Atoms and the Periodic Table
Atomic Structure
What are atoms?
• Democritus
– 4th century B.C.; Greek philosopher
• Suggested that the universe was made of invisible
units called atoms – “unable to be divided”.
• Believed movement of atoms caused the changes in
matter he observed.
– Didn’t have the evidence needed to support existance of
atoms.
• John Dalton
– 1808; English schoolteacher
• Proposed his own atomic theory
– Much evidence
Dalton’s Theory
• Every element is made of tiny, unique
particles called atoms that cannot be
subdivided.
• Atoms of the same element are exactly
alike.
• Atoms of different elements can join to
form molecules.
What’s in an Atom?
• Atoms are made of protons, neutrons, and
electrons:
– Nucleus – center of each atom.
• Carries an overall positive charge.
– Proton – a positively charged subatomic particle in
the nucleus of an atom.
– Neutron – a neutral subatomic particle in the
nucleus of an atom.
– Electron – a tiny negatively charged subatomic
particle moving around outside the nucleus of an
atom.
Atoms have no overall charge.
• A helium atom has two protons and two
electrons.
– The atom is neutral because the positive
charge of the two protons cancel the negative
charge of the two electrons.
•
•
•
•
Charge of two protons:
Charge of two neutrons:
Charge of two electrons:
Total charge of a helium atom
+2
0
-2
0
Models of the Atom
• Bohr model
• Niels Bohr – 1913; Danish scientist.
– Suggested electrons in an atom move in set paths
around the nucleus.
• Much like planets orbit the sun in our solar
system.
–Energy levels – any of the possible energies
an electron may have in an atom.
»Gain energy to move to a higher level
»Lose energy to move to a lower level
Electron Facts
• Electrons behave more like waves on a
vibrating string than like particles.
• It is impossible to determine both the exact
location of an electron in an atom and its
speed and direction.
– Calculating the change or probability.
• Electron cloud – whole shaded area of model
• Orbitals – regions in an atom where
electrons are found.
• Valence electrons – outermost electrons.
Atomic Structure Review 3.1
1. Draw and label the parts of a helium atom.
Include the mass and charge of each
subatomic particle.
2. Describe the three main ideas of Dalton’s
atomic theory in your own words.
3. Compare the outermost electrons of an
atom with the inner electrons of an atom in
terms of energy.
4. Compare the positions of the electrons in
Bohr‘s model of the atom with their
positions according to modern atomic
theory.