Definitions
What is the definition of electron configuration?
Electron configuration is the arrangement of electrons in an atom’s orbitals, written in order of increasing energy.
It shows which orbitals the electrons occupy and how many electrons are in each orbital (for example: 1s² 2s² 2p⁶).
Atomic Radius Trend
Increases down each group
Decreases across a period (left → right)
What is the name of Group 1 elements?
Alkali Metals
(H, Li, Na, K, Rb, Cs, Fr)
Why does atomic radius decrease as you move from left to right across a period even though electrons are being added?
Atomic radius decreases from left to right because the nucleus gains more protons, increasing its effective nuclear charge (Zₑff).
Even though electrons are being added, they enter the same energy level, so shielding doesn’t increase much.
The stronger nuclear pull pulls the electrons closer, making the atom smaller.
In simple terms:
More protons + same shell = electrons pulled in tighter → smaller radius.
Rank from smallest to largest atomic radius:
N, P, As
N < P < As
What does shielding mean?
Shielding (also called screening) is when inner (core) electrons block or reduce the attraction between the nucleus and the outer (valence) electrons.
Because the inner electrons “shield” the valence electrons from the full positive charge of the nucleus, the outer electrons feel a weaker pull.
Ionization Energy Trend
Decreases down a group
Increases across a period
What is the name of Group 2 elements?
Alkaline Earth Metals
(Be, Mg, Ca, Sr, Ba, Ra)
Why does ionization energy increase across a period but decrease down a group?
Across → electrons held tighter → IE ↑
Down → electrons farther away + more shielding → IE ↓
Rank the neutral atoms from largest to smallest:
Li, C, F
Li > C > F
What is the definition of isoelectronic?
Elements that have the same number of electrons and the same ground-state electron configuration.
Electron Affinity Trend
Generally becomes more negative across a period
Slightly less negative down a group
Noble gases = near zero
What is the name of Group 17 elements?
Halogens
(F, Cl, Br, I, At)
Why are anions always larger than the neutral atoms they come from, while cations are always smaller?
This is because gaining electrons increases electron–electron repulsion and expands the electron cloud, while losing electrons reduces repulsion and often removes an entire energy level.”
Rank the following by increasing size:
Li⁺, Be²⁺, B³⁺
B³⁺ < Be²⁺ < Li⁺
What does electron affinity mean?
Electron affinity is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion.
Electronegativity Trend
Increases across a period
Decreases down a group
Fluorine is the most electronegative
What is the name of Group 18 elements?
Noble Gases
(He, Ne, Ar, Kr, Xe, Rn)
Why does fluorine have a much more negative electron affinity than oxygen, even though both are in Group 16 and 17?
This element has a more negative electron affinity because its higher effective nuclear charge pulls in an added electron more strongly than oxygen does.
Rank the following by increasing ionic radius:
Cl⁻, S²⁻, Ar
Ar < Cl⁻ < S²⁻
What does the Effective nuclear charge mean?
Effective nuclear charge (Zₑff) is the net positive charge felt by an electron in an atom after accounting for shielding by the other electrons.
Ionic Radius Trend
Cations (positive ions) are smaller than their neutral atoms.
Anions (negative ions) are larger than their neutral atoms.
Within a group of ions with the same charge, ionic radius increases down the group.
What is the name of the d-block
Transition Metals
(Groups 3–12)
Magnesium has a higher first ionization energy than sodium.
Explain why, even though Mg is to the right of Na.
This element requires more energy to remove an electron because its nucleus has a higher effective nuclear charge and its 3s² configuration is more stable than a 3s¹ configuration.
Rank the ions from largest to smallest:
Fe²⁺, Fe³⁺
Fe²⁺ > Fe³⁺