The document discusses atomic number, mass number, and isotopes. It defines atomic number as the number of protons in an atom's nucleus, which determines an element's chemical properties. Mass number is the sum of protons and neutrons. Isotopes are atoms with the same number of protons but different neutrons. Isotopes are identified by their mass number written as chemical symbol followed by mass number. The document also discusses molecular formula, empirical formula, and structural formula as examples of chemical formulas that show the number and arrangement of atoms in a compound. It defines molecular, empirical, and structural formulas and provides an example of a structural formula. Finally, it describes ball-and-stick and space-filling molecular models used to

1. ATOMIC NUMBER
MASS NUMBER
ISOTOPES
Atoms, Molecules and Ions

2. ■ The atomic number of an atom is the number of protons in the nucleus of
an atom.The atomic number determines the chemical properties of an
element and its place in the periodic table.
■ Mass number is the sum of the protons and neutrons in a nucleus.
■ Atomic mass is the mass of an atom of a chemical element expressed in
atomic mass units. It is approximately equivalent to the number of
protons and neutron in the atom. It can be also computed by getting the
average number allowing for the relative abundances of different
isotopes.
■ Isotopes are atoms that have the same number of protons but different
number of neutrons.
– For example – protium, deuterium and tritium. All three of them are
hydrogen. It means that they have an atomic number of 1 and has 1
proton each. However, protium has no neutrons, deuterium has 1
neutron and tritium has 2 neutrons.

3. ■ Isotopes are identified by their mass number.
– Let us have an example. Oxygen – 15 is an isotope of oxygen.This
isotope of oxygen has a mass number of 15. Carbon – 14 is an isotope of
carbon.This isotope of carbon has a mass number of 14.
■ Isotopes are written in this format:
– Where:
■ X – chemical symbol of the element
■ A – mass number
■ Z – atomic number

4. ■ For example, let us take tritium, one of the isotopes of hydrogen.Tritium
has an isotopic symbol of:
■ Following the format this isotope has a mass number of 3 and an atomic
number of 1.
■ Therefore, this isotope has a 1 proton and 2 neutrons

5. MOLECULAR FORMULA,
EMPIRICAL FORMULA
AND STRUCTURAL
FORMULA
Atoms, Molecules and Ions

6. ■ Molecular formula, empirical formula and structural formula are all
examples of chemical formulas. All of them tell you the number of
atoms of each element found in the compound.
■ Molecular formulas are chemical formulas that tell us the actual
number of the different elements in a compound
■ Empirical formula is defined as the simplest ratio of whole numbers
of elements that make up a compound.This type of formula is
derived from experimental data.

7. ■ Molecular formula, empirical formula and structural formula are all
examples of chemical formulas. All of them tell you the number of
atoms of each element found in the compound.
■ Molecular formulas are chemical formulas that tell us the actual
number of the different elements in a compound
■ Empirical formula is defined as the simplest ratio of whole numbers
of elements that make up a compound.This type of formula is
derived from experimental data.

8. ■ Structural formulas show both the actual number of atoms of the
elements in a compound, how the atoms are arranged and which
atoms are bonded to one another.
– This is an example of a structural formula. From here, not only we
can see that it has one carbon atom and 4 hydrogen atoms, we can
also see how the carbon and hydrogen atoms are connected to
each other.

9. ■ Molecular Models are physical models that represent molecules.
They are used to visualize molecules.
■ There are two types of molecular models – the ball-and-stick model
and space-filling models.
– Ball-and-stick model makes use of wooden balls or plastics to
represent the atoms. Sticks or springs are used to represent
chemical bonds

10. – Space filling models are models wherein atoms are represented
by truncated balls held together by snap fasteners.
*The balls used in the ball-and-stick model are of the same size while
the balls in space-filling models are proportional to the size of the
atoms.