Counting Atoms


Number of protons in the nucleus
Abbreviated Z
◦ May be “Z” from the German “Atomzahl” (General
Chemistry Online: FAQ: Atoms, elements, and ions:
Why is atomic number called "Z"? Why is mass
number called "A"?)


Each element has a unique atomic number
In neutral atom (not ion), Z also equals the
number of electrons




Sum of protons and neutrons in the nucleus
of an atom
Mass of the nucleus
Is NOT the same for all atoms of one element
Abbreviated A



Atoms of the same element with different
mass numbers
Most elements have more than one form
(isotope)
Nuclide:
◦ General term for an isotope

Designated by naming and by isotope
notation
◦ Naming: Helium-4
◦ Isotope notation




How many protons, electrons and neutrons
are there in an atom of chlorine-37?
How many protons, electrons and neutrons
are there in an atom of bromine-80?
How many protons, electrons and neutrons
are there in an atom of carbon-13?
Write the hyphen notation for an atom with
15 electrons and 15 neutrons.