Chapter 1-6 Notes: Basic Atomic Structure and Bonding

About 92 naturally occurring elements; bodies are made of chemicals; the body is built from cells, so these elemental components show up across chapters.
You don’t need to memorize every tiny detail, but avoid adopting a “I’ve got it, now I’ll dump it” mindset; these concepts reappear across topics.
An element is defined by the number of protons in its nucleus; the atomic number also tells the typical number of electrons (since protons and electrons are usually equal in a neutral atom).
Most atoms are neutral overall, meaning they have no net charge because the number of protons equals the number of electrons.
Analogy to visualize atoms vs. elements: think of ice cream flavors to differentiate elements vs. combinations of elements (vanilla vs. chocolate represent different elements; mixing them yields a new, different mixture).
When you start combining different elements, the resulting substances have properties that are new and different from the individual elements.
Example using the ice cream metaphor: vanilla plus chocolate yields a mixed product with new color/characteristics.

The nucleus contains protons (positive charge) and neutrons (no charge); electrons are in the surrounding cloud and can move.
The atomic number tells you:
- the number of protons, and
- in a neutral atom, the number of electrons as well.
Electron shells/energy levels:
- First shell can hold up to 2 electrons.
- Second shell can hold up to 8 electrons.
- Third shell can technically hold up to 18 electrons, but in many cases, 8 is enough to satisfy the shell’s stability.
The general pattern: shells fill up as electrons are added; the outermost shell determines reactivity.
The octet rule:
- An atom is most stable when its outer (valence) shell has 8 electrons. This is referred to as the octet.
- When an outer shell has eight electrons, the element is considered stable or unreactive and tends not to gain, lose, or share electrons to change that shell.
- The term “octet” reflects the prefix ext{oct-} meaning eight.
Examples: some elements have six in their outer shell (e.g., oxygen), some have four (e.g., carbon), etc.; not enumerating all elements here but noting the concept of outer-shell electron counts and stability.

An ion is an element (or atom) that has a net electric charge due to gaining or losing electrons.
Ion formation occurs when an atom gains or loses electrons; the nucleus (protons and neutrons) stays the same, but the electron count changes.
Cations: positively charged ions (due to loss of electrons). Mnemonic: the letter T in “cation” helps remind you of a positive charge.
Anions: negatively charged ions (due to gain of electrons). Mnemonic: there are two Ns in “anions” to remind of a negative charge.
Example: Sodium in its neutral state is Na; it is often an electron donor and becomes Na⁺ after losing an electron.
- In neutral form:
- Element: ext{Na}
- After losing an electron:
- Ion: ext{Na}^+
Core concept: the protons and neutrons in the nucleus define the element; electrons outside can move and be transferred, creating ions.
Sodium often donates an electron, explaining why Na⁺ is a common form.
When discussing more complex ions, you might see hydrogen, carbon, and oxygen combining with a negatively charged partner to yield an ionic bond or ionic compound (e.g., salts).

Ionic bonds form when electrons are transferred from one atom to another, creating oppositely charged ions that attract each other.
Ionic compounds formed this way typically appear as crystals and are called salts.
Not all salts have identical electron distributions; however, the overall ionic compound tends to be neutral (no net charge) because the charges balance out.
Example: A simple salt is table salt, formed by the ionic bond between sodium and chloride ions, giving the neutral compound ext{NaCl}.
If you have multiple atoms (e.g., hydrogen, carbon, oxygen) together with a negatively charged ion, they can form an ionic bond or an ionic compound (a salt) with crystalline structure.
The statement about charge: salts formed by ionic bonds result in a substance that is neutral overall, even though the constituent ions carry positive or negative charges.
Visual/structural note: ionic compounds typically appear as crystals; this crystal form is characteristic of salts.

Covalent bonds involve sharing electrons between atoms rather than transferring them.
In covalent bonds, the shared electrons count for both participating atoms.
A common covalent example mentioned is carbon dioxide, ext{CO}_2.
Some elements may only need to share a single pair of electrons (a single bond).
Some elements may need to share two pairs of electrons (a double bond).
The shared-electron concept is a way to achieve stable electron configurations when outer shells are not already eight)
Real-world intuition: covalent bonding explains why molecules like CO₂ form distinct, neutral molecules rather than separated ions.

The atomic number is the key to identifying the element and, for neutral atoms, also equals the number of electrons.
Neutral atoms have no overall charge; ions carry positive or negative charges based on electron gain/loss.
The octet rule explains why many atoms tend to gain, lose, or share electrons to achieve eight electrons in their outer shell, leading to stability.
Ionic bonding leads to the formation of salts, typically crystalline solids, composed of positively and negatively charged ions in a neutral overall structure.
Covalent bonding explains the formation of discrete molecules where electrons are shared rather than transferred.
Mnemonics from the lecture help memory:
- Cation: positive charge (T in cation helps remember plus sign)
- Anions: negative charge (two Ns remind of negative)
The nucleus defines the element; electrons reside in shells around the nucleus and can be arranged to satisfy the octet in the outer shell, influencing bonding behavior.
The content uses practical analogies (vanilla vs. chocolate ice cream) to differentiate elements and describe how combining different elements yields new properties.
Salts (ionic compounds) are examples of how electron transfer creates new substances with unique structures and properties, distinct from the constituent elements.
Some details (like bicarbonate or phosphate) were mentioned as examples to illustrate ions but weren’t explored in depth in this segment.

Maximum electrons per shell:
- First shell: 2
- Second shell: 8
- Third shell: 18 (but often limited to the practical case of eight for stability)
Octet rule: outer shell stability when it contains 8 electrons, i.e., the octet: 8 electrons.
Neutral atom implies equal numbers of protons and electrons; if ionized, the balance is disturbed.
Ion notation examples:
- Sodium ion: ext{Na}^+
- Table salt formula: ext{NaCl}
Covalent bond example: ext{CO}_2 (carbon dioxide) as a covalently bonded molecule.
Ionic bond/ionic compound concept: formation of crystals (salts) from transfer of electrons and attraction of oppositely charged ions.