Trends for Structure and Bonding Flashcards
What is ionisation energy?
Ionisation energy is the minimum amount of energy required to remove one electron from each atom in one mole of gaseous atoms. Since this process requires energy, the process is always endothermic.
Ionisation energy trend across a period
INCREASES ACROSS A PERIOD
Going across a period, there is an increase in the nuclear charge and the same energy level is being filled with electrons. The attraction between the nucleus and the outermost electrons is stronger due to the increase in nuclear charge and hence the electrons are closer to the nucleus. This greater attraction towards the nucleus means more energy is needed to remove the outermost electron and hence the ionisation energy is increased.
Electrons further from the nucleus means that they have less stability and therefore are more easily removed.
As you go across a period, the shielding remains constant but the nuclear charge increases as well as the ionisation energy hence it is harder to remove an electron.
Ionisation energy trend down a group
DECREASES DOWN A GROUP
As the distance between the outermost electron and the positive nucleus increases, the attraction between the nucleus and the outermost negative electron will decrease. Also, the outermost electrons are repelled by all the other electrons in the atom besides being attracted by the positive nucleus. So the outer electrons are shielded from the attraction to the nucleus by the inner electrons and attraction from the nucleus is decreased. This means less energy is required to remove the outermost electron so ionisation energy is therefore decreased. i.e. strength of shielding increases down a group.
Special cases that break the trends of normal ionisation energy
Ionisation energies of noble gases are high and this reflects the stability of fully energy levels
Non-metals have high ionisation energies. Metals have low ionisation energies. This reflects how readily metals form cations compared to non-,metals. The lower the ionisation energy the more reactive the metal is.
What is shielding?
Shielding is the extent to which the valence electrons are shielded from the attraction of the positive nucleus by the inner electrons.
It is easier to remove an electron from an orbital that is a long distance from the positive nucleus than one that is closer to the positive nucleus.
What is atomic radii?
The actual size of atoms which is calculates from the average distance between neighbouring atoms.
Atomic radii trend across a period
DECREASES ACROSS A PERIOD
Across a period, electrons are being added to the same energy level at about the same distance from the nucleus. At the same time, protons are being added to the nucleus, that is, the nuclear charge is increasing. Therefore, there is greater attraction between the valence electrons and the nucleus.The increase in nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. Therefore the atomic radius decreases across a period.
The atomic radius decreases because the nuclear charge is changing and shielding remains constant as the electrons being added are only valence electrons not shielding electrons.
Atomic radii trend down a group
INCREASES DOWN GROUP
Down a group, the valence electrons are further from the nucleus since the valence electrons are in higher energy levels (the number of energy levels increases down a group). The increase cannot be overcome by the increased nuclear charge so therefore atoms lower in a group are larger. The inner filled levels of the electrons also tend to shield the valence electrons from the nucleus so the attraction between balance electrons and the nucleus is decreased.
Shielding is more significant than nuclear charge as you go down a group. Shielding increases and electron-electron repulsions also increase as you go down a group. This is because while the electrons are attracted to the positively charged nucleus, they are repelled by the negatively charged electrons in lower energy levels (like charges repel each other). This means that for every additional energy level, there are more and more electrons in lower energy levels that will repel the electrons in the highest energy level of an atom.
Transitional metals and atomic size trend
In transitional metals (found in the middle of the periodic table), moving left to right (across a period), there is a trend of INCREASING ATOMIC RADIUS which levels off and becomes constant. In transitional metals/elements, the number of electrons are increasing but in a particular way. The number of electrons increases going across a period, thus, there is more pull of the electrons towards the nucleus. However, with the d orbital electrons, there is some added electron repulsion e.g. Chromium has a promotion of one of the 4s electrons to half fill the 3d sub level; the electron electron repulsions are less and the atomic size is smaller. The opposite hold true for the latter part of the row.
Ionic radii trend of cation
When an atom loses an electron, the cation formed will have an overall positive charge (extra proton), the ionic radius gets smaller.
The loss of electrons to form the cation results in the loss of a complete energy level. The ion has one less occupied energy level than the atom therefore the outermost electrons of the ion are closer to the nucleus so that the ion is smaller.
Ionic radii trend of anion
When an atom gains an electron, the anion formed will have an overall negative charge (extra electron/s), the ionic radius gets bigger (also greater electron-electron repulsion)
When one or more electrons are added to the outer shell of an atom to form a negative ion, there is an increase in the repulsion between electrons due to the increased number of electrons in the valence shell. This increased repulsive effect of the electrons in the ion results in an increase in ionic radius.
What is electronegativity?
Electronegativity is the tendency an atom has to attract a shared electron pair in a covalent bond. Electronegativity depends on the size of an atom and its nuclear charge.
Electronegativity trend across a period
INCREASES ACROSS A PERIOD
Shielding is constant.
Across a period, there is an increased nuclear charge and attraction of valence electrons towards the nucleus is increased. Therefore attraction between bonding electrons and the nucleus is increased.
Electronegativity trend down a group
DECREASES DOWN A GROUP
Down a group, the valence electrons, therefore the electrons that are involved in the bond formation, are further from the nucleus. There is also shielding from the inner shells of electrons. So the attraction between the bonding electrons and the nucleus is decreased.
Other electronegativity trends
Metal elements have low electronegativity values. The nuclei of these elements do not exert a strong attractive forces for electrons.
Non-metals elements have higher electronegativity values. The nuclei of these elements exert a strong attractive force for bonding electrons (FONCl).
Group 18 elements are not assigned values as they do not form covalent bonds.
