The Atomic Theory, Subatomic Particles, Isotopes and Ions

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.