Download Atom, Molecule, and Ions

ATOM, MOLECULE, AND IONS
by Hak Sok Chea
Academic year: 2008 ~ 2009
Atomic theory
‰
Democritus concept: Greek philosopher, 5th
Century BC
¾ All matter is composed of very small, indivisible particles, which he called
Atomos.
‰
Dalton theory: English Teacher, 1808
¾ Elements are composed of extremely small particles, called atoms.
¾ All atoms of given element are identical, having the same size,
mass, and chemical properties. The atoms of one element are
different from the atoms of all other elements.
¾ Compounds are composed of atoms of more than one element. In
any compound, the ratio of the numbers of atoms of any two of
the elements present is either an integer or a simple fraction.
¾ A chemical reaction involves only the separation, combination, or
rearrangement of atom; it does not result in their creation or
destruction
Schematic representative for the first
two hypothesis
Atom of
element X
Atom of
element Y
Compound of
element X and Y
Atomic structure
ATOM
ATOM
Electron
Electron
Nucleus
Nucleus
Proton
Proton
Neutron
Neutron
- Atom: define as the basic unit of an element that can enter into
chemical combination.
- Electron: is a fundamental subatomic particle that carries a
negative electric charge. (Thomson, 1898-1903)
Atomic structure
The structure of the Atom
Electron configuration in atoms
The electron configuration is the arrangement of
electrons in an atom, molecule, or other physical structure
(e.g., a crystal).
The state of an electron in an atom is given by four quantum
numbers:
¾ Principle quantum number (n)
¾ Azimuthal quantum number (l)
¾ Magnetic quantum number (m)
¾ Spin quantum number (s)
The structure of the Atom
Principle quantum number (n)
The principal quantum number (n) describes the size
of the orbital.
Ex: Orbitals for which n = 2 are larger than those for
which n = 1,.
Energy must therefore be absorbed to excite an electron
from an orbital in which the electron is close to the
nucleus (n = 1) into an orbital in which it is further from
the nucleus (n = 2).
The structure of the Atom
Principle quantum number (n)
n definite the electronic shell:
- n =1 correspond to K shell,
- n =2 correspond to L shell,
- n =3 correspond to M shell,
- n =4 correspond to N shell,
The maximum electron in the shell correspond to 2n2
(where n is the number of the energy level)
Ex: n =1, the electrons in the K shell are maximum 4;
n = 2, the electrons in L shell are maximum 2 x 22 =8
The principal quantum number (n) cannot be zero. The allowed
values of n are therefore 1, 2, 3, 4, and so on.
The structure of the Atom
Azimuthal quantum number (l)
The angular quantum number (l) describes the shape
of the orbital.
Orbitals have shapes that are best described as spherical (l
= 0), polar (l = 1), or cloverleaf (l = 2). They can even
take on more complex shapes as the value of the
angular quantum number becomes larger.
The angular quantum number (l) can be any integer
between 0 and n - 1. If n = 3, for example, l can be
either 0, 1, or 2.
The structure of the Atom
l definite the electronic sub-shell;
l =0 sub-shell s (1 orbital, maximum 2 electrons),
l =1 sub-shell p (3 orbitals, maximum 6 electrons),
l =2 sub-shell d (5 orbitals, maximum 10 electrons),
l =3 sub-shell f (7 orbitals, maximum 14 electrons),
.....
Although there is no pattern in the first four letters (s, p,
d, f), the letters progress alphabetically from that point
(g, h, and so on).
The structure of the Atom
Magnetic quantum number (m)
The magnetic quantum number (m), to describe the
orientation in space of a particular orbital. (It is called the
magnetic quantum number because the effect of different
orientations of orbitals was first observed in the presence
of a magnetic field.)
The magnetic quantum number (m) can be any integer
between -l and +l. If l = 2, m can be either -2, -1, 0, +1, or
+2.
The three quantum numbers (n, l, and m) that describe an
orbital. And an orbital contain only two electrons.
The structure of the Atom
To distinguish between the two electrons in an orbital, we
need a fourth quantum number. This is called the spin
quantum number (s) because electrons behave as if they
were spinning in either a clockwise or counterclockwise
fashion.
One of the electrons in an orbital is arbitrarily assigned an
s quantum number of +1/2, the other is assigned an s
quantum number of -1/2. The spins of the electrons have to be
opposed.
Thus, it takes three quantum numbers (n, l, m) to define
an orbital but four quantum numbers (n, l, m, s) to
identify one of the electrons that can occupy the orbital.
Mass and charge of subatomic particles
Proton: The positively charge particles in the nucleus.
Neutron: electrically neutral particles having a mass
slightly greater than that of proton.
Particles
mass, g
mass, u
Charge
Symbol
Electron
9.1093897 × 10–28
5.48579903 × 10–4
1–
–10e
Proton
1.6726231 × 10–24
1.007276470
1+
10H+, 10p
Neutron
1.6749286 × 10–24
1.008664904
0
10n
Mass relationship of Atoms
Atomic number, Mass Number, and Isotopes
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutron
AX
A: mass number, Z: atomic number
Z
X: symbol of an atom of element
- The atomic number (Z): is the number of protons in nucleus of each
atom of an element.
- In neutral atom the number of proton = the number of electron.
- The mass number (A): is the total number of neutrons and
protons present in the nucleus of an atom of an element.
Mass relationship of Atoms
Isotope: Atoms that have the same atomic number (same
number of proton) but different mass number (different
number of neutron).
Ex: The isotope of the hydrogen:
1 H
2H
3H
1
1
1
hydrogen
deuterium tritium
Two common isotopes of uranium:
235 U
238 U
92
92
The chemical properties of an element are determined by the protons
and electrons in its atoms; neutrons do not take part in in chemical
changes under normal condition.
Mass relationship of Atoms
Hydrogen Isotope
Atomic Mass
- The mass of an atom is related to the number of
electrons, protons, and neutrons in the atom.
- The first information of atomic mass came from the work
of Dalton, Gay-Lussac, Lavoisier, Avogadro, and
Berzelius.
- In the modern system of atomic mass (1961), is based
on 12C (carbon-12) as the standard.
- In this system 12C has six protons and six neutrons that
is assigned a mass of exactly 12 atomic mass units (amu),
and the mass of all other atoms are given relative to this
standard.
Atomic Mass
- This carbon-12 atom serve as standard,
mass of one carbon-12 atom
so 1 atomic mass unit =
12
Ex: Experiment has shown that, on average, a hydrogen
atom is only 8.4 percent as massive as the standard
carbon-12 atom, then the atomic mass (that is, the mass
of atom in atomic mass unit) of hydrogen must be
0,084 x 12 = 1.008 amu.
Average atomic mass
In the natural, the occurring elements have more than one
isotope, so the atomic mass of element, we must
generally settle for the average mass
Ex: The natural abundances of carbon-12 and carbon-13
are 98.89 % and 1.11%, respectively. And the atomic
mass of C-13 is 13.00335. The average atomic mass of
carbon can be calculated as follows:
average atomic mass of natural carbon= (0.9889 x12
amu) + (0.0111 x 13.00335 amu) = 12.0
Molar mass of an element and
Avogadro’s Number
• Mole: is the amount of substance that contains as many
elementary entities (atoms, molecules, or other
particles) (Unit defined by SI).
1 mole = 6.022045 x 1023 particles ( called Avogadro’s
number.
• Molar mass is the mass of one mole of a chemical
element or chemical compound.
Ex: 1 mol of H contains 6.022 x 1023 H atoms
1 mole of C-12 atoms has exact mass 12 grams (called
molar mass) and contains 6.022 x 1023 atoms.
Molecules: Atoms in combination
A molecule is an aggregate of at least two atoms in
definite arrangement held together by chemical
forces.
Ex: O2, O3, Cl2, F2, N2, CO, NH3... Molecular Formula
simple molecule
(diatomic molecule)
Molecule:
polyatomic
molecule
Molecules: Atoms in combination
Molecule Hydrogen is diatomic molecule
Molecules: Atoms in combination
Molecules: Atoms in combination
Molecular Mass
The molecular mass is the sum of the atomic masses (in amu) in the
molecule.
Ex: the molecular mass of H2O is
2 atomic mass of H + atomic mass of O
2 (1.008 amu) + 16.00 amu = 18.02 amu
The molar mass of a compound is the mass (in gram or in kilogram)
of 1 mole of the compound.
Notice: Molar mass of a compound (in gram) is numerically equal
to its molecular mass.
Ex: Molar mass of 1 mole of H2O is 18.02 g and
1 mole of H2O (weight 18.02 g) contains 6.022 x 1023 molecules
Molecules: Atoms in combination
EX: Calculate the molecular masses of the following compounds:
- sulfur dioxide (SO2)
- ascorbic acid or vitamin C (C6H8O6)
Answer:
Ex: Methane (CH4) is the principle component of natural gas. How
many moles of CH4 are present in 6.07 g of CH4?
Answer:
Ex: How many Hydrogen atoms are present in 25.6 g of sucrose or
table sugar (C12H22O11)? The molar mass of sucrose is 342.3 g.
Answer:...
Ions and Ionic compound
The octet rule: atoms gain, lose or share valence shell electrons
(valence electrons) in order to end up with eight valence
electrons.
(an octet “8 valence electrons” is a stable electron arrangement).
An ion is an atom or group of bonded atoms which have
lost or gained one or more electrons, making them
negatively or positively charged.
To reach an octet, metal lose electrons and nonmetal gain
electrons.
that bears a net positive charge is called a cation
Ion
whose net charge is negative is called an anion.
Ions and Ionic compound
An atom can lose or gain more than one electron.
Ex: Na can lose one electron to become the cation (Na+)
Na
Na+ + eCl atom can gain an electron to become an anion (Cl-)
Cl + eCl-
Monoatomic ion
Halogen group have seven valence electrons, and require just one
more to reach octet.
Oxygen and other group VIA (S, Se) nonmetal atoms always form
monoatomic anions with a charge of 2- (each has six valence
electrons when neutral)
Nonmetal atoms gain electrons to reach an octet
Ions and Ionic compound
Electronegative of an element: is a tendency of the
element to attire the electrons (or the ability of an atom in
a molecule to attract shared electron to itself ).
The elements the most electro negativity are: the elements
have a tendency to captor a or the electrons to form the
cation. Ex: Halogen, oxygen...
Electropositive: the elements less electro negativity to
form the cation is called electro positive.
The metal is strongly electropositive, which mean it has a
strong tendency to give up electrons.
Periodic table
Ions and Ionic compound
In contrast, metal atoms lose electrons to form
cation.
Alkaline metal atoms form cation with a 1+ charge.
Alkaline earth metal atoms (two valence electrons) and
group IIIA metal atoms (three valence electrons) form
cation with 2+, 3+ charges.
Ex: Mg lose two valence electrons in n =3 energy to form Mg2+
Al lose three valence electrons in n =3 energy to form Al3+
O gain two valence electrons to form O2P gain 3 valence electrons to form P3-
Ions and Ionic compound
Polyatomic and molecular ions are often formed by the
combination of elemental ions such as H+ with neutral molecules or
by the loss of such elemental ions from neutral molecules.
Hydrogen Carbonate HCO3−
Ammonium ion NH4+,
Hydronium ion H3O+
Ions and Ionic compound
- Many ions are polyatomic ions, which mean they are
built from two or more atoms.
- When the electrons are removed or added to a group of
atoms is called polyatomic ions.
- The charge on polyatomic ions is due to an imbalance
in the total number of protons and electrons that are
present.
Ex. NH4+ has 11 protons, only 10 electrons
CO32- has 30 protons and 32 electrons.
Ions and Ionic compound
Polyatomic ion also form ionic compound.
Ex: Food preservative Na2SO3 (sodium sulfite) is the combination
of Na+ and SO32-.
Table salt (sodium chloride)
NaCl is an ionic compound
contains Na+ and Cl-
Concept map