Download 1A-2 The Atom – Building Blocks of Matter

1A-2 The Atom – Building Blocks of Matter
 Historical investigation into the composition of matter led to
these classifications:
Pure Substances
Homogeneous composition
Element
A single type of
atom
Compound
Atoms in a
definite
composition
Physically and
Physically
chemically
indivisible but
indivisible
chemically
e.g. sodium (Na) divisible to the
elements
e.g. table salt
(NaCl)
Mixtures
Composed of more than one type
of pure substances
Homogeneous
Heterogeneous
mixture
mixture
Each part
Each part is
represents the
distinct
whole
e.g. salt water
e.g. sand
 Through subsequent divisions and purifications of matter, we
arrive at single types of atoms, from which all matter is
composed.
 It became apparent that atoms are extremely small and
important features of the chemistry of matter, so models were
developed to understand and predict the structure and function
of atoms. [See table 1.1 page 11]
Key milestones along the way
 J. Dalton proposes the “billiard ball” model of the atom; different
atoms were represented by different spheres that could be joined
to make molecules; atoms are indivisible
 J.J. Thomson – atoms are divisible; found negatively and positively
charged sub-atomic particles (electrons and protons); “plum
pudding” model
 Rutherford’s gold foil experiments (McGill U) showed that the
positive particle (proton) is infinitesimally small but very massive
surrounded by a lot of negatively charged electron space; initiated
the planetary model of electrons orbiting the positively-charged
nucleus that contained the protons.
 Bohr – extended Rutherford’s model with Boltzmann’s, Planck’s
and Einstein’s work on photoelectric effect; electrons exist in
specific energy levels around the nucleus; electron shell model
(birth of the quantum mechanical model)
 Schrodinger – electrons exist in regions of space around the
nucleus according to specific mathematical wave functions, not in
fixed orbits; electron cloud model.
Representing the Atom
 Having a working model of the atom, especially regarding the
electrons, allows us to make some hypotheses about the
structure and reactivity of elements and compounds.
 Although our understanding of the atom is constantly being
refined, earlier models like the Bohr-Rutherford models are still
used to visualize these concepts.
 In this model, the total number of electrons in the neutral atom
equals the number of protons in its nucleus (i.e. the atomic
number, Z).
 So, hydrogen (Z = 1) has one electron in the lowest energy shell;
carbon (Z = 6) has six electrons in two shells.
Filling the Electron Energy Shells
 Each energy shell can accommodate a maximum of 2n2 electrons,
where n is the shell energy level starting at the lowest energy. So,
the first level (n = 1) can fit 2 electrons; the second level (n = 2)
can fit 8 electrons, and so on.
 Also, electrons must fill a lower energy level before they begin to
fill a higher energy level. (This gets a little complicated as Z
increases  Wave or Quantum Mechanical Model)
 So the electron orbital diagrams for hydrogen and carbon are:
H
C
 Electrons in an outer shell of an atom are called the valence
electrons. It is usually these electrons that are responsible for the
reactive properties of the element.
 How many valence electrons does hydrogen have? Carbon?
Neon?
 Valence electrons take so much focus that we usually draw only
these electrons for an atom – these are called Lewis Diagrams.
(see Figure 1.5 page 13)
 Notice that there is significance to how the electrons are drawn in
these diagrams. The first four are drawn as single electrons, their
negative charges repelling each other. Subsequent electrons begin
to pair up with the previous ones until the entire shell is filled.
 Unpaired valence electrons are most important as these are
generally more important in chemical bond formation.
The Nucleus
 As mentioned earlier, the total number of electrons in a neutral
atom is equal to its atomic number. Since the atom is neutral, this
also corresponds to the number of protons in the atom’s nucleus.
 The nucleus also contains almost all of the atoms mass (see
Rutherford), which is comprised of protons and neutral particles
called neutrons.
 The atomic mass (A) is the sum of the number of protons and
neutrons in the atom’s nucleus, such that an atom can be
symbolically expressed using its chemical symbol (X), atomic
number (Z) and atomic mass (A):
𝐴
𝑍𝑋
1
𝐻.
 So hydrogen is symbolized as 1
 Atoms of the same element can vary in the number of neutrons
contained in the nucleus. These different types of the same
element differ only by their atomic mass, and are called isotopes.
 For example, deuterium is an isotope of hydrogen that contains a
neutron in addition to its proton. The symbol for deuterium is
2
3
𝐻
.
Tritium
1
1𝐻 with 2 neutrons is another isotope of hydrogen.
 Most elements occur as more than one isotope, that is, different
numbers of neutrons in their nuclei. For example, the most
abundant isotope of carbon is carbon-12 (atomic mass = 12)
having 6 protons and 6 neutrons: 12
6𝐶 . Carbon also exists as
isotopes having 7 or 8 neutrons in the nucleus:
 Some isotopes are unstable; they can lose neutrons and decay
into other stable isotopes  nuclear chemistry.
Finding the Average Atomic Mass of an Element
 The atomic mass of an element is given in atomic mass units,
which works out to slightly less than 1 u for every proton or
neutron. For example, the atomic mass of 12
6𝐶 is about 11.99
units (u).
 The reported atomic mass of an element must take into account
the abundance of each naturally occurring isotope, and so is
reported as an average of the atomic masses of each isotope.
 To find this average atomic mass of an element, multiply each
isotopes atomic mass by its abundance, and then add these all
together.
Example – Average Atomic Mass of Copper
 Copper has two main isotopes: copper-63 (mass = 62.93 u;
abundance = 69%) and copper-65 (mass = 64.93 u; abundance =
31%). The average atomic mass of copper is:
http://www.youtube.com/watch?v=NSAgLvKOPLQ
http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html
1.1 The Atom – What have we learned?




Models of the atom
Electronic structure and energies
The nucleus – protons and neutrons
Isotopes, atomic number and atomic mass
Assigned Questions – BLBMW: p. 70 #9 – 11, 13 – 15, 19 – 27, 29, 31, 35