Periodic Trends Density Atomic and Ionic Radii

Chapter 10Periodic Trends

DensityAtomic and Ionic Radii

Density

Density• Iridium (Z = 77) has the highest density.• Why not Meitnerium (Z = 109?)• We can predict that an element is more

dense if it is closer to Iridium.

Density

Effective Nuclear Charge (Zeff)

• The effective nuclear charge (Zeff) of an atom is basically how well it is able to hold on to its most loosely held electron.

Atomic radius (radii)

• The atomic radius is essentially the size of an atom.

• The largest atom is Francium (Z = 87).

Atomic Radius

Xylophone monkey

Rank the atoms from smallest to largest

Na, K, O, N

O < N < Na < K

Atomic Radius and Effective Nuclear Charge

• The atomic radius decreases as effective nuclear charge increases.

Effective Nuclear Charge (Zeff)

• The effective nuclear charge of an atom is primarily determined by:

1. The nuclear charge 2. The shielding effect

The Nuclear Charge (Z)

• Example: Carbon vs. Nitrogen• Which atom would you predict to be

smaller?

The Nuclear Charge (Z)Carbon Nitrogen

Both atoms are in the 2nd row (period) of the periodic table. What does this tell us?

The Nuclear Charge (Z)Carbon Nitrogen

The greater the number of protons in the nucleus the greater the effective nuclear charge.

Conclusion: Nitrogen is a smaller atom than carbon because nitrogen has 7 protons to pull in its two levels of electrons whereas

carbon only has 6 protons holding it’s two levels of electrons.

Carbon Nitrogen

For atoms with the same number of energy levels the greater the number of protons in the nucleus the greater the effective nuclear charge.

The Nuclear Charge (Z)

• The size of atoms in the same period (row) is determined by the nuclear charge (number of protons in the nucleus).

Atomic Radius

Xylophone monkey

Nuclear charge explains why atoms get smaller across a period

Shielding Effect.

• The shielding effect is when electrons between the nucleus and the outermost electrons in an atom shield or lessen the hold of the nucleus on the outermost electrons.

Shielding Effect.Why is He smaller than Ne?

HeNe

Conclusion: Neon is larger than helium because it has an increased shielding effect from having two energy levels whereas helium only has one energy level.

HeNe

Atomic Radius

Xylophone monkey

Shielding Effect explains why atoms get larger down a group

Chemical Reactivity

Chemical Reactivity

Chemical Reactivity• Metals tend to lose electrons when reacting.

– Large metal atoms are more reactive.• Nonmetals tend to gain electrons when

reacting.– Small nonmetal atoms are more reactive.

Chemical Reactivity • Metals

increase in reactivity left and down.

• Nonmetals become more reactive up and to the right.

• Most reactive metal is?

• Most reactive nonmetal is?

Fr

F

Ionic Radius

• Ionic Radius is the size of an ion.

Size Change in Ion Formation

Rules for Ionic Radius

• Anions (negative ions) are “always” larger than cations (positive ions).

• Ionic Radius goes by the same rules as atomic radius (ions get larger as we move down and to the left).

• However it is necessary to treat anions and cations separately. (Anions are bigger).

Rank the ions from smallest to largest

K+, N3-, Na+, O2-

Na+ < K + < O2- < N3-

Homework

• Worksheet: Density, Atomic and Ionic Radii