Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Proton number
The proton number, Z, is the number of protons in the nucleus. It is also called atomic number. Every atom of one element has the same proton number.
Mass number
The mass number, A, is the total number of protons and neutrons. It is a whole number for one isotope. Neutron number is calculated as mass number minus proton number.
Nuclide symbols
In nuclide notation, mass number is written at the upper left and proton number at the lower left of the element symbol. Ionic charge, when present, appears at the upper right.
Calculating electrons
A neutral atom has electrons equal to protons. A positive ion has lost electrons, while a negative ion has gained electrons. The nucleus is unchanged during ordinary ion formation.
Mass number and relative atomic mass
Mass number belongs to one isotope and is an integer. Relative atomic mass is a weighted mean for the natural mixture of isotopes and may contain a decimal.
Practical or data skill
Complete missing entries in nuclide tables containing A, Z, protons, neutrons, electrons and charge.
Examination tip
Neutrons = A – Z. For ion electrons, adjust for the charge only after finding proton number.
Review questions and suggested answers
Question 1
An atom has A=31 and Z=15. Find neutrons.
Suggested answer
16.
Question 2
How many electrons are in Mg2+?
Suggested answer
10.
Question 3
Where is charge written?
Suggested answer
At the upper right of the symbol.
