CHAPTER 3: Part 1

Structure of the Atom

Electronic Configurations

SUBATOMIC PARTICLES (1)

• The atom consists of a massive and compact nucleus and electrons moving about the nucleus.

• The nucleus contains protons and neutrons.

• The atom's identity is uniquely defined by the number of protons (Z = atomic number) in the nucleus

SUBATOMIC PARTICLES (2)

• Proton: positive charge (+e); mass of 1.67E-27 kg; located in nucleus

• Neutron: zero charge; mass of 1.67E-27 kg; located in nucleus

• Electron: negative charge (-e); mass of 9.1E-31 kg; moves in space around nucleus

• In a neutral atom #p+ = #e-• If Z = 11, atom = Na. If Z = 80, atom = Hg

ATOMIC STRUCTURE (1)

• What about the neutrons? Atoms of the same element have the same # p+ and #e- but may have a different number of neutrons. These atoms are called isotopes.– All carbon atoms have 6 p+ but 12C or carbon-12

has 6 n; 13C has 7 n; 14C has 8 n

– All chlorine atoms have 17 p+ but 35Cl has 18 n and 37Cl has 20 n

– H: normal (1H), deuterium (2H), tritium (3H)

ATOMIC STRUCTURE (2)

• Given that atom consists of positive and negative particles, what holds it together?

• Ans: A balance of electrostatic and centrifugal forces.

• Nucleus consists of about 99.9%= of mass of atom but only about 1E-13% of volume. Recall atom diameter = 1E-10 m and nucleus diameter = 1E-15 m.

ELECTRON CONFIGURATION (1)

• How are the electrons (e-s) actually arranged around the nucleus? It turns out that e-s occupy shells around the nucleus with e-s in the shell closest to the nucleus being held most tightly. The e-s in the shell farthest from the nucleus are held most loosely. We call the latter e-s valence electrons.

ELECTRON CONFIGURATION (2)

• Valence electrons are the outermost e-s and are most involved in chemical bonding.

• They also determine the size of the atom. As Z increases, more shells are filled and the larger the atom.

• The group number above a column on the Periodic Table is equal to the number of valence electrons in the atom

ELECTRON CONFIGURATION (3)

• The occupancy numbers for these shells are 2, 8, 18 (T3.1)

• How do the electrons fill these shells? They fill from the innermost shell (closest to the nucleus) to the outermost shell until all of the e-s are placed around the nucleus.

OCTET OF ELECTRONS

• Atoms with 8 electrons in the valence shell are extra stable meaning that they do not react readily with other elements.

• Group VIIIA atoms have an octet of e-s.• These are the Noble Gases and are found in

nature as single atoms (unreactive)• He (duet of electrons), Ne, Ar, Kr, Xe• Uses: lighting, fireworks