Week 1 - Atomic Structure and Bonding Flashcards
(11 cards)
What is Stellar Nucleosynthesis?
It’s the process of forming new elements inside stars through nuclear fusion.
First, a gas cloud (like the solar nebula) collapses under gravity, forming a dense, hot core (a proto-star).
Once temperatures and pressures in the core are high enough, nuclear fusion begins—light atomic nuclei fuse to form heavier elements, mainly hydrogen fuses into helium.
Over time, fusion produces heavier elements (like carbon, oxygen, and up to iron) as the star evolves.
This is how most elements in the universe were created.
What is cosmic ray spallation, and why aren’t lithium and boron formed in stars?
Cosmic ray spallation is a process where high-energy cosmic rays (mostly protons) hit atomic nuclei (like carbon, nitrogen, or oxygen) in space.
These collisions break apart the larger nuclei, creating lighter elements like lithium (Li), beryllium (Be), and boron (B).
Stars don’t produce much Li, Be, or B because:
These elements are easily destroyed by fusion at the high temperatures inside stars.
They don’t survive long enough to build up in stellar cores.
Why does nuclear fusion stop at iron (Fe)?
Fusion releases energy when lighter elements combine into heavier ones—up to iron (Fe).
Fusion of elements lighter than iron is exothermic—it releases energy, which powers the star and supports it against gravity.
But fusion of iron or heavier elements is endothermic—it requires energy input.
Iron has the most tightly bound nucleus of all elements, highly stable, so its fusion requires energy consumption.
Thus, when a star’s core fills with iron, fusion stops as it drains the star’s core. The star can no longer maintain its structure, leading to core collapse and a supernova explosion.
What is the process of neutron capture?
When nuclear fusion can’t form heavier elements (beyond iron), neutron capture takes over.
It’s a process where an atomic nucleus absorbs a free neutron, increasing its mass but not its charge (neutrons are neutral).
This allows the creation of very heavy elements like gold and uranium.
Two types:
S-process (slow): Neutrons are captured slowly enough that beta decay (neutron → proton) happens between captures. Occurs in ageing stars. (beta-minus decay)
R-process (rapid): Neutrons are captured so quickly that multiple neutrons are absorbed before decay occurs. Happens in extreme events like supernovae or neutron star mergers.
What is beta decay?
Beta decay is a process where a neutron changes into a proton (or a proton into a neutron), emitting a particle in the process.
In beta-minus (β⁻) decay:
A neutron → proton,
An electron (β^-) and an antineutrino are emitted.
This increases the atomic number by 1 (new element).
In beta-plus (β⁺) decay:
A proton → neutron,
A positron (β⁺) and a neutrino are emitted.
This decreases the atomic number by 1. This changes the element, but the mass number stays the same.
How are very heavy elements (like uranium) formed if fusion stops at iron?
Nuclear fusion can only form elements up to iron (Fe); beyond that, it becomes endothermic (consumes energy).
Heavier elements are formed through a combination of neutron capture and radioactive (beta) decay.
In neutron capture, a nucleus absorbs free neutrons:
This increases its mass number without changing its charge.
Beta decay then converts neutrons into protons, increasing the atomic number (new element).
Repeating this process—especially in extreme environments like supernovae or neutron star mergers—can form elements up to and beyond uranium (U).
What are electron orbitals?
Orbitals are regions around the nucleus where electrons are arranged.
Orbitals have different energy levels, and electrons are filled in these in a specific sequence based on energy levels. Based on the Aufbau Principle:
They occupy the lowest energy orbitals first to create a stable atom, before moving to higher energy orbitals.
How do shells, orbitals, the Aufbau principle, and the periodic table all relate?
Atoms have shells (n = 1, 2, 3…) made of different orbital types (quantum number n = 1,2,3):
n = 1 → s
n = 2 → s, p
n = 3 → s, p, d
n ≥ 4 → s, p, d, f
The Aufbau Principle says:
Electrons fill the lowest-energy orbitals first (not always in perfect shell number order).
This filling order explains the layout of the periodic table:
s-block (Groups 1–2): filling s orbitals
p-block (Groups 13–18): filling p orbitals
d-block (transition metals): filling d orbitals
f-block (lanthanides/actinides): filling f orbitals
The period number (row) tells you which shell is being filled — though d and f sometimes “lag behind” due to energy order.
Thus, the periodic table’s shape comes directly from the Aufbau filling order — it’s a visual map of how electrons fill orbitals by energy.
Chemical reactivity largely determined by electrons in outermost orbitals (valence electrons). These are the most intimately involved in bonding+ionisation.
What is a Lewis dot diagram?
A simplified way to show valence electrons (outer shell) of an atom.
Electrons are drawn as dots around the element symbol.
Helps predict bonding behavior.
What is the Octet Rule?
The octet rule states that atoms tend to gain, lose, or share electrons to have 8 electrons in their outer (valence) shell.
This makes them stable, like noble gases (e.g., neon, argon).
Examples:
Sodium (Na):
Has 1 valence electron → loses 1 → becomes Na⁺ with a full inner shell (8 e⁻)
Chlorine (Cl):
Has 7 valence electrons → gains 1 → becomes Cl⁻ with 8 e⁻
Oxygen (O):
Has 6 valence electrons → shares 2 electrons → forms 2 bonds (e.g., in H₂O)
Carbon (C):
Has 4 valence electrons → shares 4 → forms 4 bonds (e.g., in CH₄)
Exceptions:
Hydrogen (H) and Helium (He) want 2 electrons, not 8.
Some atoms (like P, S, and Xe) can hold more than 8 — these are called expanded octets.
What does it mean when an atom has an expanded valence shell, and which elements can do it?
An expanded valence shell occurs when an atom holds more than 8 electrons in its outer shell.
This is possible for elements in Period 3 or beyond on the periodic table, because they have access to empty 3d orbitals.
These orbitals can hold extra electrons beyond the s and p subshells.
Common elements that can expand their valence shells:
Sulfur (S)
Phosphorus (P)
Chlorine (Cl)
Bromine (Br)
Iodine (I)
Xenon (Xe)
Example: SO₄²⁻ (sulfate ion)
Sulfur forms 4 bonds with 4 oxygens → more than 8 electrons
This is allowed because sulfur can use its 3d orbitals → expanded shell
Only elements in Period 3 or higher can expand their octets — never Period 2 elements like C, N, or O.
