An element is a pure substance: It cannot be broken down into simpler substances. Because an element is pure, every atom of that element is like every other atom of that element with the exact same physical and chemical properties.

Atoms, the smallest units of matter, are made of subatomic particles: protons, neutrons and electrons. Electrons surround the nucleus and are negatively charged particles with almost no mass. Protons and neutrons, at the center of the atom, do have mass. Protons are positively charged particles, while neutrons are neutral.

In most atoms, the number of electrons and protons are the same, balancing the atom's electrical charge. Sometimes, however, a few of the electrons can become shared with another atom. This creates a new molecule called a compound. Compounds have the atoms of two or more different elements.

Hydrogen bromide is a chemical compound. It’s formed from atoms of hydrogen and bromide, two

Elements are grouped together into categories based on their physical and chemical properties. The two main categories are metals and non-metals. Non-metals include metalloids and noble gases.

Iron, gold, and copper are three examples of metals. They share some common properties: They are all malleable, which means they can be flattened or hammered into different shapes; they are all ductile, which means they can be drawn out into wire; they are all good conductors of electricity and heat; and they are all shiny. Some properties that may vary for each metal include boiling point, density, solubility and color. Mercury, for instance, is a metal that is liquid at room temperature, while lead is an especially dense metal.

Non-metals are not shiny; they're not malleable or ductile; and most are poor conductors of heat and electricity.

Silver is an element that can conduct electricity and heat and is shiny. Silver is most likely

Each element on the periodic table has its own box and chemical symbol. The chemical symbol of hydrogen is H, and gold is Au. The little number at the top of the box is the element's atomic number, which is the number of protons that one atom of that element has. The little number at the bottom is the atomic mass of this element, which is the combined number of protons and neutrons.

Each horizontal row is called a period. The properties of the elements in each row follow a repeating, or periodic, pattern from left to right. In every row the first elements are metals, followed by non-metals, metalloids and, finally, noble gases.

Each column is called a group. Elements in a group share similar chemical and physical properties. For example, all the elements in group 1, like lithium and sodium, are very reactive metals.

On the periodic table, gold has the atomic number 79. This means that