Atomic Theory

Greek Philosophers• Aristotle

4Elements of matter

-Earth -Fire -Air -Water

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.

• Thought electrons traveled in fixed paths around the nucleus called energy levels.

Atomic Structure• Arrangement and number of sub-atomic particles

•Protons•Neutrons•Electrons

• Center or core of an atom• Contains 99.9% of the weight of an

atom• Contains protons and neutrons

• Positively Charged Sub-atomic particles• Found in the nucleus• All protons are a like

• Neutrally charged (no charge) sub-atomic particle• Found in nucleus

• 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

• Also called atomic weight or Mass Number• Sum weight of protons and neutrons in an atom

Calculate the mass number of each of the following atoms

O 8

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 + neutrons

so…..• 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.

Symbol

•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

The Periodic Table of Elements

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

NaSodium

23

11_________________________________

Example 2 If an element :Magnesium

a) number of protons

b) number of neutrons

c) number of electrons

d) complete symbol

=_____________

= _____________

= _____________

_____________

ATOM

The smallest particle of an element with the same properties of the element.

MOLECULE

The smallest part of a substance made from more than one atom that binds together.

Let’s Build SomeMolecules

Let's start with a couple of Molecules• Hydrogen Gas H2

H H

• Oxygen Gas O2

O O

• Ammonia Gas NH3

H H

HN

• Water H2O

HH

O

Elements Substance that contains only one kind

of atom

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

How compounds are formed ?-Compounds hold together because atoms bond or

attach to one another

How atoms can attach or bond with each other?

They either

Share electrons

Gain Or

lose electrons