Atomic Number, Mass Number and Isotopes: Mechanism

≈ 30 min

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to Atomic Number, Mass Number and Isotopes.

Two numbers pin down an atom:

Atomic number ( $Z$ ) = number of protons. It defines which element the atom is — every carbon atom has $Z = 6$ .
Mass number ( $A$ ) = number of protons + neutrons.

So number of neutrons = $A - Z$ . We write an atom as $^{A}_{Z}\text{X}$ , for example $^{23}_{11}\text{Na}$ : 11 protons, $23 - 11 = 12$ neutrons.

Isotopes are atoms of the same element (same $Z$ ) with different numbers of neutrons (different $A$ ). Chlorine has two common isotopes, $^{35}\text{Cl}$ and $^{37}\text{Cl}$ — both have 17 protons, but 18 and 20 neutrons.

Chemistry — Grade 10 — Mechanism: How many neutrons are in an atom of $^{23}_{11}\text{Na}$ ?

Chemistry — Grade 10 — Mechanism: Two atoms both have 17 protons, but one has 18 neutrons and the other 20. They are…

different elements

isotopes of the same element

ions

compounds

Chemistry — Grade 10 — Mechanism: An element has atomic number $Z = 6$ and mass number $A = 14$ . How many neutrons does this atom have?

Name the original topic being extended by this mechanism lesson.

Which statement is the best evidence-led starting point for Atomic Number, Mass Number and Isotopes: Mechanism?

Two numbers pin down an atom: - Atomic number ( $Z$ ) = number of protons. It defines which element the atom is — every carbon atom has $Z = 6$ . - Mass number ( $A$ ) = number of protons + neutrons. So number of neutrons = $A - Z$ . We write an atom as $^{A}_{Z}\text{X}$ , for example $^{23}_{11}\text{Na}$ : 11 protons, $23 - 11 = 12$ neutrons

Pure Substances and Mixtures

Elements and Compounds

A conclusion needs no observations, calculations or stated assumptions.