Chemistry - Chapter 3

Subatomic particles

Smaller particles that make up atoms, including protons, neutrons, and electrons

Nucleus

The dense, positively charged center of an atom containing protons and neutrons

Electron cloud

The region surrounding the nucleus where electrons are likely to be found

Electron

A negatively charged subatomic particle located outside the nucleus

Proton

A positively charged subatomic particle found in the nucleus

Neutron

A neutral subatomic particle found in the nucleus

Electrical neutrality

The condition in which an atom has equal numbers of protons and electrons

Cathode-ray tube

A sealed glass tube used to study electrons by passing electricity through low-pressure gas

Cathode

The negatively charged electrode that emits electrons in a cathode-ray tube

Anode

The positively charged electrode that attracts electrons in a cathode-ray tube

Cathode rays

Streams of negatively charged particles observed in cathode-ray tube experiments

Evidence cathode rays have mass

The paddle wheel in the tube moved when struck by the rays

Evidence cathode rays are negative

Rays were repelled by negative charges and attracted to positive charges

J. J. Thomson

Scientist who discovered the electron using cathode-ray tube experiments

Electron charge-to-mass ratio

A value measured by Thomson showing electrons are identical in all atoms

Robert Millikan

Scientist who determined the mass and charge of the electron

Mass of an electron

9.109 × 10⁻³¹ kg, about 1/1837 the mass of a hydrogen atom

Alpha particles

Positively charged particles with large mass used in Rutherford’s experiment

Rutherford gold foil experiment

An experiment where alpha particles were fired at gold foil to study atomic structure

Unexpected gold foil result

Some alpha particles were deflected at large angles or bounced back

Rutherford’s conclusion

Atoms contain a small, dense, positively charged nucleus

Atomic empty space

Most of an atom’s volume is empty space

Nuclear charge

The positive charge of the nucleus caused by protons

Atomic identity

The number of protons in an atom determines which element it is

Simplest hydrogen atom

An atom with one proton and one electron and no neutrons

Relative mass of a proton

About 1 amu, much larger than the mass of an electron

Relative mass of a neutron

About 1 amu, slightly larger than a proton

Nuclear forces

Strong attractive forces that hold protons and neutrons together in the nucleus

Atomic radius

The distance from the center of the nucleus to the outer edge of the electron cloud

Picometer (pm)

A unit used to measure atomic size; equal to 10⁻¹² meters

Nuclear density

Nuclei are extremely dense compared to the rest of the atom

Atom

The smallest particle of an element that still retains the chemical properties of that element

Matter

Anything that has mass and takes up space

Element

A substance that cannot be broken down into simpler substances by chemical means

Compound

A substance formed when two or more elements chemically combine in fixed ratios

Chemical reaction

A process in which substances change into new substances by rearranging atoms

Democritus

Ancient Greek philosopher who proposed that matter is made of indivisible particles called atoms

Aristotle

Greek philosopher who believed matter was continuous and not made of atoms

Law of conservation of mass

Mass is neither created nor destroyed during a chemical reaction or physical change

Law of definite proportions

A compound always contains the same elements in the same fixed ratio by mass

Law of multiple proportions

When two elements form more than one compound, the masses combine in small whole-number ratios

John Dalton

Scientist who proposed the first modern atomic theory based on experimental evidence

Dalton’s atomic theory

A theory stating that matter is made of atoms that combine in whole-number ratios and are rearranged in reactions

Dalton theory: atoms

All matter is composed of extremely small particles called atoms

Dalton theory: identical atoms

Atoms of the same element are identical, while atoms of different elements differ

Dalton theory: indivisible atoms

Atoms cannot be created, destroyed, or subdivided in chemical reactions

Dalton theory: compound formation

Atoms of different elements combine in simple whole-number ratios

Dalton theory: reactions

Chemical reactions involve the rearrangement of atoms

How Dalton explains conservation of mass

Atoms are not created or destroyed, so total mass remains constant

How Dalton explains definite proportions

Compounds always contain the same combination of atoms

How Dalton explains multiple proportions

Different compounds form when atoms combine in different whole-number ratios

Modern atomic theory

Updated atomic theory that recognizes subatomic particles while keeping the idea that matter is made of atoms