Ions
In the previous section, we focused our attention on the electron configuration of neutral atoms.
In a neutral atom, the number of protons is the same as the number of electrons. But what
happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element?
A change in the number of electrons of an atom does not change the type of atom that it is. However, the charge of the atom will change. If electrons are added, then the atom will become more negative. If electrons are taken away, then the atom will become more positive. The atom that is formed in either of these cases is called an ion. Put simply, an ion is a charged atom.
Definition: Ion
An ion is a charged atom. A positively charged ion is called a cation e.g. Na+,
and a
negatively charged ion is called an anion e.g. F−. The charge on an ion depends
on the
number of electrons that have been lost or gained.
453
Look at the following examples. Notice the number of valence electrons in the neutral atom, the number of electrons that are lost or gained, and the final charge of the ion that is formed.
Lithium
A lithium atoms loses one electrons to form a positive ion (figure 3.11).
The arrangement of electrons in a lithium ion.
In this example, the lithium atom loses an electron to form the cation Li+. Fluorine
A fluorine atom gains one electron to form a negative ion
The arrangement of electrons in a fluorine ion.
Atomic structure
As a result of the models that we discussed in section 3.1, scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand things like why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom.
So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by one or more negatively charged electrons. These electrons orbit the nucleus.
The Electron
The electron is a very light particle. It has a mass of 9.11 x 10−31 kg. Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it can’t be broken down into anything smaller. The electron also carries one unit of negative electric charge which is the same as 1.6 x 10−19 C (Coulombs).
The Nucleus
Unlike the electron, the nucleus can be broken up into smaller building blocks called protons and neutrons. Together, the protons and neutrons are called nucleons.
The Proton
Each proton carries one unit of positive electric charge. Since we know that atoms are electrically neutral, i.e. do not carry any extra charge, then the number of protons in an atom has to be the same as the number of electrons to balance out the positive and negative charge to zero. The total positive charge of a nucleus is equal to
the number of protons in the nucleus. The proton is much heavier than the electron (10 000 times heavier!) and has a mass of 1.6726 x 10−27 kg.
When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons.
The Neutron
The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is 1.6749 x 10−27 kg (slightly heavier than the proton).
Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! Also, if protons were the only particles in the nucleus, then a helium nucleus (atomic number 2) would have two protons and therefore only twice the mass of hydrogen. However, it is actually four times heavier than hydrogen. This suggested that there must be something else inside the nucleus as well as the protons. To make sure that the atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass.
1.3 Unlike the electron which is thought to be a point particle and unable to be broken up into smaller pieces, the proton and neutron can be divided. Protons and neutrons are built up of smaller particles called quarks. The proton and neutron are made up of 3 quarks each.
Достарыңызбен бөлісу: |
