Ch. 4.4 Atomic Number and Mass Number

Atomic Number

• The atomic number is the number of protons in an atom; all atoms of an element share the same atomic number.

• Examples: H = 1, C = 6, Cu = 29.

• It is a whole number and appears above the symbol in the periodic table.

• In a neutral atom, the number of electrons equals the number of protons: Z = \text{number of protons} and N_e = Z.

• For a neutral atom, example: Aluminum has 13 protons and 13 electrons; net charge = 0: 13^+ + 13^- = 0.

Mass Number

• The mass number represents the total number of particles in the nucleus: protons + neutrons.

• Symbolically: A = Z + N\,, where Z = protons, N = neutrons.

• Mass number is always a whole number.

• The mass of a single atom does not appear in the periodic table.

Neutrons and Isotopes

• Neutrons count: N = A - Z . (Z - protons, N - neutrons)

• Mass numbers are given for specific isotopes only, not for the element as a whole.

Composition of Some Atoms (Table 4.6)

• Hydrogen (H): Z = 1, A = 1, p = 1, n = 0, e = 1

• Nitrogen (N): Z = 7, A = 14, p = 7, n = 7, e = 7

• Oxygen (O): Z = 8, A = 16, p = 8, n = 8, e = 8

• Chlorine (Cl): Z = 17, A = 37, p = 17, n = 20, e = 17

• Iron (Fe): Z = 26, A = 58, p = 26, n = 32, e = 26

• Gold (Au): Z = 79, A = 197, p = 79, n = 118, e = 79

Quick Reference Formulas (Learning Tips)

• Protons = atomic number: Z = \text{number of protons}

• Mass number = protons + neutrons: A = Z + N

• Neutrons: N = A - Z

• Mass numbers refer to specific isotopes only.