Atomic theory has evolved over time as scientists have discovered more about the internal structure of atoms. Atoms are now understood to be the smallest particle of an element that retains the properties of that element. Atoms contain a nucleus of protons and neutrons surrounded by electrons. The number of protons determines the element, and atoms of the same element have the same number of protons but may vary in the number of neutrons forming different isotopes. Forces such as electromagnetic force hold the structure of the atom together.

1. Atomic Theory
• Democritus (440 BCE)- realized that if you
continued to cut something, eventually you
would end up with something that couldn’t
be cut anymore, atomos – meaning not
able to divide
• Atoms are the smallest particle that an
element can be divided and still be the
same substance
• All matter is made of atoms

2. John Dalton (1803)
• realized that atoms combine in very specific
proportions (ratios) based on mass
• all substances are made of atoms and they can
not be created, divided or destroyed because
they were made of a single substance
• All atoms of the same element are exactly alike
and different from other elements, they are
unique
• Atoms join with other atoms to form new
substance

4. J. J. Thomson (1897)
• discovered that there were small particles
inside the atom, meaning that atoms can
be divided into smaller substances
• Electrons – negatively charged particles
attracted to positively charged particles
• Plum pudding model – electrons are
mixed throughout the atom, soft blobs of
matter

6. Ernest Rutherford (1909)
• Discovered that an atom contains a
nucleus with positively charged particles
and that the electrons must be “floating”
around the nucleus

9. Niels Bohr (1913)
• Proposed that electron moved around the
nucleus in energy levels (shells), but no
electrons between the energy level (think
ladder)
• Electrons can jump from one level to
another
• Travel in a definite path

11. Modern Atomic Theory
• Erwin Shrodinger & Werner Heisenberg
• Electrons have no predictable pattern and
move in a region where electrons are
likely to be found called the electron
cloud

13. Atoms
• All atoms have a nucleus
– protons (+),
– neutrons (no chg)
– electrons (-)
• Same number of protons and electrons an atom
has no charge
• More protons (+) than electrons (-) the atom has
a positive ion is formed (more positives than negatives)
• More electrons (-) than protons (+) a negative
ion is formed (more negatives than positives)

14. • 117 different element that are unique and
all things known to exist come from a
combination of these elements in specific
mass ratios
• Simplest atom is made of one proton, and
1 electron – hydrogen (has no neutrons)

15. • All additional element will have protons,
neutrons and electrons
• The atomic number of an element is
determined by the number of protons,
– 1 is hydrogen, 6 is carbon, hydrogen has 1
proton, carbon has 6 protons (you can not
change the number of protons)
• To find neutrons take the mass number
(rounded) and subtract the protons.

16. Isotopes
• Isotopes have the same number of protons but
additional neutrons which causes the atomic
mass to be different
• Isotopes can be stable (maintain there structure)
and unstable (fall apart over time)
• Unstable isotopes are radioactive and will decay
over time giving off particles and energy
(radioactive)

17. • Mass number determines the isotope, the
number of protons and neutrons added
together
• Most elements have isotopes
• All isotopes of an element have the exact
properties of the element

18. Forces in atoms
• Gravitational force – pulls objects toward
each other—depends on mass and
distances between the objects—very small
force in atoms
• Electromagnetic force –– proton (+) and
electrons (-) have strong attraction which
keeps the electrons in motion around the
nucleus of atoms

19. • Strong force – force which keeps protons
from flying apart due to close distance
between protons and neutrons
• Weak force – relevant to radioactive
atoms- allows neutrons to change into
proton and electron