Atom



Atoms (Greek άτομον) are the basic units that make up all matter. They are sub-microscopic structures and the smallest part of a substance that can take part in chemical reactions. Atoms exist in more than 110 different substances that are known as elements. These elements are arranged into groups in the Periodic Table of Elements based on both common and progressively changing but related properties.

Composition
Each atom contains a nucleus and associated electrons. The nucleus is at the center of the atom. It is composed of particles called protons, which are positively charged, and neutrons, which have no charge. Therefore the nucleus always has a positive charge. Both neutrons and a protons are referred to as nucleons.

The electrons have a negative charge. When atoms have the same number of electrons and protons, they have a neutral charge. When atoms have more protons than electron they have a positive charge. By contrast, if there are more electrons than protons, the atom is negatively charged. Charged atoms are called ions.

Proton


A proton (from the Greek πρωτος, -η, -ον meaning "first") is one of the three types of subatomic particles, the other two being neutrons and electrons. Protons have an electric charge of +1. A proton has slightly less mass than a neutron.

Neutron


Neutrons are one of the three types of subatomic particles, the other two being protons and electrons. A neutron has slightly more mass than a proton and unlike the electrons and protons has no charge.

Electron
Electrons are the subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.

Identity

 * Main Article: Elements

Atoms are the smallest parts of a chemical element that still have the properties of that element. Elements are therefore identified by the properties of their atoms and in particular, the number of protons in the nucleus. All hydrogen atoms have one proton, all carbon atoms have six, all oxygen atoms have eight, etc.

The number of protons that an element has is called its atomic number. The atomic number of the various elements can be found above the element's chemical symbol in the periodic table. The atoms that have same number of protons and electrons but different number of neutrons are called isotopes. Since isotopes have different number of neutrons, they frequently have different physical properties. Many dating techniques involve measuring isotope quantities in a sample. (See radiometric dating)

Periodic Table

 * Main Article: Periodic Table of Elements

Elements are arranged into the periodic table based on properties they share in common. Each cell of the table contains a different element. However, the cells of most tables contain the name and symbol of each element, the atomic number (the number of protons), and the atomic mass (number of protons + neutrons). Some periodic tables will also identify the electron shell configuration of each element, and their state at room temperature (liquid, gas, or solid).

Electrons

 * Main Article: Electron

The electrons of atoms orbit at varying distances from the nucleus, which are known as shells or energy levels. The elements in the periodic table are arranged into rows (known as groups or families) based on how many shells of electron an atom possesses. They are also arranged into columns (known as periods) with other elements that have the same number of electron in the outer-shell, or valence. The electron configurations of atoms primarily determines their chemical reactivity, particularly the valence electrons. It is said that the fewer electrons the atom contains in its valence, the less stable it is and the more likely it is to react.

Size
The mass of an atom is determined by the number of protons and neutrons in the nucleus. The lightest element in existence is hydrogen, which has only one proton. The combined number of protons and neutrons possessed by an element is knows as its atomic mass. The average atomic mass of the elements on Earth can be found displayed in the periodic table. Unlike a proton, a neutron has no charge, but its mass is about the same as that of a proton. The mass of the proton or neutron is 1836 times bigger than that of the electron.

The size of the atoms is about 1~2 Å. Compared to the overall size of the atom, the nucleus is about the size of a raindrop in a playground. A nucleus’ volume is only 1014 that of the atom. Empty space takes up most of the space occupied by an atom.