The neon in an illuminated sign reacts with nothing; metallic sodium, on the other hand, reacts violently on contact with water. Bohr's model, with electrons orbiting in exact paths like planets, cannot explain this difference: we need the orbital model, which describes electrons using orbitals, regions where they are most likely to be found, organised into main energy levels and sublevels.
From Bohr's model to the orbital model: levels, sublevels and orbitals
When different gases are heated in a discharge tube, each element emits light in very specific, separate coloured lines (line emission spectra) instead of a continuous rainbow: how are the electrons arranged in the atom so that they only absorb or emit such specific packets of energy?
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In Bohr's model, electrons followed exact orbits around the nucleus, like planets around the Sun. Today, the quantum model refers to orbitals. The figure compares the…
In Bohr's model, electrons followed exact orbits around the nucleus, like planets around the Sun. Today, the quantum model refers to orbitals. The figure compares the two models.
Explain in your own words what an orbital is and how it differs from an orbit in Bohr's model.
Sublevels are not filled in the order "$1s$, $2s$, $2p$, $3s$, $3p$, $3d$...". From level $3$ onwards, the $4s$ sublevel has lower energy than the $3d$, so it is occupied first. The Aufbau principle says that electrons fill the lowest-energy sublevel available first. The diagram that shows this order is the Aufbau diagram. Where sublevels contain several orbitals of equal energy (such as the three $p$ orbitals), electrons occupy them singly before pairing up (Hund's rule).
Writing the electron configuration of an atom
The atomic number $Z$ tells us how many electrons a neutral atom contains, but atoms with very different atomic numbers behave chemically in the same way if they share the same outer structure: how do we write down and organise all the electrons of an element in order to predict its bonding?
Move the electron slider until it matches the number of protons: check that the interactive shows "neutral atom". Those $17$ electrons are the ones that have to be shared out among the sublevels following the Aufbau order.
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A space probe analyses the atmosphere of a planet and finds atoms whose electron configuration is $1s^2\ 2s^2\ 2p^4$. a) How many electrons does each of those atoms…
A space probe analyses the atmosphere of a planet and finds atoms whose electron configuration is $1s^2\ 2s^2\ 2p^4$.
a) How many electrons does each of those atoms have?
b) Knowing that in a neutral atom the number of electrons equals the atomic number $Z$, which element is it?
Atoms tend to gain or lose electrons to reach the stable configuration of a noble gas, with a full outermost level (the octet rule). Atoms with few valence electrons usually lose them (Na becomes $\text{Na}^+$). Atoms that are only a few electrons short of a full level usually gain them: chlorine ($3s^2\ 3p^5$) is one electron short, so it forms $\text{Cl}^-$ with $3s^2\ 3p^6$, the configuration of argon.
Reduce the electrons in the interactive from $11$ to $10$: a charge of $+1$ appears. This is how sodium forms the $\text{Na}^+$ ion, losing its single valence electron and ending up with the configuration of a noble gas.
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An $\text{X}^{2+}$ ion has the electron configuration $1s^2\ 2s^2\ 2p^6$. a) Calculate how many electrons the ion has and deduce the atomic number of element X. b) A…
An $\text{X}^{2+}$ ion has the electron configuration $1s^2\ 2s^2\ 2p^6$.
a) Calculate how many electrons the ion has and deduce the atomic number of element X.
b) A $\text{Y}^{-}$ ion has exactly the same configuration: deduce its atomic number as well.
c) Using the periodic table, name elements X and Y.
Bonding is a continuum. The difference in electronegativity (Pauling scale) between two atoms tells you where a bond lies: a large difference gives an ionic bond (NaCl), a small difference gives a polar covalent bond (O–H in water), and no difference gives a pure covalent bond (Cl–Cl).
Substance
Structure
Conductivity
Melting and boiling points
Solubility in water
State at room temperature
NaCl
Lattice of ions
Not as a solid; molten or dissolved it does, because the ions can move
High (strong attraction between ions)
Soluble
Solid
Copper
Metallic lattice
Conducts, because of mobile electrons
High
Insoluble
Solid
Diamond
Covalent lattice
Does not conduct
Very high (strong bonds in all directions)
Insoluble
Solid
Water
Molecules
Poor (pure water)
Low (only forces between molecules are overcome)
Not applicable
Liquid
Not all salts are soluble in water. A molecular substance can also form ions when it dissolves, like HCl. To investigate conductivity, use data or a supervised low-voltage school set-up; never the mains supply.
A dot-and-cross diagram for sodium chloride shows the ionic case: sodium transfers its single valence electron to chlorine.
The diagram links the main ideas:
Back to the beginning: Neon ($Z = 10$, $1s^2\ 2s^2\ 2p^6$) has its valence level full with $8$ electrons and does not need to bond; sodium ($Z = 11$, $[\text{Ne}]\ 3s^1$), on the other hand, loses its single valence electron very easily to reach that same stability, which causes its violent reactivity with water.
Summary
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