1. Atomic Structure
An atom is the smallest particle of an element that retains its identity in a chemical reaction.
Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.

2. Atomic Structure
Ancient Greeks believed that everything was made up of the four elements
Water, Fire, Earth, Air
Democritus stated that everything is made up of tiny, invisible particles
He said the particles were indivisible
He called them “atomos” – which means unable to divide

3. Atomic Structure John Dalton developed the first atomic theory
His theory was successful because it was supported with scientific evidence

4. Atomic Structure
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

5. Atomic Structure
Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. 

6. Atomic Structure Much of Dalton’s atomic theory is accepted today.
One important change, however, is that atoms are now known to be divisible.
Also, atoms of the same element are not identical.

7. Atomic Structure
By the late 1800’s, JJ Thomson contributed to the development of the model
He performed an experiment with a cathode ray tube
His experiment led to the discovery of the electron (negative particles)
Protons (positive particles) and neutrons (neutral particles) were discovered later

8. Atomic Structure
Since the mass of an atom is very small chemists created a new unit to describe the mass of atoms
Amu (atomic mass unit) is 1/12 the mass of a Carbon – 12 atom
Protons are amu
Neutrons are amu
However we round each one to 1 amu

9. Atomic Structure Properties of Subatomic Particles Particle Symbol
Relative charge
Relative mass (mass of proton = 1)
Electron e– 1–
1/1840 ( 0 )
Proton p+ 1+ 1
Neutron n0

10. Atomic Structure
Elements are different because they contain different numbers of protons.
The atomic number identifies an element.
Atomic number = protons

11. Atomic Structure
The total number of protons and neutrons in an atom is called the mass number.
Mass number = protons + neutrons
The mass listed in the periodic table is the average atomic mass
It is a weighted average of the atomic masses of naturally occurring isotopes

12. Atomic Structure Remember that atoms are electrically neutral.
In an atom, protons = electrons
Protons, neutrons, and electrons can be calculated from atomic number and mass number.
How many protons, electrons, and neutrons are in each atom?
boron -10, fluorine - 19, sodium - 23

13. Atomic Structure
Isotopes are atoms of the same element with different masses
Neutrons were actually discovered due to isotopes
In an isotope the number of protons and electrons are the same – only the neutrons differ

14. Atomic Structure For example, hydrogen has 3 isotopes: Isotope Proton
Neutron
Electron
Hydrogen - 1
Hydrogen – 2
Hydrogen - 3
Note that the correct way to write an isotope is to write the name, followed by the mass number.

15. Atomic Structure
Another way to specify an isotope is to use the nuclear symbol.
The nuclear symbol includes the chemical symbol, atomic number, and atomic mass.
Write the nuclear symbol for:
boron -10, fluorine - 19, sodium - 23

16. Atomic Structure
The value shown in the periodic table is the average atomic mass
It is a weighted average
For example: Chlorine has two isotopes
Chlorine-35 and Chlorine-37
The abundance is
Cl-35 has an amu of with an abundance of %
Cl-37 has an amu of with an abundance of %

17. Atomic Structure
In the early 1900’s, Ernest Rutherford performed the “Gold Foil Experiment”
He observed that the alpha particles went through the gold foil

18. Atomic Structure From his results, he proposed:
The atom is almost entirely empty space
The nucleus was extremely tiny and contained the protons and the neutrons
The electrons are distributed around the nucleus and occupy almost all the volume of the atom.