
neutral particles called neutrons.
b. Electrons: they are negatively charged particles which revolve around the nucleus in energy levels with high speed.
The atomic number: is the number of protons in the
nucleus .
The mass number: is the number of protons + neutrons
in the atom.
4. The number of protons = the number of electrons in a neutral atom
Energy levels: are imaginary places around the nucleus in which the electrons move according to their energy
1. Electrons revolve around the nucleus in fixed orbits
called energy levels.
2. The smallest atom (hydrogen) has one energy level.
3.The biggest atom has 7 energy levels
The electronic configuration:
1. The rule 〖𝟐𝒏〗^𝟐 is used to find the number of electrons that fill each energy level. n is the number of the level.
2. The first energy level is filled with : 〖𝟐(𝟏)〗^𝟐 = 2 electrons.
3. The 2nd energy level is filled with : 〖𝟐(𝟐)〗^𝟐 = 8 electrons.
4. The 3rd energy level is filled with 〖𝟐(𝟑)〗^𝟐 = 18 electrons.
5. The 4th energy level is filled with 〖𝟐(𝟒)〗^𝟐 2 = 32 electrons.
6. Levels 5-7 are filled with 32 electrons only .
The rule 〖𝟐𝒏〗^𝟐 isn't applied because the level can only contain 32 electrons. The presence of more than 32 electrons in an energy level would make the atom unstable.
Valency: is the number of electrons gained or lost or shared by an atom in a chemical reaction
Many attempts are made by scientists for classification of elements :
1-to be easily studied.
2- find the relation between elements and their physical
and chemical properties.
Points of the lesson
1- Mendleev's periodic table.
2- Moseley's periodic table.
3- Modern periodic table.
Mendeleev’s periodic table is considered as the first real periodic table for classification of the elements.
1-In 1871. he recorded on single cards: the names of the elements, their atomic weights and their important properties.
2- He arranged the similar elements in vertical columns called “groups” later.
3- He discovered that the elements were arranged in an ascending order according to their atomic weights from left to right in the horizontal rows, which were later called “periods”
4-The properties of elements are repeated periodically at the beginning of each new period.
5-Mendeleev explained his periodic table in his book “Principles
of Chemistry” In 1871, and classified the known elements until this time to be (67) elements.
Advantages of Mendeleev’s table Mendeleev
1- predicted the ability of discovering new
elements. So he left spaces (empty cells) in his table,
2-he corrected the wrong estimated atomic weights
of some elements.
disadvantages of Mendeleev’s
1-He had to make a disturbance in the ascending order of atomic weights for some elements, due to putting them in groups which suit their properties,
2- he also would have to deal with the isotopes of one element as different elements because they are different in their atomic weights.
Moseley periodic table
*From the discoveries which have helped Moseley to put his periodic table are :
1. Radiation activity phenomena.
2. Getting of -rays.
3.The more knowledge about the arrangement of electrons in
atoms.
1-In 1913, the New Zealand Scientist Rutherford discovered that the nucleus of atom contains positively charged protons,
2-The British Scientist Moseley discovered in the same year after studying the properties of x rays that the periodicity of elements properties is related to their atomic number not their atomic weights as Mendeleev believed
1-So Moseley rearranged the elements in an ascending order according to their atomic number such that the atomic number of an element increases by one than the element
before it in the same period
2- He added the inert gases in the (0) zero group .
3- He allocated a place below his periodic table for the two groups
lanthanides and actinides.

Emad Younan
عماد يونان

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