A neutral atom is one that lacks an electrical charge due to a trade-off between the number of its protons and electrons. Both are electrically charged subatomic particles.
Protons are agglomerated with neutrons, and make up the nucleus; while the electrons are blurred defining an electronic cloud. When the number of protons in an atom, equal to its atomic number (Z), equals the number of electrons, it is said that there is a trade-off between the electrical charges within the atom.
Hydrogen atom. Source: Mets501 via Wikipedia.
For example, you have a hydrogen atom (upper image), which has a proton and an electron. The proton is positioned in the center of the atom as its nucleus, while the electron orbits the surrounding space, leaving regions of lower electron density as it moves away from the nucleus.
This is a neutral atom because Z is equal to the number of electrons (1p = 1e). If the H atom lost that single proton, the atomic radius would shrink and the charge of the proton would prevail, becoming the cation H + (hydron). If, on the other hand, it gained an electron, there would be two electrons and it would become the H - (hydride) anion.
Neutral atom vs ion
For the example of the neutral atom of H, it was found that the number of protons equals the number of electrons (1p = 1e); a situation that does not occur with ions derived from the loss or gain of an electron.
Ions are formed by a change in the number of electrons, either due to the atom gaining them (-) or losing them (+).
In the atom of the cation H + the valence charge of the single proton predominates over the total absence of an electron (1p> 0e). This is true of all the other heavier atoms (np> ne) in the periodic table.
Although the presence of a positive charge may seem insignificant, it diagonally changes the characteristics of the element in question.
On the other hand, in the atom of the anion H - the negative charge of the two electrons predominates against the only proton of the nucleus (1p <2e). Likewise, the other anions of greater mass have an excess of electrons compared to the number of protons (np
Na vs Na
A better known example is that of metallic sodium. Its neutral atom, Na, with Z = 11, has 11 protons; therefore there must be 11 electrons to compensate for the positive charges (11p = 11e).
Sodium, being a highly electropositive metallic element, very easily loses its electrons; in this case, it loses only one, that of its valence layer (11p> 10e). Thus, the Na + cation is formed, which electrostatically interacts with an anion; as the chloride, Cl -, in the sodium chloride salt, NaCl.
Metallic sodium is poisonous and corrosive, while its cation is even present within cells. This shows how the properties of an element can change dramatically when it gains or loses electrons.
On the other hand, the anion Na - (sodium, hypothetically) does not exist; and if it could be formed, it would be extremely reactive, since it goes against the chemical nature of sodium to gain electrons. Na - would have 12 electrons, exceeding the positive charge of its nucleus (11p <12e).
Neutral molecules
Atoms are covalently linked to give rise to molecules, which can also be called compounds. Within a molecule there cannot be isolated ions; instead, there are atoms with formal positive or negative charges. These charged atoms affect the net charge of the molecule, transforming it into a polyatomic ion.
For a molecule to be neutral, the sum of the formal charges of its atoms must be equal to zero; or, more simply, all its atoms are neutral. If the atoms that make up a molecule are neutral, this will be too.
For example, you have the water molecule, H 2 O. Its two H atoms are neutral, as is the oxygen atom. They cannot be represented in the same way as shown in the image of the hydrogen atom; since, although the nucleus does not change, the electronic cloud does.
The hydronium ion, H 3 O +, on the other hand, possesses a partially positively charged oxygen atom. This means that in this polyatomic ion it loses an electron, and therefore its number of protons is greater than that of its electrons.
Examples
Oxygen
The neutral oxygen atom has 8 protons and 8 electrons. When it gains two electrons, it forms what is known as an oxide anion, O 2-. In it, negative charges predominate, with an excess of two electrons (8p <10e).
Neutral oxygen atoms have a high tendency to react and bond with themselves to form O 2. It is for this reason that there are no "loose" O atoms out there on their own and without reacting with anything. All known reactions for this gas are attributed to molecular oxygen, O 2.
Copper
Copper has 29 protons and 29 electrons (in addition to neutrons). Unlike oxygen, its neutral atoms can be found in nature due to its metallic bond and relative stability.
Like sodium, it tends to lose electrons rather than gain them. Given its electronic configuration and for other aspects, it can lose one or two electrons, becoming cuprous cations, Cu +, or cupric, Cu 2+, respectively.
The Cu + cation has one less electron (29p <28e), and the Cu 2+ has lost two electrons (29p <27e).
Noble gases
The noble gases (He, Ne, Ar, Kr, Xe, Rn), are one of the few elements that exist in the form of their neutral atoms. Their atomic numbers are: 2, 10, 18, 36, 54, and 86, respectively. They do not gain or lose electrons; although, xenon, Xe, can form compounds with fluorine and lose electrons.
Metal alloys
Metals if they are protected from corrosion can keep their atoms neutral, joined by metallic bonds. In alloys, solid solutions of metals, the atoms remain (mostly) neutral. In brass, for example, there are neutral atoms of Cu and Zn.
References
- Jetser Carasco. (2016). What is a neutral atom? Recovered from: introduction-to-physics.com
- Markings, Samuel. (April 25, 2017). Non Neutral Atoms Examples. Sciencing. Recovered from: sciencing.com
- Chem4kids. (2018).Looking at Ions. Recovered from: chem4kids.com
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.