2. What is an atom?
Atom is the smallest particle of an element that retains its
identity in a chemical reaction
Democritus, a Greek philosopher, believed that atoms were
indivisible and indestructible.
But he didn’t support his ideas experimentally and was not based
on a scientific method.
Dalton, a Chemist and school teacher, transformed Democritus
ideas on atoms into a scientific theory.
Dalton’s atomic theory:
1. All elements are composed of tiny indivisible particles
called atoms.
2. Atoms of the same element are identical.
3. Atoms of different elements can physically mix together
or can chemically combine to form compounds.
4. Chemical reactions occur when atoms are separated,
joined, or rearranged.
4. One important change occurred, is that atoms are now known
to be divisible.
Atoms can be broken into smaller particles, called
subatomic particles.
Subatomic particles are of three kinds, electrons, protons,
and neutrons.
Electrons are negatively charged subatomic particles.
Thomson experiment proved that electrons are negatively
charged.
Textbook, page 104 explains this experiment in detail.
5. Since the atom is neutral, so there are other particles that
neutralizes the electrons.
Scientists thought about the protons that are positively
charged particles, and neutrons with no charge but with a
mass nearly equal to that of a proton.
Rutherford’s Gold-Foil Experiment is explained in detail page: 107
Conclusion:
1. Atom is mostly empty space
2. All the positive charge and the majority of the atom’s mass
are concentrated in a small central region called, nucleus.
Nucleus: is the tiny central core of an atom and is composed
of protons and neutrons.
The Rutherford Atomic Model
The protons and neutrons are located in the nucleus. The
electrons are distributed around the nucleus and occupy
almost all the volume of the atom.
7. Atomic Number
Atoms are composed of protons, neutrons, and electrons.
Protons and neutrons make up the nucleus, and electrons surround the
nucleus.
is the number of protons in the nucleus of an atom.
Elements are different because they contain different
numbers of protons, so the atomic number identifies an
element.
Mass Number
is the total number of protons and neutrons in an atom.
Since the atom of an element is neutral, the number of protons
equal to the number of electrons.
So the number of neutrons in an atom is the difference between
the mass number and atomic number.
Number of Neutrons = Mass Number – Atomic Number
Find the number of protons, electrons, and neutrons for each of the
following atoms?
a. Ne b. C d. Kc. U
8. Isotopes
are atoms that have the same number of protons but
different numbers of neutrons.
Since the number of neutrons is different, the mass number
is also different.
e.g. 1H, 2H, 3H 16O, 17O
Isotopes are chemically alike, since the number of protons
and electrons are identical.
9. Average Atomic Mass
is a weighted average mass of the atoms in a naturally
occurring sample of the element.
e.g. Consider the two stable isotopes of Chlorine,
35Cl with a relative abundance of 75% and 37Cl with a
relative abundance of 25%
Calculate the average atomic mass of Chlorine.
To calculate the atomic mass of an element, multiply the
mass of each isotope by its natural abundance, and then add
the products.
How can you find the exact atomic mass of these isotopes?
10. Average Atomic Mass
is a weighted average mass of the atoms in a naturally
occurring sample of the element.
e.g. Consider the two stable isotopes of Chlorine,
35Cl with a relative abundance of 75% and 37Cl with a
relative abundance of 25%
Calculate the average atomic mass of Chlorine.
To calculate the atomic mass of an element, multiply the
mass of each isotope by its natural abundance, and then add
the products.
How can you find the exact atomic mass of these isotopes?
