Tag: Atomic structure

MONOATOMIC IONS

An ion is a chemical species with a net electrical charge. It can be composed of a single atom (monoatomic ion) or a group of atoms (polyatomic ion).

When the net charge is positive, the ion is called an ANION. In the presence of a net positive charge, the ion is called a CATION.

Example:

  • Na+; Cl–; Ca2+; O2- (monoatomic ions).
  • NH4+, VO2+, CO32-, SO42-, PO43- (polyatomic ions).

CATIONS

Let’s focus on the following monoatomic cation:

The positive charge +1 indicates that the number of protons exceeds the number of electrons by one.

This is obtained by subtracting one electron from the neutral atom, NOT by adding one proton.

Li+   3 protons; 2 electrons.

From the current information, it’s not possible to determine the number of neutrons.

(If the calculation of subatomic particles is unclear, click here).   

                                 

A positive ion, obtained by the removal of one or more electrons, is called a CATION

ANIONS

Let’s focus on the anion F– obtained by the following neutral element:

Fluorine is composed of 9 protons, 10 neutrons, 9 electrons. The addition of one electron, results in the anion F– . 

F–      9 protons; 10 neutrons 10 electrons                             

A negative ion, obtained by the addition of one or more electons, is called an anion.

A mistake that must be absolutely avoided, is adding or subtracting a proton instead of an electron. This mistake results in a change of the atomic number.

KEY POINTS:

  • A monoatomic cation is a positive ion, composed of a single atom, obtained by the removal of one or more electrons from a neutral element.
  • A monoatomic anion is a negative ion, composed of a single atom, obtained by the addition of one or more electrons from a neutral element.

SOLVED EXERCISE:

Calculate the number of protons, neutrons and electrons of the ion S2- from the following neutral atom:

The atomic structure of the neutral atom is composed of 16 protons, 16 neutrons e 16 electrons;

It’s necessary to add or subtract a number of electrons consistent with the charge of the ion;

Therefore, the structure of the ion is made up of di 16 protons, 16 neutrons e 18 electrons.

THE ATOMIC STRUCTURE

An atom is made up of subatomic particles called protons, neutrons and electrons.

  • Protons are positively charged particles;
  • Neutrons have no electrical charge;
  • Electrons are negatively charged particles.

As shown in Figure 1 below, an atom consists of an inner nucleus with protons and neutrons with electrons located outside the nucleus.

Figure 1 – Representation of the atomic structure.

All the positive charge is concentrated in the center of the atom, while the negative charge is distributed externally. The role of the neutron is to increase the distance between the protons and prevent proton-proton repulsive interactions from destabilizing the entire structure.

The masses of subatomic particles are extremely small.

  • Proton mass: 1.673×10−27 kg
  • Neutron mass: 1,675×10−27 kg
  • Electron mass: 9,109×10−31 kg

The mass of the electron is nearly 2,000 times smaller than that of the proton and neutron, which is why it can be considered negligible.

Even the charges of subatomic particles are significantly small.

  • Proton charge: + 1.602×10−19 C
  • Neutron charge: 0 C
  • Electron charge: – 1.602×10−19 C

The charge of a proton is equal to the charge of an electron, though with the opposite sign. The overall charge of the atom is neutral, meaning the positive charge of the nucleus is balanced by the negative charge of the electrons.

Since the charge of a proton is equal to the charge of an electron, it’s easy to deduce:

IN AN ATOM THE NUMBER OF PROTONS IS EQUAL TO THE NUMBER OF ELECTRONS.

ATOMIC NUMBER AND MASS NUMBER

Atoms can be identified by two types of numbers:

  • ATOMIC NUMBER (Z)  which represents the number of protons;
  • MASS NUMBER (A) which represents the sum of protons and neutrons.

The difference between the atomic number (Z) and the mass number (A) gives the number of neutrons.

As shown in Figure 2, a chemical element is described by two numbers: the atomic number (Z), which is listed in the lower left, and the mass number (A), which is listed in the upper left.

Figure 2 – Representation of a chemical element.

KEY POINTS:

  • MASS NUMBER (protons + neutrons) is always listed in the upper left;
  • ATOMIC NUMBER (protons) is always listed in the lower left;
  • In an atom the number of protons is equal to the number of electrons;
  • The number of neutrons is obtained by subtracting the atomic number from the mass number.

SOLVED EXERCISE:

Find the number of protons, neutrons, and electrons in the following atom:

  • Atomic number, reported in the lower left, represents the number of protons. p+= 6
  • The number of electrons is equal to the number of protons. e–= 6
  • (Mass number – Atomic Number)= Number of neutrons. n = 12-6 = 6

SOLUTION:

p+= 6;

n = 6;

e–= 6