This Java enum (=singleton) provides information about the elements of the periodic table (PTE). (Symbol, full name, atomic number, average atomic mass and electron negativity by Pauling) It can be searched by symbol and atomic number.

Java Periodic table of elements.java

package divo.felix.chem.pte;

import java.util.HashMap;
import java.util.Map;

/**
 * Holds information of the periodic table of elements.
 * 
 * @author Felix Divo
 */
public enum Element {

	/* SYMBOL NAME ATOMIC_WEIGHT ELECTRONEGATIVITY */
	H("Hydrogen", 1.00794f, 2.1f), He("Helium", 4.0026f, 0), Li("Lithium",
			6.941f, 0.98f), Be("Beryllium", 9.01218f, 1.57f), B("Boron",
			10.811f, 2.04f), C("Carbon", 12.011f, 2.55f), N("Nitrogen",
			14.0067f, 3.04f), O("Oxygen", 15.9994f, 3.44f), F("Fluorine",
			18.9984f, 3.98f), Ne("Neon", 20.1797f, 0), Na("Sodium", 22.98977f,
			0.93f), Mg("Magnesium", 24.305f, 1.31f), Al("Aluminum", 26.98154f,
			1.61f), Si("Silicon", 28.0855f, 1.9f), P("Phosphorus", 30.97376f,
			2.19f), S("Sulfur", 32.066f, 2.58f), Cl("Chlorine", 35.4527f, 3.16f), Ar(
			"Argon", 39.948f, 0), K("Potassium", 39.0983f, 0.82f), Ca(
			"Calcium", 40.078f, 1), Sc("Scandium", 44.9559f, 1.36f), Ti(
			"Titanium", 47.88f, 1.54f), V("Vanadium", 50.9415f, 1.63f), Cr(
			"Chromium", 51.996f, 1.66f), Mn("Manganese", 54.938f, 1.55f), Fe(
			"Iron", 55.847f, 1.83f), Co("Cobalt", 58.9332f, 1.88f), Ni(
			"Nickel", 58.6934f, 1.91f), Cu("Copper", 63.546f, 1.9f), Zn("Zinc",
			65.39f, 1.65f), Ga("Gallium", 69.723f, 1.81f), Ge("Germanium",
			72.61f, 2.01f), As("Arsenic", 74.9216f, 2.18f), Se("Selenium",
			78.96f, 2.55f), Br("Bromine", 79.904f, 2.96f), Kr("Krypton", 83.8f,
			0), Rb("Rubidium", 85.4678f, 0.82f), Sr("Strontium", 87.62f, 0.95f), Y(
			"Yttrium", 88.9059f, 1.22f), Zr("Zirconium", 91.224f, 1.33f), Nb(
			"Niobium", 92.9064f, 1.6f), Mo("Molybdenum", 95.94f, 2.16f), Tc(
			"Technetium", 98, 1.9f), Ru("Ruthenium", 101.07f, 2.2f), Rh(
			"Rhodium", 102.9055f, 2.28f), Pd("Palladium", 106.42f, 2.2f), Ag(
			"Silver", 107.868f, 1.93f), Cd("Cadmium", 112.41f, 1.69f), In(
			"Indium", 114.82f, 1.78f), Sn("Tin", 118.71f, 1.96f), Sb(
			"Antimony", 121.757f, 2.05f), Te("Tellurium", 127.6f, 2.1f), I(
			"Iodine", 126.9045f, 2.66f), Xe("Xenon", 131.29f, 2.6f), Cs(
			"Cesium", 132.9054f, 0.79f), Ba("Barium", 137.33f, 0.89f), La(
			"Lanthanum", 138.9055f, 1.1f), Ce("Cerium", 140.12f, 1.12f), Pr(
			"Praseodymium", 140.9077f, 1.13f), Nd("Neodymium", 144.24f, 1.14f), Pm(
			"Promethium", 145, 1.13f), Sm("Samarium", 150.36f, 1.17f), Eu(
			"Europium", 151.965f, 1.2f), Gd("Gadolinium", 157.25f, 1.2f), Tb(
			"Terbium", 158.9253f, 1.1f), Dy("Dysprosium", 162.5f, 1.22f), Ho(
			"Holmium", 164.9303f, 1.23f), Er("Erbium", 167.26f, 1.24f), Tm(
			"Thulium", 168.9342f, 1.25f), Yb("Ytterbium", 173.04f, 1.1f), Lu(
			"Lutetium", 174.967f, 1.27f), Hf("Hafnium", 178.49f, 1.3f), Ta(
			"Tantalum", 180.9479f, 1.5f), W("Tungsten", 183.85f, 2.36f), Re(
			"Rhenium", 186.207f, 1.9f), Os("Osmium", 190.2f, 2.2f), Ir(
			"Iridium", 192.22f, 2.2f), Pt("Platinum", 195.08f, 2.28f), Au(
			"Gold", 196.9665f, 2.54f), Hg("Mercury", 200.59f, 2), Tl(
			"Thallium", 204.383f, 2.04f), Pb("Lead", 207.2f, 2.33f), Bi(
			"Bismuth", 208.9804f, 2.02f), Po("Polonium", 209, 2), At(
			"Astatine", 210, 2.2f), Rn("Radon", 222, 0), Fr("Francium", 223,
			0.7f), Ra("Radium", 226.0254f, 0.89f), Ac("Actinium", 227, 1.1f), Th(
			"Thorium", 232.0381f, 1.3f), Pa("Protactinium", 231.0359f, 1.5f), U(
			"Uranium", 238.029f, 1.38f), Np("Neptunium", 237.0482f, 1.36f), Pu(
			"Plutonium", 244, 1.28f), Am("Americium", 243, 1.3f), Cm("Curium",
			247, 1.3f), Bk("Berkelium", 247, 1.3f), Cf("Californium", 251, 1.3f), Es(
			"Einsteinium", 252, 1.3f), Fm("Fermium", 257, 1.3f), Md(
			"Mendelevium", 258, 1.3f), No("Nobelium", 259, 1.3f), Lr(
			"Lawrencium", 262, 0), Rf("Rutherfordium", 261, 0), Db("Dubnium",
			262, 0), Sg("Seaborgium", 263, 0), Bh("Bohrium", 262, 0), Hs(
			"Hassium", 265, 0), Mt("Meitnerium", 266, 0), Uun("ununnilium",
			269, 0), Uuu("unununium", 272, 0), Uub("ununbium", 277, 0);

	/**
	 * Holds mappings of the Elements.
	 * <p>
	 * There's a bit of java kung fu going on here that deserves an explanation.
	 * The map is put inside a static inner (holder) class so it gets
	 * initialized before the enum instances are initialized, that way they can
	 * add themselves to it. If not in the inner static class, it would not be
	 * initialize, because the first thing initialized in the enum class must be
	 * the instances, but static inner classes are initialized before the class
	 * is initialized. (Trick from <a
	 * href="http://stackoverflow.com/a/22738610/2210921">here</a>.)
	 */
	static final class Holder {
		/** Mapps the atomic number to the Element */
		static final Map<Integer, Element> map_atomicNumber = new HashMap<Integer, Element>();
		/** Mapps the symbol to the Element */
		static final Map<String, Element> map_symbol = new HashMap<String, Element>();
	}

	/**
	 * Creates a new Element and mapps it to 'Holder.map_atomicNumber' and
	 * 'Holder.map_symbol'.
	 * 
	 * @param fullName
	 *            the full name of the element (not the symbol)
	 * @param atomicMass
	 *            the average atomic mass of the element in 'u'
	 * @param electroNegativity
	 *            the electron negativity by Pauling
	 */
	private Element(String fullName, float atomicMass, float electroNegativity) {

		this.fullName = fullName;
		this.atomicMass = atomicMass;
		this.electroNegativity = electroNegativity;

		Element.Holder.map_atomicNumber.put(this.getAtomicNumber(), this);
		Element.Holder.map_symbol.put(this.name(), this);
	}

	private final String fullName;
	private final float atomicMass;
	private final float electroNegativity;

	/**
	 * Get the atomic number of this Element.
	 * 
	 * @return this Element's atomic number in 'u'
	 */
	public int getAtomicNumber() {
		return this.ordinal() + 1;
	}

	/**
	 * Get the symbol of this Element.
	 * 
	 * @return this Element's symbol (1 to 3 chars long)
	 */
	public String getSymbol() {
		return this.name();
	}

	/**
	 * Get the full name of this Element
	 * 
	 * @return this Element's full name (not the symbol)
	 */
	public String getFullName() {
		return this.fullName;
	}

	/**
	 * Get the atomic mass of this Element.
	 * 
	 * @return this Element's average atomic mass in 'u'
	 */
	public float getAtomicMass() {
		return this.atomicMass;
	}

	/**
	 * get the electron negativity of this Element.
	 * 
	 * @return this Element's electron negativity in Pauling
	 */
	public float getElectroNegativity() {
		return this.electroNegativity;
	}

	/**
	 * Get a String representation of this Element.
	 * <p>
	 * For example:<br>
	 * <i>He[fullName="Helium", atomicMass="4.0026", electroNegativity="0"]</i>
	 * 
	 * @return a String representation of this Element.
	 */
	public String toString() {
		return name() + "[fullName=\"" + this.fullName + "\", atomicMass=\""
				+ this.atomicMass + "\", electroNegativity=\""
				+ this.electroNegativity + "\"]";
	}

	/**
	 * Search for the Element by the given atomic number.
	 * 
	 * @param atomicNumber
	 *            ths atomic number to search by
	 * 
	 * @return the Element with the given atomic number or null if it couldn't
	 *         be found
	 */
	public static Element getByAtomicNumber(int atomicNumber) {
		return Element.Holder.map_atomicNumber.get(atomicNumber);
	}

	/**
	 * Search for the Element by the given symbol.
	 * 
	 * @param symbol
	 *            ths symbol to search by, must be correctly typed (case
	 *            sensitive)
	 * 
	 * @return the Element with the given symbol or null if it couldn't be found
	 * 
	 * @see {@link #toCorrectSymbol(String symbol)}
	 */
	public static Element getBySymbol(String symbol) {
		return Element.Holder.map_symbol.get(symbol);
	}

	/**
	 * Get the symbol of the Elemment with the specified atomic number.
	 * 
	 * @param atomicNumber
	 *            ths atomic number to search by
	 * 
	 * @return the element's symbol or null if the element couldn't be found
	 */
	public static String getSymbolByAtomicNumber(int atomicNumber) {

		Element e = Element.Holder.map_atomicNumber.get(atomicNumber);
		if (e != null)
			return e.getSymbol();
		else
			return null;
	}

	/**
	 * Get the atomic number of the Elemment with the specified symbol.
	 * 
	 * @param atomicNumber
	 *            ths symbol to search by, must be correctly typed (case
	 *            sensitive)
	 * 
	 * @return the element's atomic number or 0 if the element couldn't be found
	 * 
	 * @see {@link #toCorrectSymbol(String symbol)}
	 */
	public static int getAtomicNumberBySymbol(String symbol) {

		Element e = Element.Holder.map_atomicNumber.get(symbol);
		if (e != null)
			return e.getAtomicNumber();
		else
			return 0;
	}

	/**
	 * Finds out, whether an Element with the given symbol exists.
	 * 
	 * @param symbol
	 *            the symbol to search by, must be correctly typed (case
	 *            sensitive)
	 * 
	 * @return true, if an Element with the given symbol exists.
	 * 
	 * @see {@link #exists(int atomicNumber)}
	 * @see {@link #toCorrectSymbol(String symbol)}
	 */
	public static boolean exists(String symbol) {
		if (null != Element.Holder.map_symbol.get(symbol))
			return true;
		else
			return false;
	}

	/**
	 * Finds out, whether an Element with the given atomic number exists.
	 * 
	 * @param symbol
	 *            the atomic number to search by
	 * 
	 * @return true, if an Element with the given atomic number.
	 * 
	 * @see {@link #exists(String symbol)}
	 */
	public static boolean exists(int atomicNumber) {
		if (null != Element.Holder.map_symbol.get(atomicNumber))
			return true;
		else
			return false;
	}

	/**
	 * Turns a, for example, lowercase Symbol ("cl") into the correctly spelled
	 * symbol ("Cl"). Assumes that the symbol corresponds to a existing element
	 * of the periodic table of elements.
	 * <p>
	 * Examples:<br>
	 * "cl" -> "Cl"<br>
	 * "Cl" -> "Cl"<br>
	 * "cL" -> "Cl"<br>
	 * "CL" -> "Cl"<br>
	 * 
	 * @param symbol
	 *            the symbol to convert (1 to 3 chars long)
	 * 
	 * @return the correctly spelled symbol
	 */
	public static String toCorrectSymbol(String symbol) {

		String correct_symbol = "";

		correct_symbol += Character.toUpperCase(symbol.charAt(0));

		if (2 == symbol.length()) {
			correct_symbol += Character.toLowerCase(symbol.charAt(1));

		} else if (3 == symbol.length()) {
			correct_symbol += Character.toLowerCase(symbol.charAt(1));
			correct_symbol += Character.toLowerCase(symbol.charAt(2));
		}

		return correct_symbol;
	}
}