Structure of atoms and molecules

Chemical bondsProperties of compounds

Eva Samcova

Periodic table

• Each horizontal row is called a period• Each vertical group is called a group• The vertical groups have both numbers and letters (IA).

These A groups and 0 are called representative elements. The B series plus group VIII are called the transition elements . The two rows of elements placed outside the table are the inner elements .

• IA – alkali metals, IIA – alkaline earth metals, VII A are halogens (other groups are named simply after the first member ,then the boron family, the carbon family, the nitrogen family, the oxygen family).

• Metals. Nonmetals. Metalloids.

In nature there are 81 stable elements• 15 of these are present in all living things• A further 8 – 10 are only found in particular

organisms• Macroelements : almost 99% of the atoms in

the bodies of animals can be accounted for by just four elements H, O, C a N ; S a P. H2 + O = water (makes up 60 - 70% of cell mass)

• Biological important ions (0,5 %): Na+, K+, Mg2+,

Ca2+, Cl-

• Microelements (trace elements): Fe, Zn, Cu, Co, Mn ; non-metals I and Se

Atom• Atomic number, mass number

• Atomic nucleus. Isotops. Most of natural elements are formed from several isotops. Atomic weight.

• The electrons in a atom are confined to particular energy shells outside the nucleus

• 3 quantum numbers :

• principal energy level n = 1, 2, 3 …………..• subshells (atomic orbitals) l = 0, 1, 2,….(n-1)

• magnetic quantum number m = -l, 0, +l

Aufbau principle

•pro vazbu

Electron configuration

• Pauli Exclusion Principle . An orbital can hold as many as two electrons, but only if they have opposite spins.

• Orbitals have unique shapes.• Hundś Rule . Electrons at the same subshell

spread out among the subshellś orbitals as much as possible.

Electronegativity

• Ionization energy is the energy needed to remove an electron from each atom and so to form cation (the higher ionization energy, the more stable electron configuration of atom).Formation of cations .

• Electron affinity is the energy needed for admission of a electron by free atom in basic state and so to form anion (energy is usually released). Formation of anions .

• Electronegativity – average value of ionization energy and electron affinity (number without units)

The particles,with at least one unpair electron. They are formed by homolytic splitting and they are very reactive. Radical structures of atoms H, N, O and Cl cause their limited occurence in atomic state.Homolytic splitting :

X – Y → X• + Y•

Radicals Principal and excited state• Principal state is the state with the lowest

energy level.• Excited state result from absorption of

energy. (Excitation 10-8-10-9s; by radiation the atom is coming back to the principal state)

• Cl – principal state 3s2, 3p5. excited state 3s2,3p4, 3d1(nebo 3d2, 3d3)

• Excited states are important for formation of bonds (the increase of unpaired electrons eeded for formation of covalent bonds).

The chemist is interesting in ?

Full occupied layers (shells) are very stable in energy – inert.

Electrons of not completely occupied valence shells determine chemical reactivity and other physical properties of elements.

What decides about kind of bond ?

• Ionization energy• Electron affinity• Electronegativity of elements

• Tendency of atoms to attract electrons is expressed in „Activity series “- list of elements (or other substances) in the order of the ease with which they release electrons under standard conditions and become oxidized. Electrochemical series

Elements● Representative elements : they use for bonds

s, p orbitals of valence shells, ions are colourless and diamagnetic; they have relatively stable oxidation states, S2+ 4+

● Transition elements : position of electrons to the last but one sphere (shell) d-orbitals, they use for the bond valence and bellow valence shells, they have metal properties, they are colour, paramagnetic ions with variable oxidation states, Fe 2+,3+,Cr 2+,3+ 6+

● Inner transition elements : f orbital (n-2), colour, paramagnetic ions

Chemical bonds• When electron density becomes sufficiently concentrated

between two atomic nuclei there is a chemical bond –covalent bond- between them. Bond is formed between atoms with equal or very near electronegativity (difference by 0,4). Examples O2, N2, H2, bond electron pair is just in the middle of atoms

• Polar covalent bond : difference of electronegativities of atoms from 0,4 to 1,7. Electron pair is shifted in the direction of more electronegative atom. Degree of polarity – Dipole moment µ = Q.l, where Q is partial charge on the two atoms, and l is the interatomic distance. Resulting dipole moment is the vectorial sum of dipole moments of individual bonds. Symmetric molecules have zero dipole moment.

Ionic bond

• Electron transfers between atoms produce ions.

• Great difference of electronegativies (the relative ablity of an element´s atoms to draw away electron density of a covalent bond is called the electronegativity) ≥1,7. Ionic bond has electrostatic character –attraction of oppositely charged particles, e.g. lattice of NaCl – great bond power→high melting point 800°C).

Hybridization of atomic orbitals effects shape of molecules

• Spatial arrangement is clear in case of simple moleculesl H2,Cl2,HCl

• More complex molecules form hybrid orbitals different in energy, shape and direction. Number of hybrid orbitals determines valence angles inside molecule and hereby interact shapes of molecules. sp (linear), sp2 (triangle, angles 120°), sp 3 (tetrahedron, angles 109,5°)

• Delocalized bonds : unsaturated hydrocarbons with conjugated double bonds or in case of aromatic hydrocarbons, π electrons are delocalized among more atoms.

Bonds σ and π

• Bond σ arises by maximal superimposing of atomic orbitals on line; practically all simple bonds. Bond strength : s-s<s-p<hybridní orbitaly

• Bond π is occuring in case of multiple bonds. This type of bonds arises more often by side overlapping of nonhybrid orbitals π. It contains more energy and this is reason for smaller stability.

Noncovalent bonds (weak interactions)• Noncovalent bonds arise between particular parts of one

molecule or among particular part of different molecules.• Spatial arrangement• Electrostatic attraction forces cause that molecules (or

atoms) can form more complicated supramolecular structures.

• Van der Waals interaction:• Interaction awakened by London disperse forces• Interaction dipole – dipole• Interaction dipole – induced dipole• Interaction dipole-ion