Atomic radii - closest 2 atoms can approach - distance between nucleus and outer electrons.
Might predict that atoms get larger as you add electrons - not entirely the case.
Notice, it is periodic!
As the primary energy level increases (going down a group), the radius increases. Remember we said the primary energy level (n) determines the size of an orbital so this makes sense.
Why, however, does the radius decrease within a period (row) ?
1. Recall - electron (-) charge proton (+) charge
2. Let's go back to the Bohr model and ignore the different types of energy sub-levels (s,p,d,f)
Going across a period (row), the same primary energy level is being filled, but the nuclear charge is getting higher. The increased attraction to the nucleus results in them being more tightly held resulting in a smaller atomic radius.
Elements in a given group feel the same effective nuclear charge, but as one goes down a group electrons occupy orbitals further from the nucleus in higher primary energy levels.
All of this is related to chemistry - ionization energies
As the outer electron experiences less attractive force on it, it is easier to lose.
In fact, some lose it so easily they "explode."
Compare alkali metals:
To get them to react you have to give them something to give their electron to - H2O
Li + H2O
Na + H2O (electron further out than with Lithium, less tightly held, more reactive)
K + H2O
Na vs. Mg - higher nuclear charge of magnesium means less reactive.
Ionization of atoms
Atom -> Positive Ion + e-
Ions - atoms that lose or gain an electron and become charged
There is an energy, termed the ionization energy, associated with this process.
Ionization energy - energy required to remove an electron from an atom in the gaseous state.
1st ionization energy - 1st e- removed
2nd ionization energy - 2nd e- removed.
Halogens and noble gases hard to ionize!
Summary of atomic structure, its relation to the periodic table, and the properties of elements:
1. Nucleus consists of positively charge protons and uncharged neutrons. Nucleus very small, high mass density. Atoms are mainly empty space.
2. Electrons (much smaller than protons and neutrons) occupy orbitals of discrete energy and classified by a primary energy level (n) and a sub-level (s,p,d,f).
3. Electrons fill orbitals from the lowest energy up (Aufbau), two electrons per orbital with opposite spins (Pauli exclusion principle), and when given a choice of orbitals of the same energy singly before pairing up.
4. Groups in the periodic table share the same outer (valence) energy level electronic structure structure.
5. Point four implies elements of the same group have similar chemical and physical properties (as empirically observed by Mendeleev for instance). Periodicity!
6. Examples: relating ionization energy (chemical property) to electronic structure, relating atomic radii (physical property) to atomic structure.