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: +2• Charge of two neutrons: 0• Charge of two electrons: -2• Total charge of a helium atom 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 3.11. 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.