Chemistry: Elements and Atomic Theory

A fundamental law in chemistry stating that matter cannot be created or destroyed in ordinary chemical reactions. The total mass of the reactants before a reaction equals the total mass of the products after the reaction.

A chemical reaction where two solutions are mixed together, resulting in the formation of a precipitate (a solid that settles out of the solution).

A chemical compound formed by the reaction of copper sulfate (CuSO4) and sodium carbonate (Na2CO3).

A chemical compound formed by the reaction of barium chloride (BaCl2) and sodium sulfate (Na2SO4).

A chemical compound formed by the reaction of lead nitrate (Pb(NO3)2) and sodium chloride (NaCl).

One atomic mass unit (amu) is precisely one-twelfth (1/12th) the mass of a single carbon-12 atom.

The relative atomic mass of an element is the average mass of an atom of that element, compared to 1/12 the mass of a carbon-12 atom.

A molecule is formed when two or more atoms bond together through attractive forces.

Most atoms cannot exist individually; they tend to form molecules or ions.

Atoms are too small to be seen with the naked eye. They are billions of times smaller than the smallest things visible to us.

The standard way to represent an element, usually using the first one or two letters of its name, with the first letter capitalized and the second, if present, lowercase.

The mass of an atom of an element, usually measured relative to the mass of a carbon-12 atom.

The quantities of elements that combine in a specific ratio to form a particular compound.

An international organization that sets standards for scientific terminology and measurements, including element names and symbols.

Compounds always contain the same elements in the same proportions by mass, regardless of the source or how they were prepared.

The smallest indivisible particle of an element that retains the chemical properties of the element.

The idea that atoms of a particular element are identical in mass and chemical properties, while atoms of different elements have different masses and chemical properties.

Atoms combine in simple whole-number ratios to form compounds.

Atoms are the fundamental building blocks of all matter.

Atoms are incredibly tiny, millions of them stacked together would only be as thick as a sheet of paper.

The atomic radius, the distance from the center of the atom to its outer edge, is measured in nanometers.

Dalton proposed the first specific symbols for elements, using symbols like circles and lines to represent different atoms.

Berzelius refined Dalton's system by suggesting using one or two letters from the element's name to represent its symbol.

Each symbol for an element represents one atom of that element, making it a fundamental unit of chemical representation.

The modern-day symbols of different elements are derived from Berzelius's system, using one or two letters from the name of the element.

The symbols of elements are used to understand and represent chemical reactions, compounds, and the composition of matter.

When writing chemical formulas, the positive and negative charges of the ions must cancel out to ensure a neutral compound.

When a compound contains a metal and a non-metal, the metal is always written first in the formula, followed by the non-metal.

Polyatomic ions are groups of atoms that act as a single unit with a charge. In chemical formulas, they are enclosed in brackets with the number of ions outside the brackets.

Hydrogen chloride (HCl) is a compound formed by the combination of hydrogen (H) and chlorine (Cl).

Magnesium chloride (MgCl2) is a compound formed by the combination of magnesium (Mg) and chlorine (Cl). It has 2 chlorine atoms to balance the 2+ charge of magnesium.

Molecules made up of only one type of atom, like oxygen (O2) which has two oxygen atoms bonded together.

The number of atoms in a single molecule. For example, oxygen (O2) has a diatomic atomicity, while ozone (O3) has a triatomic atomicity.

Atoms of different elements combine in fixed ratios to form molecules. For example, water (H2O) always has two hydrogen atoms and one oxygen atom.

Negatively charged ions, formed when an atom gains electrons. Example: chloride ion (Cl-) in sodium chloride (NaCl).

The sum of the atomic masses of all the atoms in a molecule. It's expressed using atomic mass units (u).

A chemical formula representing the smallest repeating unit of an ionic compound.

The mass of a formula unit of an ionic compound. Calculated by summing the atomic masses of all atoms in the formula unit.

The smallest repeating unit of an ionic compound, containing the simplest whole-number ratio of ions.

Indicates multiple occurrences of the same ion in a chemical formula.

The chemical formula of a compound represents its composition using symbols and numbers, showing the types and quantities of atoms it contains.

The valency of an element indicates the number of electrons an atom of that element needs to gain or lose to achieve a stable electron configuration. For example, sodium has a valency of 1 because it needs to lose one electron.

Ionic compounds form between metals and non-metals, where one element loses electrons (becoming a positively charged cation) and the other gains electrons (becoming a negatively charged anion). The electrostatic attraction between these ions holds the compound together.

Polyatomic ions are groups of atoms that act as a single unit with a net positive or negative charge. For example, the sulfate ion (SO₄²⁻) contains one sulfur and four oxygen atoms, with an overall charge of -2.

Roman numerals in parentheses are used in the names of some metal ions to indicate their valency. For example, iron (II) denotes the Fe²⁺ ion, while iron (III) denotes the Fe³⁺ ion.

The sum of atomic masses of all the atoms in its formula. Basically, the total weight of all the atoms in a formula unit.