The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element: Its value determines the identity of the atom.  For example, any atom that contains six protons is the element carbon and has the atomic number 6, regardless of how many neutrons or electrons it may have.  A neutral atom must contain the same number of positive and negative charges, so the number of protons equals the number of electrons.  Therefore, the atomic number also indicates the number of electrons in an atom.  The total number of protons and neutrons in an atom is called its mass number (A). The number of neutrons is therefore the difference between the mass number and the atomic number: A – Z = number of neutrons.

Atoms are electrically neutral if they contain the same number of positively charged protons and negatively charged electrons.  When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an ion. The charge of an atom is defined as follows:

Atomic charge = number of protons − number of electrons

As will be discussed in more detail later in this chapter, atoms (and molecules) typically acquire charge by gaining or losing electrons.  An atom that gains one or more electrons will exhibit a negative charge and is called an anion.  Positively charged atoms called cations are formed when an atom loses one or more electrons.  For example, a neutral sodium atom (Z = 11) has 11 electrons.  If this atom loses one electron, it will become a cation with a 1+ charge (11 − 10 = 1+).  A neutral oxygen atom (Z = 8) has eight electrons, and if it gains two electrons it will become an anion with a 2− charge (8 − 10 = 2−).

TRY THIS OUT!

https://phet.colorado.edu/en/simulation/build-an-atom

1.Go to the above activity and click on the first option “Atom”. Then hide the mass number box and net charge box on the screen. Also choose the orbit model.

2.Drag and drop 7 protons and 7 electrons from the bucket and put in in the nucleus ( “X “ marked ) and dotted circles respectively.  Can you predict which element is this?

3. Now check the boxes Element, Neutral/Ion and Stable/Unstable.  You should see like the figure below.

4. What are the atomic number of the element?

Atomic #:

5. Why the atom is unstable? Add neutron one by one to the nucleus and until it becomes stable. Write the mass number of the element. Click on the box “Mass number” and compare to your values.

Mass #:

6. Now take one electrons out from the atom. Why it is called “Ion”?. How many protons and electrons are now present in the atom? Is it positive or negative ion? Predict the charge of the ion. Then open the “Net Charge” box and compare to your results

Charge:

7. Add one more electron to the neutral atom. How many protons and electrons are now present in the atom? Is it positive or negative ion? Predict the charge of the ion.

Charge: 

8. Add some more neutrons and observe the simulated picture. Can you predict when the atom will become unstable?

Example 3: Composition of an atom

Iodine is an essential trace element in our diet; it is needed to produce thyroid hormone. Insufficient iodine in the diet can lead to the development of a goiter, an enlargement of the thyroid gland.

(a) Insufficient iodine in the diet can cause an enlargement of the thyroid gland called a goiter. (b) The addition of small amounts of iodine to salt, which prevents the formation of goiters, has helped eliminate this concern in the US where salt consumption is high. 

Figure 2.11 Iodine, An Element Present in Human Body

Ref: www.openstax.org/

The addition of small amounts of iodine to table salt (iodized salt) has essentially eliminated this health concern in the United States, but as much as 40% of the world’s population is still at risk of iodine deficiency. The iodine atoms are added as anions, and each has a 1− charge and a mass number of 127. Determine the numbers of protons, neutrons, and electrons in one of these iodine anions.

Solution

The atomic number of iodine (53) tells us that a neutral iodine atom contains 53 protons in its nucleus and 53 electrons outside its nucleus. Because the sum of the numbers of protons and neutrons equals the mass number, 127, the number of neutrons is 74 (127 − 53 = 74). Since the iodine is added as a 1− anion, the number of electrons is 54 [53 – (1–) = 54].

Chemical Symbols

A chemical symbol is an abbreviation that we use to indicate an element or an atom of an element.  For example, the symbol for mercury is Hg. We use the same symbol to indicate one atom of mercury (microscopic domain) or to label a container of many atoms of the element mercury (macroscopic domain).

The symbol Hg represents the element mercury regardless of the amount; it could represent one atom of mercury or a large amount of mercury.

Figure 2.11.  The element of mercury, its chemical symbol is Hg.

Ref: www.openstax.org/

Some symbols are derived from the common name of the element; others are abbreviations of the name in another language.  Most symbols have one or two letters, but three-letter symbols have been used to describe some elements that have atomic numbers greater than 112. To avoid confusion with other notations, only the first letter of a symbol is capitalized.  For example, Co is the symbol for the element cobalt, but CO is the notation for the compound carbon monoxide, which contains atoms of the elements carbon (C) and oxygen (O). All known elements and their symbols are in the periodic table.

Table 2. Common elements and their symbols.

Traditionally, the discoverer (or discoverers) of a new element names the element.  However, until the name is recognized by the International Union of Pure and Applied Chemistry (IUPAC), the recommended name of the new element is based on the Latin word(s) for its atomic number. For example, element 106 was called unnilhexium (Unh), element 107 was called unnilseptium (Uns), and element 108 was called unniloctium (Uno) for several years.  These elements are now named after scientists (or occasionally locations); for example, element 106 is now known as seaborgium(Sg) in honor of Glenn Seaborg, a Nobel Prize winner who was active in the discovery of several heavy elements.

The atomic number is the nuclear charge, and thus the number of electrons in the neutral atom

What single parameter uniquely characterizes the atom of a given element? It is not the atom’s relative mass, as we will see in the section on isotopes below. It is, rather, the number of protons in the nucleus, which we call the atomic number and denote by the symbol Z. Each proton carries an electric charge of +1, so the atomic number also specifies the electric charge of the nucleus. In the neutral atom, the Z protons within the nucleus are balanced by Z electrons outside it.

Moseley searched for a measurable property of each element that increases linearly with atomic number. He found this in a class of X-rays emitted by an element when it is bombarded with electrons. The frequencies of these X-rays are unique to each element, and they increase uniformly in successive elements. Mosely found that the square roots of these frequencies give a straight line when plotted against Z; this enabled him to sort the elements in order of increasing atomic number.

See this example of his experimental plots.

Atomic numbers were first worked out in 1913 by Henry Moseley, a young member of Rutherford’s research group in Manchester.

You can think of the atomic number as a kind of serial number of an element, commencing at 1 for hydrogen and increasing by one for each successive element. The chemical name of the element and its symbol are uniquely tied to the atomic number; thus the symbol “Sr” stands for strontium, whose atoms all have Z = 38.

Mass number

This is just the sum of the numbers of protons and neutrons in the nucleus. It is sometimes represented by the symbol A, so

A = Z + N

in which Z is the atomic number and N is the neutron number.

Nuclides and their symbols

Because it is not always easy to display a subscript directly beneath a superscript, it is not uncommon to use constructions such as 12Mg26 , which will often be our practice in this document when it is necessary to show both Z and A explicitly.

The symbol for a specific isotope of any element is written by placing the mass number as a superscript to the left of the element symbol. The atomic number is sometimes written as a subscript preceding the symbol, but since this number defines the element’s identity, as does its symbol, it is often omitted.  For example, magnesium exists as a mixture of three isotopes, each with an atomic number of 12 and with mass numbers of 24, 25, and 26, respectively.  These isotopes can be identified as 24Mg, 25Mg, and 26Mg.  These isotope symbols are read as “element, mass number” and can be symbolized consistent with this reading.  For instance, 24Mg is read as “magnesium 24,” and can be written as “magnesium-24” or “Mg-24.” 25Mg is read as “magnesium 25,” and can be written as “magnesium-25” or “Mg-25.” All magnesium atoms have 12 protons in their nucleus. They differ only because a 24Mg atom has 12 neutrons in its nucleus, a 25Mg atom has 13 neutrons, and a 26Mg has 14 neutrons.

The symbol for an atom indicates the element via its usual two-letter symbol, the mass number as a left superscript, the atomic number as a left subscript (sometimes omitted), and the charge as a right superscript.

Figure 12. Chemical symbols for atoms.