Periodic Table Trends

Question 1

The effective nuclear charge on such an electron is estimated by the following equation:

Answer 1

Zeff = Z – S
Z = is the number of protons in the nucleus (atomic no.)
S = Screening constant – represents the portion of the nuclear charge that is screened from the above electron, by the other electrons in the atom.
= The number of non-valence electrons (i.e. average number of electrons between the nucleus and the e- in question)

Question 2

Zeff experienced by an electron depends on: (2)

Answer 2

1) The number of core electrons.

2) Its distance from the nucleus.

Question 3

• As the average number of screening electrons (S) ______, the effective nuclear charge (Zeff) decreases.
• As the distance from the nucleus _____, S increases and Zeff decreases.

Answer 3

increases

increases

Question 4

PERIODIC TRENDS FOR Zeff : (2)

Answer 4

• As we move across a period, Zeff increases.

* As we move down a group, Zeff increases slightly.

Question 5

How is a valence electron shielded? (2)

Answer 5

• Strongly shielded by electrons in inner shells, which are closer to the nucleus.
• Less strongly shielded by other electrons in the same shell
s > p > d > f

Question 6

Atomic size varies consistently through the periodic table: (2)

Answer 6

• Moving down a group, the atoms become larger.

* Moving across a period, atoms become smaller.

Question 7

There are two factors at work that influence atomic radii are:

Answer 7

• principal quantum number, n, and

* the effective nuclear charge, Zeff

Question 8

As the principal quantum number __ increases (we move down a group), the distance of the outermost
electron from the nucleus becomes larger.

Answer 8

—-> the atomic radius increases

Question 9

As we move across the periodic table, the number of core electrons remains constant but the nuclear
charge increases. There is an increased attraction between the nucleus and the outermost electrons. Therefore…

Answer 9

—-> the atomic radius decreases

Question 10

Ionic size (ionic radius) is the distance between ions in an ionic compound.

Ionic radius size depends on: (3)

Answer 10

• nuclear charge
• number of electrons
• orbitals that contain the valence electrons

Question 11

Cations are smaller than the parent atom because: (2)

Answer 11

(i) the electron/s is removed from the most spatially extended occupied orbital/s in the atom.
(ii) there is a decrease in electron-electron repulsion as an electron is lost from the atom.

Question 12

Anions are larger than the parent atom because:

Answer 12

When electrons are added to a neutral atom to form an anion, the increased electron-electron repulsions cause the electrons to spread out more in space.

Question 13

All the members of an _____ series have the same number of electrons.
As nuclear charge increases in an isoelectronic series the ions become smaller:

Answer 13

isoelectronic

Question 14

What is ionisation energy?

Answer 14

The ease with which electrons can be removed from an atom or ion.

Question 15

1. Values for a given element increase as successive electrons are removed
   i.e. I1 < I2 < I3
   Why?

Answer 15

As each successive electron is removed, more energy is required to pull an electron away from an increasingly more positive ion.

Question 16

2. There is a sharp increase in ionisation energy that occurs when an inner-shell electron is removed. Why?

Answer 16

The inner/core electrons are found closer to the nucleus and experience a greater Zeff. They are more tightly bound to the nucleus and more energy is required to remove them.

Question 17

The same factors that influence atomic size also influence ionization energy:

Answer 17

o The effective nuclear charge

o The average distance of the e─ from the nucleus

Question 18

What is the trend for ionisation energy?

Answer 18

• Ionisation energy generally increases across a period

* Ionisation energy decreases down a group

Question 19

Why does ionisation increase across a period? (2)

Answer 19

→ As we move across a period, Zeff increases and there is a decrease in the atomic radius.
→ Therefore, it becomes more difficult to remove an electron.

Question 20

Why does ionisation energy decrease down a group? (3)

Answer 20

→ This means that the outermost electron is more readily removed as we go down a group.
→ As the atom gets bigger, it becomes easier to remove an electron from the highest energy orbital.
→ The Zeff changes little down a group

Question 21

What is electron affinity?

Answer 21

• Electron affinity measures the ease with which an atom gains an electron.
• Electron affinity is the energy change when a gaseous atom gains an electron to form a gaseous ion:

Question 22

What is the trend for electron affinity?

Answer 22

In general, electron affinity becomes more exothermic as you go from left to right across a row.