Learn: Chemistry Basics: Atoms, Bonds, Water & Solutions

Element

A pure substance made of only one type of atom.

Atom

The smallest unit of an element that retains its identity.

Proton

A positively charged particle in the nucleus with a mass of about 1 amu.

Neutron

A neutral particle in the nucleus with a mass of about 1 amu.

Electron

A negatively charged particle outside the nucleus with a very small mass.

Atomic number

The number of protons in an atom; it identifies the element.

Mass number

The total number of protons and neutrons in an atom.

Ion

An atom or group of atoms with an electrical charge from gaining or losing electrons.

Positive ion

An ion formed when electrons are lost; it has more protons than electrons.

Negative ion

An ion formed when electrons are gained; it has more electrons than protons.

Isotope

An atom of the same element with a different number of neutrons.

What determines an element's identity

The number of protons in its atoms.

Molecule

Two or more atoms chemically bonded together.

Chemical formula

A combination of element symbols and numbers showing which atoms are present in a substance.

Subscript

A small number after an element symbol showing how many atoms of that element are present.

Electron shell

An energy level around the nucleus where electrons are found.

First electron shell

Holds up to 2 electrons.

Second electron shell

Holds up to 8 electrons.

Third electron shell in the simplified model

Holds up to 8 electrons for the first 20 elements.

Valence electrons

Electrons in an atom's outermost occupied shell.

Why valence electrons matter

They are involved in chemical bonding.

Stable atom

An atom with a relatively stable electron arrangement, often a full outer shell.

Octet rule

The tendency of many atoms to gain, lose, or share electrons to obtain 8 valence electrons.

Hydrogen's stable outer shell

Hydrogen's first shell is full with 2 electrons.

Ionic bond

An attraction between oppositely charged ions, usually formed after electrons transfer between atoms.

Covalent bond

A chemical bond formed when atoms share electron pairs.

Which elements usually form ionic bonds

A metal and a nonmetal.

Which elements usually form covalent bonds

Two nonmetals.

Electron transfer

The movement of electrons from one atom to another, commonly associated with ionic bonding.

Electron sharing

The sharing of electron pairs between atoms in covalent bonding.

Electronegativity

How strongly an atom attracts shared electrons in a chemical bond.

Polarity

An uneven distribution of electrical charge in a molecule.

Polar molecule

A molecule with an uneven distribution of electrical charge.

Why water is polar

Oxygen attracts shared electrons more strongly than hydrogen.

Partial negative charge (δ−)

The slightly negative end of a polar molecule, such as the oxygen end of water.

Partial positive charge (δ+)

The slightly positive end of a polar molecule, such as the hydrogen ends of water.

Hydrogen bond

A weak attraction between a partially positive hydrogen and a partially negative atom on another molecule or another part of a molecule.

Covalent bonds in water

The O-H bonds within one water molecule.

Hydrogen bonds in water

The attractions between neighboring water molecules.

Solvent

The substance that dissolves another substance.

Solute

The substance that is dissolved in a solvent.

Solution

A uniform mixture of a solute and a solvent.

Soluble

Able to dissolve in a particular solvent.

Insoluble

Does not dissolve appreciably in a particular solvent.

Hydrophilic

Water-attracting; usually polar or charged.

Hydrophobic

Does not mix well with water; often nonpolar.

Universal solvent

Water's ability to dissolve many substances, especially ionic and polar substances.

Why salt dissolves in water

Water's partial charges attract the positive and negative ions in salt.

Why oil does not mix well with water

Oil is nonpolar and does not interact favorably with polar water.

Cohesion

The attraction between molecules of the same substance.

Why water has cohesion

Hydrogen bonds attract neighboring water molecules.

Surface tension

The resistance of a liquid's surface to being stretched or broken because of cohesive forces.

Example of surface tension

Some insects can stand on the surface of water.

Adhesion

The attraction between molecules of different substances.

Example of adhesion

Water sticking to the sides of a glass.

Capillary action

The movement of water through a narrow space due to adhesion and cohesion.

Example of capillary action

Water moving through plant tissues or being absorbed by a paper towel.

Why ice floats

Ice is less dense than liquid water because its hydrogen-bonded structure is more open.

Water's density as a solid

Water expands when it freezes, making ice less dense than liquid water.

Why floating ice matters

Ice insulates the water below, helping aquatic organisms survive winter.

High specific heat

The ability to absorb a lot of heat energy before the temperature rises significantly.

Water's ability to moderate temperature

Water heats and cools slowly because it has a high specific heat.

Example of temperature moderation

Oceans change temperature more slowly than land.

Heat of vaporization

The energy required to change a liquid into a gas.

Why evaporation cools

High-energy molecules escape from a liquid, removing energy from what remains.

Evaporative cooling

Sweating cools the body as sweat evaporates and removes heat.

pH scale

A scale that measures how acidic or basic a solution is.

Acid

A substance with a pH below 7.

Neutral substance

A substance with a pH of 7.

Base

A substance with a pH above 7.

Strong acid

A substance with a very low pH.

Strong base

A substance with a very high pH.

H+

Hydrogen ion; higher concentrations are associated with greater acidity.

H3O+

Hydronium ion, formed when a hydrogen ion associates with a water molecule; higher concentrations are associated with greater acidity.

OH−

Hydroxide ion; higher concentrations are associated with basic solutions.

What happens to acidity as pH decreases

Acidity increases, and the concentration of H+ and H3O+ increases.

What happens to basicity as pH increases

Basicity increases, and the concentration of OH− generally increases.

What happens to H+ concentration as pH increases

H+ concentration decreases.

What happens to OH− concentration as pH decreases

OH− concentration generally decreases.

A one-unit change in pH

Represents a tenfold change in hydrogen ion concentration.

Scientific method

A systematic process for investigating questions using observations and experiments.

Independent variable

The factor deliberately changed in an experiment.

Dependent variable

The factor measured or observed in an experiment.

Control group

The comparison group that does not receive the experimental treatment.

Constants

The factors kept the same throughout an experiment.

Hypothesis

A testable prediction about what will happen in an experiment.

If-then hypothesis format

If the independent variable changes, then the dependent variable will change.

Bar graph

A graph used to compare separate categories.

Line graph

A graph used to show changes over time or another continuous variable.

Graph title

A description of what the graph shows.

X-axis

The horizontal axis, usually showing the independent variable.

Y-axis

The vertical axis, usually showing the dependent variable.

Graph units

Measurements such as centimeters, grams, or minutes used to label the axes.

Analyzing a graph

Identify trends, compare values, use evidence, and draw a conclusion.

Conclusion

A statement explaining what the experimental results suggest.