• Isotopes are elements that have more neutrons than protons, which increases their atomic
o A radioactive isotope is one in which the nucleus decays spontaneously, giving
off particles and energy. When the decay changes the number of protons, it
transforms the atom to an atom of a different element.
• When two atoms chemically react, their nucleuses do not touch – rather their subatomic
particles react to each other.
• Atom’s electrons have different amount of energies. Energy is the capacity to cause
change by doing work. Potential energy is the energy that matter possesses because of
its location or structure.
o The electrons of an atom have potential energy because of how they are arranged
in relation to the nucleus. It takes work to move a negative electron farther away
from the nucleus, so the more distant an electron is from the nucleus, the greater
its potential energy.
• Electrons are found on electron shells, each with a characteristic average distance and
• Electron distribution and chemical properties:
o The first shell can hold only TWO electrons. The second shell holds up to EIGHT
o The chemical behavior of an atom depends on the number of electrons in its
outermost shell, which are called valence electrons. An atom with a complete
valence shell is unreactive.
• Electron Orbitals
o The threedimensional space where an electron is found 90% of the time is called
o The first electron shell has only one spherical s orbital (called 1s), but the second
shell has four orbitals: one large spherical s orbital (called 2s) and three dumbbell
shaped p orbitals (called 2p orbitals).
o No more than 2 electrons can occupy a single orbital. So, there are 2 electrons in
the s orbital.
o The four orbitals of the second electron shell can hold up to 8 electrons, 2 in each
2.3 The formation and function of molecules depend on chemical bonding between atoms
• Atoms with incomplete valence shells interact with other atoms to complete their valence
shells. These interactions result in atoms staying close together, help by attractions called
chemical bonds. Covalent bonds and ionic bongs are the strongest kinds of chemical
• A covalent bond is the sharing of a pair of valence electrons by two atoms.
o Two or more atoms held together by covalent bonds make up a molecule. We can
use a structural formula to show the single bond (a pair of shared electrons) in H2:
H – H. A double bond is the sharing of two pairs of valence electrons: H=H.
o The bonding capacity of an atom is called the atom’s valence and usually equals
the number of unpaired electrons required to complete the atom’s outermost