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Matter is made of atoms (cannot be further divided), the word atom comes from the Greek word “atomos” which means indivisible.
Democritus
• Atoms were tiny particles that could not be divided• John Dalton proposed that an atom is a sphere of
matter that is the same throughout and can’t be seen by human eyes
• Marble Model
• Came up with the idea of the electron negatively charged particle• Thomson discovered that all atoms contain electrons, which are tiny,
negatively charged particles. Thomson proposed that an atom is a sphere of positive charge. The electrons are mixed uniformly in the sphere.
• Cookie Model
Ernest Rutherford• He hypothesized that almost all the mass and all the
positive charge of an atom is concentrated in an extremely tiny nucleus at the center of the atom.
• They are the negatively charged particles orbiting the nucleus.
• It is much smaller than the protons and the neutrons .
What is the net charge of the atom ?
• The atom is formed of positively charged proton in the nucleus and negatively charged electrons in the energy level ,but they are equal to each other in number ,making the net charge of the atom “neutral”.
No of positive protons = No of negative electrons
• They are charged atoms• Number of positive protons = Number of negative electrons.
Positive ion
Negative ion
If the atom lost an electron ,therefore the number of positive protons are more than the negative electrons making the atom positively
charged
Positive Ion
If the atom gained an electron ,therefore the number of negative electrons
are more than the positive protons making the atom negatively charged
Negative Ion
• Atomic number is the number of Protons in an atom
• All atoms of a particular element have the SAME number of protons (All Carbon atoms have 6 protons)
• In a neutral atom,
The atomic number =The number of electrons
O8
16
Atomic number
Mass number
Symbol
OxygenName
Atomic number= Number of protons= Number of electrons
Mass number= Number of protons + Number of neutrons
Calculating Neutrons:
• Mass number = protons + neutronsso…..
• Neutrons = mass number – protons
What is the number of neutrons of the following atom ?
A21
45
• In 1869 Dimitri Mendeleev was working on a way to classify elements.
• At the time, more than 60 elements had been discovered.
• And he arranged the elements according to their atomic mass
Developing a Periodic Table
100 table The periodic table is a chart of the
elements arranged according to their atomic number in rows and columns according to their
physical and chemical properties.
•The periodic table is a chart of the elements arranged into rows and columns according to their physical and chemical properties.
• It can be used to determine the relationships among the elements.
What is the periodic table?
Groups and Periods - Columns in the periodic table represents “groups”- There are 18 groups in the periodic table.- Elements of the same the same group have the same
chemical properties as the all have the same number of electron/s in the outer most energy levels.
- Rows in the periodic table represent “periods”.- There are 7 periods in the periodic table .- Elements of the same period have the same number
of energy levels
Periodic Table
A great deal of information about an element can be gathered from its position on the period table.
Understanding the organization and plan of the periodic table will help you obtain basic information about each of the 118 known elements.
Mass Number
Atomic Mass (Mass Number) is the number …………….
__________Atom p+ n0 e- Mass #
Oxygen - 8
- 33 42
- 31 15
___ ___ ______
______ ___ ___ ___
______ ___ ______
Recall - How are P, N, e- related?
• atomic number = # protons
• # protons = …………….
• # protons + # neutrons = atomic mass# neutrons = ……………. …………….
• How many protons, neutrons and electrons are found in an atom of Cs?
• _____________• …………….
• ……………. • Therefore, # Neutrons = …………….
_____________
_____________
55Cs
133
Example 1
Example 2 If an element :Magnesium
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
=_____________
= _____________
= _____________
_____________
Properties of MetalsMetals appear to the left of the dark ziz-zag
line on the periodic table. Most metals are solid at room temperature.
Compounds-Are substances made of atoms of two or more elements
that are chemically combined.-The chemical formula of a compound shows the elements
that are in it
-Example:SiO₂ (sand): It is formed of 1 Silicon atom 2 oxygen atoms
-Compounds don’t have the same properties of the elements forming them
Example:The ordinary table salt is a compound called NaClIt is formed of : 1 atom of Sodium and 1 atom of chlorine Where “Sodium” is a metal
• And “Chlorine” is a gas
But when they combine together they give a product of different properties which is the table salt