Key idea
Understand chemistry, step by step
Once these seven steps are comfortable, continue with orbitals, ionization energy, electronegativity, molecular shape and stoichiometry.
Mendeleev · Beginner pathway
Understand chemistry, step by step
A progressive pathway: atom, electrons, periodic table, ions and then bonding. Each step builds on the previous one with an example and an understanding check.
How to use this pathway
Do not try to memorize everything. At each step, make sure you can restate the idea, solve the guided example and recognize the common mistake before moving on.
1. Understand → 2. Test → 3. Connect to the periodic table
01Inside the atom: nucleus, electrons and scale02Protons, neutrons and electrons: who does what?03Electron shells: a first map04Electron configuration: the language of electrons05Why the periodic table has this shape06Valence electrons and ions: understanding charge07Why atoms bond
Mendeleev · Challenge
End-of-path challenge
Connect the ideas instead of memorizing definitions.
1. A neutral atom has 17 protons. Identify the element and give its electron count.
Show the reasoned answer
It is chlorine: Z = 17. A neutral atom has equal proton and electron counts, so it has 17 electrons.
2. The atom gains one electron. What changes?
Show the reasoned answer
Proton count remains 17; electron count becomes 18. The species is now Cl⁻.
3. Why do chlorine and sodium behave differently despite being relatively close in the periodic table?
Show the reasoned answer
Their outer electron configurations differ strongly: sodium tends to lose its outer electron, while chlorine tends to complete its valence shell.
Key idea
Understand chemistry, step by step
Guided example
Understand chemistry, step by step
Once these seven steps are comfortable, continue with orbitals, ionization energy, electronegativity, molecular shape and stoichiometry.
Put it into practice
Understand chemistry, step by step
Choose a concrete case linked to Understand chemistry, step by step. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.
Check your answer
A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.