Hesi A2 chemistry

what is the Kelvin scale and how is it related to Celsius

Kelvin scale: A temperature scale used in science that starts at absolute zero (the lowest possible temperature).

Relation to Celsius: Kelvin and Celsius have the same size units.
To convert: K = °C + 273

what is an element

A pure substance made of only one type of atom that cannot be broken down into simpler substances by chemical means

what are heterogenous mixtures

Mixtures where the components are unevenly mixed and can be seen separately

what are homogeneous mixtures

Mixtures where the components are evenly mixed and look the same throughout (also called solutions)

what are atoms and how do ions differ

Atoms:
The smallest units of an element that retain the element’s properties.

Ions:
Atoms that have gained or lost electrons, giving them a positive or negative charge.

Simple tip: Atoms = neutral, Ions = charged ⚛

what are compounds and molecules and how do they differ

Molecules:
Two or more atoms chemically bonded together (can be the same element or different).

Compounds:
Substances made of two or more different elements chemically bonded in a fixed ratio.

Difference:

• All compounds are molecules

• Not all molecules are compounds (e.g., O₂ is a molecule, not a compound)

HESI tip: Compound = different elements.

what are the main properties of solids

Main properties of solids:

• Definite shape

• Definite volume

• Particles tightly packed

• Not easily compressed

what are the main properties of liquids

Main properties of liquids:

• Definite volume

• No definite shape (take the shape of their container)

• Particles close together but able to move

• Not easily compressed

what are the main properties of gases

Main properties of gases:

• No definite shape

• No definite volume

• Particles far apart and move freely

• Easily compressed

what is the nucleus

Nucleus: The central part of an atom that contains protons and neutrons and controls the atom’s properties.

what are electrons

Tiny, negatively charged particles that orbit the nucleus of an atom and participate in chemical bonding

how is the atomic number determined

The number of protons in an atom’s nucleus.

HESI tip: Atomic number = number of protons = identifies the element

how is the mass number of an atom determined

The total number of protons and neutrons in an atom’s nucleus.

HESI tip: Mass number = protons + neutrons

what are isotopes

Atoms of the same element that have different numbers of neutrons (different mass numbers)

what are valence electrons

Electrons in the outermost shell of an atom that determine chemical reactivity

what are the groups on the periodic table and what are the periods

Groups (periodic table):
Columns; elements in the same group have similar chemical properties.

Periods (periodic table):
Rows; elements in the same period have the same number of electron shells.

what is meant by atomic radius and what is its trend in the periodic table

The distance from an atom’s nucleus to the outermost electron.

Trend in the periodic table:

• Decreases across a period (left → right) because more protons pull electrons closer

• Increases down a group (top → bottom) because more electron shells are added

HESI tip: Right = smaller, Down = bigger

what is electronegativity and what is its trend in the periodic table

Electronegativity:
An atom’s ability to attract electrons in a chemical bond.

Trend in the periodic table:

• Increases across a period (left → right)

• Decreases down a group (top → bottom)

HESI tip: Right & up = most electronegative

what is ionization energy and what is its trend in the periodic table

Ionization energy:
The energy required to remove an electron from an atom.

Trend in the periodic table:

• Increases across a period (left → right) because atoms hold electrons more tightly

• Decreases down a group (top → bottom) because electrons are farther from the nucleus

HESI tip: Right & up = highest ionization energy

what is electron affinity and what is its trend in the periodic table

Electron affinity:
The energy change when an atom gains an electron (how much an atom “wants” an electron).

Trend in the periodic table:

• Increases across a period (left → right)

• Decreases down a group (top → bottom)

HESI tip: Right & up = most eager to gain electrons.

what are some of the primary characteristics of ionic bonds

Ionic bonds:

Primary characteristics:

• Formed by transfer of electrons from one atom to another

• Between metals and nonmetals

• Create positively and negatively charged ions

• Form crystalline solids that are brittle and have high melting points

• Conduct electricity when melted or dissolved in water

HESI tip: Ionic = transfer electrons, metal + nonmetal

what are some of the primary characteristics of ionic substances

Ionic substances:

Primary characteristics:

• Crystalline solids at room temperature

• High melting and boiling points

• Brittle (shatter if force is applied)

• Conduct electricity when molten or dissolved in water

• Made of positive and negative ions held by strong electrostatic forces

HESI tip: Ionic substances = hard, high melting, conduct when liquid/solution.

what are cations and anions

Cations:
Positively charged ions (lost electrons).

Anions:
Negatively charged ions (gained electrons).

HESI tip: Cats = positive, An = negative 🐱⚡

what are some of the primary characteristics of a covalent bond

Covalent bonds:

Primary characteristics:

• Formed by sharing electrons between atoms

• Usually between nonmetals

• Do not conduct electricity

• Can be single, double, or triple bonds

• Form molecules with lower melting and boiling points than ionic compounds

HESI tip: Covalent = share electrons, nonmetal + nonmetal.

what are some of the primary characteristics of metallic bonds

Metallic bonds:

Primary characteristics:

• Electrons are shared and move freely among metal atoms (“sea of electrons”)

• Found between metal atoms

• Conduct electricity and heat

• Malleable and ductile (can be hammered or stretched)

• Usually lustrous (shiny)

HESI tip: Metallic = mobile electrons, metals, conduct & bendable

what is the kinetic theory of matter

Kinetic theory of matter:
Matter is made of tiny particles (atoms or molecules) that are always moving.

Key ideas:

• Faster particle movement = higher temperature

• Particles collide but don’t lose energy

• Explains states of matter and gas behavior

HESI tip: Matter moves, movement = energy/temperature

what is the ideal gas law

Ideal gas law:

Definition:
A mathematical relationship between pressure (P), volume (V), number of moles (n), gas constant (R), and temperature (T) of a gas.

Formula:

PV=nRTPV = nRTPV=nRT

HESI tip: Relates P, V, T, and moles for gases.

what is the heat of vaporization

Heat of vaporization:
The amount of energy needed to turn a liquid into a gas at its boiling point without changing its temperature.

HESI tip: Liquid → Gas = Heat in 🔥➡💨

what is molar mass

Molar mass:
The mass of one mole of a substance (in grams per mole, g/mol).

HESI tip: Add up the atomic masses of all atoms in the molecule.

how do endothermic and exothermic reactions differ

Endothermic reactions:

• Absorb energy (usually as heat) from the surroundings

• Feels cold

Exothermic reactions:

• Release energy (usually as heat) to the surroundings

• Feels hot

HESI tip: Endo = in (cold), Exo = out (hot) 🔥❄

what is a synthesis reaction

Synthesis reaction:
A chemical reaction where two or more substances combine to form one new compound.

General formula:

A+B→ABA + B → ABA+B→AB

what is a decomposition reaction

Decomposition reaction:
A chemical reaction where one compound breaks down into two or more simpler substances.

General formula:

AB→A+BAB → A + BAB→A+B

what is a combustion reaction

Combustion reaction:
A chemical reaction in which a substance reacts with oxygen to produce energy (usually heat and light), carbon dioxide, and water.

Example:

CH4+2O2→CO2+2H2O+energyCH_4 + 2O_2 → CO_2 + 2H_2O + energyCH4​+2O2​→CO2​+2H2​O+energy

HESI tip: Combustion = burns in oxygen, releases energy 🔥

what is a single replacement reaction

A chemical reaction where one element replaces another in a compound.

General formula:

A+BC→AC+BA + BC → AC + BA+BC→AC+B

HESI tip: Single replacement = one element swaps places

what is a double replacement reaction

A chemical reaction where the ions of two compounds swap places to form two new compounds.

General formula:

AB+CD→AD+CBAB + CD → AD + CBAB+CD→AD+CB

HESI tip: Double replacement = ions switch partners

what are acids and bases

Acids:
Substances that release hydrogen ions (H⁺) in water; taste sour and have a pH < 7.

Bases:
Substances that release hydroxide ions (OH⁻) in water; taste bitter, feel slippery, and have a pH > 7.

what is a solution

A homogeneous mixture where one substance (solute) is completely dissolved in another (solvent).

what are solutes and solvents

Solute:
The substance that is dissolved in a solution.

Solvent:
The substance that does the dissolving (usually present in greater amount)

what is molarity

Molarity (M):
A measure of solute concentration in a solution, defined as moles of solute per liter of solution.

what are alpha, beta, and gamma radiation

Alpha (α) radiation:

• Helium nucleus (2 protons + 2 neutrons)

• Heaviest, least penetrating, stopped by paper or skin

Beta (β) radiation:

• High-speed electron or positron

• Moderate penetration, stopped by plastic or thin metal

Gamma (γ) radiation:

• Electromagnetic radiation (energy)

• Highly penetrating, stopped by thick lead or concrete

what is half-life

The time it takes for half of a radioactive substance to decay.

what is fission

The process in which a heavy atomic nucleus splits into two smaller nuclei, releasing a large amount of energy

what is fusion

The process in which two light atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy.

what are the first two laws of thermodynamics

Energy cannot be created or destroyed, only transferred or converted from one form to another.

Second law of thermodynamics:
Entropy (disorder) of the universe always increases in natural processes; energy tends to spread out.

HESI tip: 1st = energy conserved, 2nd = disorder increases

which particles is responsible for all chemical reactions and what are the main bond types

Particle responsible for all chemical reactions:
Electrons, especially valence electrons in the outermost shell.

Main bond types:

1. Ionic bonds – transfer of electrons (metal + nonmetal)

2. Covalent bonds – sharing of electrons (nonmetal + nonmetal)

3. Metallic bonds – “sea of electrons” shared among metal atoms

HESI tip: Valence electrons = reactive, bonds = how atoms stick together.

characteristics of non-metals

Characteristics of non-metals:

• Poor conductors of heat and electricity

• Brittle when solid

• Dull appearance (not shiny)

• Can be gases, liquids, or solids at room temperature

• Gain electrons easily to form anions

HESI tip: Non-metals = bad conductors, brittle, gain electrons

what is the effect of pressure on a liquid solution

Effect of pressure on a liquid solution:
Pressure has little effect on liquids because they are almost incompressible.

HESI tip: Pressure mostly affects gases, not liquids

what does the law of sate conservation of mass state

Law of Conservation of Mass:
Mass cannot be created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.

HESI tip: Atoms are just rearranged, not lost or gained ⚖

which type of bonding results from transferring electrons between atoms

Ionic bonding:
Occurs when electrons are transferred from one atom to another, forming positive and negative ions that attract each other.

HESI tip: Ionic = transfer electrons, metal + nonmetal

what is pH solution

pH of a solution:
A measure of how acidic or basic a solution is, based on the concentration of hydrogen ions (H⁺).

• pH < 7 → acidic

• pH = 7 → neutral

• pH > 7 → basic (alkaline)

HESI tip: Low pH = acid, High pH = base ⚖