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=number of protonsZ = \text{number of protons} and Ne=ZN_e = Z.

  • For a neutral atom, example: Aluminum has 13 protons and 13 electrons; net charge = 0: 13++13−=013^+ + 13^- = 0.

Mass Number

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

  • Symbolically: A=Z+N ,A = Z + N\,, where ZZ = protons, NN = 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−ZN = 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=1Z = 1, A = 1, p = 1, n = 0, e = 1

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

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

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

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

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

Quick Reference Formulas (Learning Tips)

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

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

  • Neutrons: N=A−ZN = A - Z

  • Mass numbers refer to specific isotopes only.