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

• Atoms of the same element with different numbers of neutrons.