Atomic Number and Mass Number

 

Atomic Number (Z) or Proton Number

Definition

Atoms of each element contain a characteristic number of protons. In fact, the number of protons determines what atom we are looking at (e.g., all atoms with six protons are carbon atoms); the number of protons in an atom is called the atomic number. In contrast, the number of neutrons for a given element can vary to produce isotopes. The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms). For instance, iron, Fe, can exist in its neutral state, or in the +2 and +3 ionic states.

“The number of protons present in the nucleus of an atom is called Atomic Number (denoted as Z)”. For this reason, it's sometimes called the proton number.

OR

“The number of electrons revolving in the orbits of neutral atom is called Atomic Number (as neutral atoms of an element contain an equal number of protons and electrons).

 Atomic Number (Z)  = Number of Protons (P) = Number of electrons (e)

All the atoms of a particular element have the same number of protons, and hence the same atomic number but atoms of different elements have different atomic numbers. For example, all carbon atoms have the atomic number of 6, whereas all atoms of Oxygen have 8 protons in their nucleus.

 

Representation

Atomic number is written as SUBSCRIPT on the LEFT HAND SIDE of the chemical symbol of element. e.g. ₃Li, ₆C, ₇N etc.

 

Origin of Symbol Z

“Atomic number" in German is "Atomzahl", so the Z symbol for atomic number probably comes from "Zahl" (meaning number).

The letter Z is one of the signs for the highest god in Greek mythology, Zeus.

In modern physics Z represents the greatest energy, nuclear power, in its potential form, nuclear charge."

 

Example

1. Atomic number of hydrogen is 1 because its nucleus contains 1 proton.

2.  Atomic number of chlorine is 17 owing to the presence of 17 protons.

Range of Atomic Number

Because protons are units of matter, atomic numbers are always whole numbers. At present, they range from 1 (for hydrogen) to 118 (the number of the heaviest known element). 

Atomic number and mass number are always whole numbers because they are obtained by counting whole objects (protons, neutrons, and electrons). 

Importance

1.    It identifies the element i.e. it distinguishes one element from another. The proton number is unique to each element so no two elements have the same number of protons. Electrons come and go during chemical processes but the proton number doesn’t change.

2.   The modern periodic table is organized according to increasing atomic number. Thus it determines the position of the element on the Periodic Table.

3.   It is a key factor in determining the properties of an element. (Note, however, the number of valence electrons determines chemical bonding behaviour).

          Atomic Mass Number (A) or                Nucleon Number

Definition

The sum of protons and neutrons in the nucleus of an atom is called Mass Number or Nucleon Number denoted as “A”.

OR

the total number of "nucleons" (Protons and Neutrons) in the nucleus of an atom is called mass number. (protons and neutrons are collectively called nucleons).

e.g. Mass number of Na is 23 because its nucleus contains 11 protons and 12 neutrons.  

The nucleon number minus the proton number gives you the number of neutrons of an atom.

Representation

Mass number is written as superscript on the left hand side of the chemical symbol of element. e.g. ¹²C, ¹⁴N

Notation of Atom

To write the notation of an atom, we need to know the symbol of the element, the atomic number and the mass number. The mass number of the atom goes above the symbol (superscript) and the atomic number is written as a subscript below they symbol.

Calculating PEN (Protons-Electrons-Neutrons) Numbers

1. The atomic number is equal to the number of protons in an atom.

2.  Since atoms are neutral, then it is also the same as the number of electrons.

3.  The mass number is the number of protons plus neutrons.

4. The number of neutrons can thus be calculated by subtracting the atomic number from the mass number.

Example

Beryllium for example has an atomic mass of 4, therefore it has 4 protons and 4 electrons.

The mass number of beryllium is 9, so it has 9 – 4 = 5 neutrons.

The PEN numbers for beryllium are thus:

p = 4

e = 4

n = 9 – 4 = 5