The Atomic Theory
1.
Dalton’s Atomic Theory
(The solid Sphere Model)
-
He pictured atom as solid, indestructible spheres with a mass
like a billiard ball.
-
He postulated the Atomic Theory:
a. All matter is composed of
extremely small, indivisible, indestructible particles called atoms.
b. All atoms of each element
are exactly alike but they differ from atoms of other elements. The atoms of
different elements have different mass and other properties.
c. When atoms of different
elements combine to form a compound, the constituent atoms are always present
in the same fixed numerical ratio.
d. A chemical
reaction involves a rearrangement of atoms. No atom is created or destroyed.
2.
Joseph John Thomson (The
Raisin Bun Model)
-
He said that
negatively charged electrons of atoms were embedded in a positively charged
mass similar to the way raisins embedded in a loaf of bread. His model is also
known as the Plum-Pudding Model.
3. Ernest Rutherford (Nuclear Model of an Atom)
-
He discovered the nucleus through his gold foil experiment.
-
Based on the observations made he come up with the following
conclusions:
a. The gold atoms in the foil
must be mostly empty space because most of the α (alpha) particles were able to penetrate the foil.
b. The atom has a positive
central core (which later on he called nucleus which means “little nut” in
Latin), where most of its mass is intense. The nucleus was so dense that the
alpha particles would bounce of it.
c. The negatively charged
electron in the atom circulates around the nucleus. The electrons were so tiny,
and spread out at such great distances, that the alpha particles would pass
right through this area of the atom.
4. Neils Bohr (Planetary Model of an Atom)
-
He suggested that electrons could travel around the nucleus
without radiating energy provided that they remained in certain restricted
orbits. He proposed that an electron could move from one orbit to another by
gaining or losing one or more quanta of energy.
-
He described atoms like a solar system wherein the electrons
revolve around the nucleus.
 |
5.
Neils Bohr and Arnold
Sommerfeld (Bohr – Sommerfeld Atomic Model)
-
Proposed that the orbits if the electrons move in elliptical
orbits rather than circular orbits.
THE SUBATOMIC PARTICLES: PROTONS, NEUTRONS AND ELECTRONS
Particle
|
Location
|
Charge (C)
|
Mass(g)
|
Mass (amu)
|
Proton (p+)
|
Inside the nucleus
|
+
1.602 x 10 -19
|
1.673
x 10-24
|
1.0073
≈1
|
Neutron (n0)
|
Inside the nucleus
|
0
|
1.675
x 10-24
|
1.0083
≈1
|
Electron (e-)
|
Outside the nucleus
|
-
1.602 x 10 -19
|
9.109
x 10-28
|
0.0006≈0
|
Atomic Symbol – a shorthand notation used to represent a certain
element and its atomic properties .
(Z) 56
|
Ba
|
2+
|
(M)141
|
||
Atomic Symbol – a two or three letter representation of the element
derived from the name of the element (Ba)
Charge – this is the number indicating how many electrons can
an element gain or lose (2+)
Mass Number (M) – the sum of the numbers of the
protons and the neutrons present in the nucleus of an atom (141)
Atomic Number (Z) – the number of protons the atom
contains (56).
Number of protons = atomic number
Number of electrons = atomic number
Number of neutrons = mass number –
atomic number
IONS
-
When an atom loses or gains electrons, it acquires a net
electrical charge and is called an ion. An ion that has more electrons than
protons has a positive charge. An ion that has fewer electrons than protons has
a positive charge.
Charge
of the Ion = Number of Protons – Number of Electrons
ISOTOPES
-
these are atoms that have the same number of protons and
electrons but different number of neutrons. Most elements have at least two
isotopes, one of which is much more common than the other.
