• Nb of protons = Z

• Nb of electrons (neutral atom) = Z

• Nb of neutron = A-Z

Interactions between atoms

• 2 types of interactions

$$ \underbrace{\text{🤢 Repulsive } \qquad {\color{cyan}\text{or}} \qquad \text{😍 Attractive }}{\normalsize\text{🧲 Atoms bind together}} \\ \overbrace{\underbrace{{\text{👨‍👩‍👧‍👦 In large numbers}}}{\text{🧊 Form solids}} \quad {\color{cyan}\text{or}} \quad \underbrace{\text{🧑🏼‍🤝‍🧑🏿 In small numbers}}_{\text{🍇 Form molecules}}} $$

Repulsion

• Atoms repel each other when they get too close

Steep dependence

Attraction - Ionic

<aside> <img src="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/97271424-4a68-4e9f-b2eb-e0c2af3a2769/Ions.png" alt="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/97271424-4a68-4e9f-b2eb-e0c2af3a2769/Ions.png" width="40px" /> Ions: Atoms are ionised if they don’t have the same amount of electrons as protons ( $Nb(e^-)\ne Z$ )

</aside>

• Electrostatic potential energy $V\propto \frac{e^2}{r}$

It is a long range force, so to calculate the total binding energy we need to sum for all nearby atoms

$$ V(r)=\frac{e^2}{4\pi\epsilon_0}\sum_i \frac{1}{|r-r_i|} $$

🗞️ Example: Na$^+$Cl$^-$, Melting temp ~ 1100 K which is very high since ionic bonds are strong

Attraction - Van der Waals

• Neutral atoms have fluctuating electric dipole momentum $\vec p$

🗞️ Example: hydrogen

$$ \text{if we have: }\quad {e^-}\overset{\vec d}{\rightarrow}e^+ \quad\text{ inst. dipole moment: } \vec p = e\vec d $$

• It creates a dipolar electric field $|\vec E|\propto 1/r^3$