Atomic number and mass number, isotopes, relative atomic mass from abundances, ions, and electrons in shells.
What you will learn
- Work out protons, neutrons and electrons from a symbol
- Calculate a relative atomic mass from isotope abundances
- Write the electronic structure of the first 20 elements
Two numbers on every symbol
The atomic number (bottom) is the number of protons.
The mass number (top) is protons + neutrons.
- Sodium-23: atomic number 11, mass number 23
- protons 11, electrons 11, neutrons 23 − 11 = 12
Isotopes are atoms of the same element with different numbers of neutrons: chlorine-35 and chlorine-37. Ions have gained or lost electrons: Na⁺ has 10 electrons, Cl⁻ has 18.
A positive ion has lost electrons, a negative ion has gained them, and the size of the charge is the number of electrons moved. So electrons in the ion = protons − charge for a positive ion, and protons + charge for a negative one: Be²⁺ has 4 − 2 = 2 electrons, P³⁻ has 15 + 3 = 18.
The rest of this lesson and its 13 practice questions are locked.
In this lesson
- Two numbers on every symbol
- Relative atomic mass
- Electrons in shells
- Try it: From symbol to particles
13 practice questions, marked as you go; about 25 minutes in all.
Key terms
Atomic number, Mass number, Isotopes, Ions, Electronic structure
In this unit: Atomic structure and the periodic table
Atoms, formulae and equations, separating mixtures, the model of the atom, isotopes, electrons in shells and the groups of the periodic table.