general chemistry unit 1: Atomic Structure and Properties

https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties

structure of an atom

a tiny, dense central nucleus made of protons and neutrons, surrounded by a cloud of moving electrons

average atomic mass

the weighted average mass of all its naturally occurring isotopes, measured in atomic mass units (amu

physical properties

State of matter: elements exist as solids, liquids, or gases at room temperature. a few like mercury & bromine are liquids, and others like oxygen & nitrogen are gases at room temp.

Conductivity: metals are good conductors of heat and electricity. non-metals are poor conductors.

Malleability and ductility: metals can be hammered into thin sheets (malleable) or drawn into wires (ductile). non-metals are brittle & solid.

Luster: metals have a shiny appearance, while non-metals usually look dull.

chemical properties

Electronegativity: measures how strongly an atom pulls shared electrons toward itself in a chemical bond.

Ionization energy: the energy required to remove an electron from an atom. low values mean an atom loses electrons easily (common in metals) and vice.

Reactivity: dictated by valence electrons (outer shell electrons). elements seek a stable, full outer shell. ex) alkali metals are extremely reactive, yet noble gases are almost completely inert

pure substance

a form of matter that has a constant chemical composition & uniform properties throughout. there are two kinds of pure substances:

elements: made of only one type of atom; cannot be broken down into simpler substances.

compounds: made of two or more different elements chemically bonded together in a fixed ratio. ex) NaCl

mixtures

a mixture contains two or more substances physically combined in variable proportions where each component keeps its own chemical identity and properties.

components are not chemically joined, meaning no new substance forms upon mixing.

because components retain individual physical properties (like boiling point or solubility), you can separate them using physical methods like filtration, distillation, or chromatography.

homo vs hetero mixtures

Homogeneous Mixtures: uniform in composition throughout (also called solutions, like salt water or air). You cannot visually distinguish the individual parts.

Heterogeneous Mixtures: non-uniform composition with visibly distinct regions or phases (like oil and water or a salad).

how to measure mixtures?

• Mass Percent: The mass of an individual component divided by the total mass of the mixture, multiplied by 100.

• Mole Fraction: The ratio of the moles of a single component to the total moles of all components in the mixture.

• Molarity / Molality:

periodic trends

predictable patterns in element properties across rows (periods) and columns (groups) of the periodic table.

atomic radius: ½ the distance between the nuclei of two identical atoms bonded together. it decreases from left to right across a period because increasing nuclear charge pulls electrons tighter. it increases down a group as new electron shells add distance.

ionization energy: the minimum energy required to remove an electron from a gaseous atom. it increases from left to right and bottom to top because smaller atoms hold their electrons more tightly.

electronegativity: an atom's tendency to attract shared electrons in a chemical bond. It increases from left to right and bottom to top, with fluorine as the most electronegative element

electron affinity: the energy change when a neutral atom gains an electron. It generally becomes more negative (releasing more energy) moving from left to right and bottom to top

ionic compounds

neutral chemical substances made of positively charged metal ions and negatively charged non-metal ions held together by strong electrical forces

how to ionic compounds form?

• Electron transfer: Metal atoms give away electrons to become positive ions called cations.

• Anions: Non-metal atoms take those electrons to become negative ions called anions.

• Lattice structure: The positive and negative ions pull on each other and form a repeating, rigid 3D crystal structure.

properties of ionic compounds

• High melting and boiling points: It takes a lot of heat energy to break the strong bonds between the ions.

• Hard and brittle: The crystals break apart easily when hit because like-charges push away from each other.

• Conductivity: Solid ionic compounds do not conduct electricity. They conduct electricity well when melted or dissolved in water because the ions can move freely.

• Water solubility: Most ionic compounds dissolve easily in water.

common ionic compounds

• Sodium chloride (NaCl): Table salt.

• Calcium carbonate (CaCO₃): Chalk.

• Sodium bicarbonate (NaHCO₃): Baking soda.

covalent compounds

forms when two or more nonmetal atoms bond together by sharing valence electrons

how do covalent compounds form?

Sharing Electrons: Unlike ionic bonds where electrons are transferred, atoms in covalent bonds share outermost electrons so each atom fills its valence shell

The Octet Rule: Atoms share electrons to reach a stable state, usually holding eight electrons in their outer shell (or two for hydrogen)

Between Nonmetals: These bonds typically form between two or more nonmetal elements that have similar tendencies to attract electrons

• Single Bond: Two atoms share one pair (two) of electrons.

• Double Bond: Two atoms share two pairs (four) of electrons.

• Triple Bond: Two atoms share three pairs (six) of electrons.

common covalent compounds

• Water (H₂O): Oxygen and hydrogen atoms share electrons.

• Carbon dioxide (CO₂): Carbon and oxygen atoms share electron pairs.

• Methane (CH₄): A simple organic fuel gas made of carbon and hydrogen

properties of covalent compounds

• Low melting and boiling points: Most covalent compounds melt and boil at lower temperatures than ionic compounds.

• Poor electrical conductivity: They do not conduct electricity well because they lack free ions.

• Soft or brittle solids: Solid forms tend to be softer or break easily compared to hard ionic crystals.