化学专业外语.ppt

Lesoon 1 Size of Atoms and Ions
Lesoon 1 Size of Atoms and Ions
The size of atoms decreases from left to right across a period in periodic table.
For example, on moving from lithium to beryllium, the number of charges on the nucleus is increased by one, so that all the orbital electrons are pulled in closer to the nucleus.
In a given period, the alkali metal is the largest atom and the halogen the smallest.
When a row of ten transition elements or fourteen inner transition elements occurs in a horizontal period, the contraction in size is even more marked.
On descending a group in the periodic table such as lithium, sodium, potassium, rubidium, caesium, the size of the atoms increases due to the effect of extra shells of electrons being added; this outweighs the effect of increased nuclear charge.
A positive ion is formed by removing one or more electrons from an atom.
Normally the whole of the outer shell of electrons is removed in this way, and since the remaining inner shells do not extend so far in space, the cation is much smaller than the metal atom.
In addition, the ratio of positive charges on the nucleus to the number of orbital electrons is increased.
Thus the effective nuclear charge is increased and the electrons are pulled in.
A positive ion is therefore smaller than the corresponding atom and the more electrons removed (that is, the greater the charge on the ion), the smaller it es.
Atomic radius Na Atomic radius Fe
Ionic radius Na+ Ionic radius Fe2+
Ionic radius Fe3+
When a negative ion is formed, one or more electrons are added to an atom, the effective nuclear charge is reduced and hence the electron cloud expands. Negative ions are bigger than the corresponding atom.
Van der Walls non-bounded radius Cl
Ionic radius Cl-
范德瓦尔斯(1837-1923),荷兰物理学家、化学家、生物学家,1910年获得诺贝尔物理奖,他提出了包括气态和液态的“物态方程”。
氢 Hydrogen 氦 Helium 锂 Lithium铍 Beryllium硼 Boron
碳 Carbon氮 Nitrogen 氧 Oxygen氟 Fluor