An atom is a basic constituent of matter that consists of a positively-charged core (nucleus) surrounded by a cloud of negatively-charged electrons. By definition, atoms are neutral entities because the positive charge of the nucleus is cancelled by the negative charge of the electron cloud. However, the gain or loss of an electron can lead to the formation of an ion, also known as a charged atom.
The Charge of Elements
An element is an example of an atom with a fixed number of positive protons within the nucleus. For example, sodium is an element with 11 protons within the nucleus and 11 electrons. Another example of an element is carbon, which has six protons within the nucleus and six electrons. In both cases, these elements have a neutral charge. An atom becomes charged when the number of protons does not equal the number of electrons. For example, if an element has six protons but only five electrons, the net charge of the element is +1. Conversely, if an element has six protons but seven electrons, then the net charge of the element is -1. In reality, all elements are neutral in their natural state, and it is the gain or loss of electrons that determines their charge.
The Orbits of Electrons Around the Nucleus
The electrons that surround atoms can only sit in well-defined shells. Each shell can only hold a fixed number of electrons, and the atoms are more stable when these shells are filled. It is possible to predict which charge an atom will gain by looking at the how the electrons sit around the atom. The first shell of an atom can only hold two electrons, the second shell can hold eight electrons and the third shell can hold 16 electrons. If a shell is less than half full, then it is easier for an atom to lose electrons in order to become more stable. In this case, the atom becomes a positive ion. Alternatively, if a shell is more than half full, it is easier for an atom to gain electrons in order to become more stable. This leads to a negative ion.
Example -- Sodium
Sodium has 11 electrons that orbit the nucleus. The first two shells within sodium are full and only one electron occupies the third shell. Therefore, it is easier for sodium to lose an electron and become positive.