Atomic Radius Rules and Examples

Atomic Radius Measures how large an atom is. Most atoms are in the picometer ( meters) range (really, really small) Very difficult to measure single atoms Groups of atoms (like a block of iron) are bonded to each other…that bonding will change the size

Three basic rules To get to the point… – Levels Add – Protons Pull – Electrons repel You will see how his works now…

Levels Add Within any one level, the electron orbitals are more or less the same size The bigger the level, the bigger the orbital So 1s< 2s < 3s And 2s is just bit smaller than 2p 3s is a bit smaller than 3p and 3d 4p 3d 4s 3p 3s 2p 2s 1s smallest smaller larger

Protons Pull Consider periodic 2 (Li to Ne) Within a level, increasing atomic number means more protons Protons pulls electron in closer to the nucleus This makes the atom smaller There are more electrons also, but they are in the same level and so do not interfere with each other too much. Li-3 protons Be-4 prrtons B-5 protonsC-6 protonsN-7 protonsO-8 protonsF-9 protonsNe-10 protons 90 picometers125 picometers 81 picometers 85 picometers 74 picometers77 picometers 72 picometers 71 picometers

The Pattern Continues The next period, Period 3 starts at a higher level But as the atomic number goes up from Na to Ar, the radius becomes Smaller Then with a new period (4), it gets larger again

The trend in the periodic table

Electrons Repel Atom Ion O vs O pm 140 pm Br vs Br pm 196 pm As electrons are added to an atom (and becoming an ion) the element gets much bigger The extra electrons are repelled by the original electrons, pushing them away. That makes the negative ion bigger Compare:

Missing Electrons Atom + Ion 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 Na Na pm 97 pm [Kr]5s 2 [Kr] SrSr pm 113 pm If a positive ion is formed, the element becomes smaller Often an orbital will become empty. It was assume the electron configuration of a noble gas. Compare:

Final Exercise Compare the size…. Ca 2+ S 2- Ar Answer…. Smallest Largest Medium (all have same number of electrons, Calcium has the most protons Compare the following three elements Which is larger? Hint: give the electron configuration for each, and then look p the number of protons.

So What? Properties influenced by atomic size: – Ionization Energy – Electron Affinity – Electronegativity – Melting and Boiling Points (partly) – Density (partly) – Bonding Type (partly) – Polarity (partly) What difference, for chemistry, does the size make? Plenty Remember it is the outermost electrons that react with other atoms The way they react, or react at all, is determined by how strongly they are held to the nucleus. And how strongly they are held is determined more than anything else, by how close the outermost electrons are to the nucleus (or their size)

Example: Ionization Energy Ionization Energy is the energy needed to remove an electron Energy + Atom  Ion 1- + electron The bigger the atom  the further away the outermost electron  The easier to ionize  The LESS energy needed + e Energy in Atom 0 Ion e More Energy in Atom 0 Ion 1+ But with a smaller atom…. Energy is added, an ion is created

Ionization Energies for the first electron of each element Which atoms have the highest ionization energies? Which the lowest? Does it get larger or smaller as you go down the periodic table? How does that compare with Atomic Radius?