Chemistry : Predict Chemical and Physical Properties of Elements

What is the relationship between the number of protons and electrons in a neutral atom?

The number of protons equals the number of electrons.

How are electrons organized in an atom?

Electrons are organized into atomic shells (energy levels).

How many shells do elements in the 1st period have? Give examples.

1 shell (e.g., H and He).

How many shells do elements in the 2nd period have?

2 shells (e.g., Li to Ne).

What is the general rule for the number of shells in the nth period?

Elements in the nth period have n shells.

What are the four types of subshells?

s, p, d, f.

How many electrons can an s orbital hold?

2 electrons.

Describe the shape of an s orbital.

Spherical and non-directional.

How many p orbitals are there, and what is their shape?

Three p orbitals (px, py, pz), dumb-bell shaped and directional.

What is unique about the energy levels of p orbitals?

All three p orbitals are degenerate (same energy level).

How many d orbitals are there, and what is their energy level compared to s and p?

Five d orbitals; higher energy than s and p orbitals.

What is the maximum number of electrons a d subshell can hold?

10 electrons.

What is the Aufbau principle?

Electrons fill the lowest energy orbitals first (e.g., 1s before 2s).

State the order of orbital filling up to 4p.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p.

What is the Pauli exclusion principle?

Two electrons in the same orbital must have opposite spins (↑↓).

What is Hund's rule?

Electrons occupy degenerate orbitals singly with parallel spins before pairing.

Why is chromium's electron configuration an exception to the Aufbau principle?

It favors a half-filled 3d subshell for stability ([Ar] 3d⁵ 4s¹).

Define atomic radius.

Distance from the nucleus to the outermost electron.

How does atomic radius change across a period? Why?

Decreases; increased effective nuclear charge pulls electrons closer.

How does atomic radius change down a group? Why?

Increases; more electron shells are added.

Which is larger: Na or Cl? Why?

Na; it has fewer protons and less effective nuclear charge than Cl.

Arrange in increasing atomic radius: Li, K, Cs.

Li < K < Cs.

Define ionization energy.

Energy required to remove an electron from a gaseous atom.

How does IE change across a period? Why?

Increases; stronger attraction between nucleus and electrons.

How does IE change down a group? Why?

Decreases; outermost electrons are farther from the nucleus.

Why do noble gases have very high IE?

Stable full electron shells make electron removal difficult.

Why does IE drop from Be to B?

B's electron is in a higher-energy 2p orbital (easier to remove).

Why does IE drop from N to O?

O's 4th electron in 2p is paired, causing repulsion (easier to remove).

Arrange in increasing IE: Li, K, Cs.

Cs < K < Li.

Define electronegativity.

Ability of an atom to attract shared electrons in a bond.

How does EN change across a period? Why?

Increases; higher effective nuclear charge attracts electrons more.

How does EN change down a group? Why?

Decreases; more electron shells shield the nucleus's pull.

Which element has the highest EN?

Fluorine (EN = 4.0).

Arrange in increasing EN: Li, K, Cs.

Cs < K < Li.

What type of bond forms if ΔEN > 2?

Ionic bond (e.g., NaCl).

What type of bond forms if ΔEN < 2?

Polar covalent bond (e.g., HCl).

What type of bond forms if ΔEN = 0?

Non-polar covalent bond (e.g., H₂).

Define electron affinity.

Energy change when an electron is added to a gaseous atom.

What does a negative EA value indicate?

Energy is released (process is favorable).

How does EA change across a period? Why?

Increases; stronger nuclear attraction for added electrons.

How does EA change down a group? Why?

Decreases; added electrons are farther from the nucleus.

Why does Cl have a high EA (-349 kJ/mol)?

It readily gains an electron to achieve a stable octet.

Arrange in increasing EA: Li, K, Cs.

Cs < K < Li.

Why do metals (e.g., Na) have low IE?

They easily lose electrons to achieve stability.

Why do non-metals (e.g., Cl) have high EA?

They readily gain electrons to achieve stability.

What happens when Na reacts with Cl?

Na loses an electron (low IE), Cl gains it (high EA), forming ionic NaCl.

Which group has the lowest IE?

Alkali metals (Group 1).

Which group has the highest EA?

Halogens (Group 17).

What makes an electron configuration stable?

Full/half-filled subshells (e.g., Zn's full 3d, N's half-filled 2p).

Why is 4s filled before 3d?

4s has lower energy than 3d (except in transition metals like Cr/Cu).