Periodic Table Cheat Sheet

Element

A pure substance consisting only of atoms that have the same number of protons in their nuclei.

Example: Hydrogen (H), Oxygen (O), Gold (Au)

Atomic Number

The number of protons in the nucleus of an atom, which determines the chemical properties of an element and its place in the periodic table.

Example: Hydrogen (H) has an atomic number of 1.

Atomic Mass

The average mass of atoms of an element, usually expressed in atomic mass units (amu).

Example: Carbon (C) has an atomic mass of approximately 12.01 amu.

Symbol

A one- or two-letter abbreviation for an element.

Example: Na is the symbol for Sodium.

Alkali Metals (Group 1)

Highly reactive metals that readily lose one electron to form a +1 ion.

Example: Lithium (Li), Sodium (Na), Potassium (K)

Alkaline Earth Metals (Group 2)

Reactive metals that readily lose two electrons to form a +2 ion.

Example: Magnesium (Mg), Calcium (Ca)

Halogens (Group 17)

Highly reactive nonmetals that readily gain one electron to form a -1 ion.

Example: Fluorine (F), Chlorine (Cl), Bromine (Br)

Noble Gases (Group 18)

Inert (unreactive) gases with a full outer electron shell.

Example: Helium (He), Neon (Ne), Argon (Ar)

Period Trend: Atomic Size

Atomic size generally decreases from left to right across a period due to increasing nuclear charge.

Example: Sodium (Na) is larger than Chlorine (Cl) in Period 3.

Period Trend: Electronegativity

Electronegativity generally increases from left to right across a period, meaning elements become more likely to attract electrons.

Example: Fluorine (F) is more electronegative than Lithium (Li) in Period 2.

Metals, Nonmetals, and Metalloids

The periodic table is divided into metals (left), nonmetals (right), and metalloids (along the staircase).

Example: Silicon (Si) is a metalloid.