Unit 2 Chemistry Study Guide

Comprehensive vocabulary flashcards covering atomic structure, subatomic particles, ions, chemical formulas, isotopes, periodic table groups and trends, and key atomic discoveries.

Proton

A positively charged ($+1$) subatomic particle located in the nucleus.

Neutron

A neutral ($0$ charge) subatomic particle located in the nucleus.

Electron

A negatively charged ($-1$) subatomic particle located in the electron cloud.

Nucleus

The center of an atom containing protons and neutrons.

Atomic number

The number of protons in an atom, which identifies the element.

Mass number

The total number of protons and neutrons in an atom ($\text{protons} + \text{neutrons}$).

Average atomic mass

The weighted average of the isotope masses of an element, usually expressed as a decimal because isotopes have different abundances.

Neutral atom

An atom that has an equal number of protons and electrons.

Cation

A positively charged ion formed when an atom loses electrons.

Anion

A negatively charged ion formed when an atom gains electrons.

Atom

The smallest unit of an element.

Element

A substance composed of only one type of atom.

Molecule

Two or more atoms bonded together, usually by covalent bonds.

Compound

A substance composed of two or more different elements chemically bonded.

Ionic compound

A chemical substance formed by positive and negative ions attracted to each other, having an overall charge of zero.

Coefficient

The number written in front of a chemical formula that multiplies the entire formula and indicates the number of molecules or formula units.

Subscript

A small number written after an element symbol that counts its atoms.

Formula unit

The simplest whole-number ratio of ions in an ionic compound.

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons.

Group (Family)

A vertical column on the periodic table; elements in the same group generally have the same number of valence electrons and similar chemical properties.

Period

A horizontal row on the periodic table.

Alkali metals

Very reactive metals located in Group $1$ of the periodic table.

Alkaline earth metals

Reactive metals located in Group $2$ of the periodic table.

Halogens

Reactive nonmetals located in Group $17$ of the periodic table.

Noble gases

Mostly unreactive elements located in Group $18$ of the periodic table.

Transition metals

Elements located in the middle of the periodic table that are good conductors.

Metalloids

Elements that display properties of both metals and nonmetals.

Valence electrons

Electrons located in the outer shell of an atom.

Atomic radius

A measure of atomic size; it decreases from left to right across a period and increases down a group.

Electronegativity

The ability of an atom to attract bonding electrons; it generally increases from left to right across a period and decreases down a group.

Ionization energy

The energy needed to remove an electron from an atom; it generally increases from left to right across a period and decreases down a group.

Dmitri Mendeleev

The scientist who organized elements by properties and atomic mass, leaving gaps for elements not yet discovered.

Rutherford's gold foil experiment

An experiment showing that atoms are mostly empty space with a small, dense, positively charged nucleus.

Why atomic models change

New experiments and evidence improve our understanding of atoms

atom

smallest unit of an element

element

one type of atom

proton

positive particle

neutron

neutral particle

electron

negative particle

nucleus

center containing protons and neutrons

atomic number

protons

protons

mass number

isotope

same element, different neurons

average atomic mass

number in front

mass number

protons + neutrons

ion

charged atom/group

cation

positive ion

anion

negative ion

molecule

bonded atoms

compound

different elements chemically bonded

ionic compound

positive and negative ions

coefficient

number in the front

subscript

atom count

formula unit

simplest ion ratio

group/family

column

period

row

valence electrons

outer-shell electrons

atomic radius

atomic size

electronegativity

attraction for bonding electrons

ionization energy

energy to remove an electron