Details on atomic number, atomic mass number, actual mass of an atom, and relative atomic mass.

Definition: The total number of protons present in the nucleus of an atom of an element is called its atomic number. It is denoted by the English letter Z.

Key Characteristics:

• In an electrically neutral atom: Atomic Number (Z) = Number of Protons (p) = Number of Electrons (e).
• The atomic number is the fundamental identity of an element; it remains completely unchanged during chemical changes.

Notation Rules & Example:
The atomic number is written as a subscript on the lower-left side of the elemental symbol.

• Example: The atomic number of carbon is 6. It is written as ₆C.
• Electronic Configuration: 1s² 2s² 2p² [often expressed using brackets as C(6) = 2, 4].

Definition: The total sum of the number of protons (Z) and neutrons (n) present in the nucleus of an atom is called its atomic mass number (or nucleon number). It is designated by A.

Notation Rules & Example:
The mass number is written as a superscript on the upper-left side of the elemental symbol.

• Example: Two isotopes of carbon with mass numbers 12 and 14 are represented as ¹²₆C and ¹⁴₆C.

Essential Calculations & Mathematical Examples:

Determination of Neutron Number (n = A − Z):

• Neutron number of ¹²₆C, n = 12 − 6 = 6
• Neutron number of ¹⁴₆C, n = 14 − 6 = 8

Internationally, any chemical species is represented using the following standard notation:

• X = Symbol of the element
• Z = Atomic number (bottom-left)
• A = Mass number (top-left)
• n = Number of atoms in the molecule / Atomicity (bottom-right)
• ±q = Charge of the ion (top-right)

Examples:

• An atom of oxygen: ¹⁶₈O
• A molecule of oxygen: ¹⁶₈O₂ (or O₂)
• An oxide ion of oxygen: ¹⁶₈O²⁻ (or O²⁻)

Because atoms are unimaginably tiny, their masses cannot be measured directly using ordinary laboratory balances or optical microscopes. Their actual masses have been determined through precise physical experiments:

• Actual mass of 1 Hydrogen (¹H) atom: 1.673 × 10⁻²⁴ g (or 1.673 × 10⁻²⁷ kg)
• Actual mass of 1 Carbon (¹²C) atom: 1.9926 × 10⁻²³ g (or 1.9926 × 10⁻²⁶ kg)
• Actual mass of 1 Oxygen (¹⁶O) atom: 2.656 × 10⁻²³ g (or 2.656 × 10⁻²⁶ kg)

Since the actual mass of an atom is exceedingly minute, relative atomic mass is used for convenience of calculations by determining a ratio with respect to a standard reference scale. In 1961, IUPAC unanimously adopted the Carbon-12 scale.

Definition: The comparative number indicating how many times heavier an atom of an element is compared to 112th of the mass of a Carbon-12 (¹²C) isotope is called the relative atomic mass of that element.

Measuring Unit (amu or Dalton):
The value of 112th part of the mass of one ¹²C atom is:

This value is universally termed 1 amu (Atomic Mass Unit) or 1 Da (Dalton).