Sub-atomic Particles in an Atom
Identify sub-atomic particles in an atom
Dalton thought that atoms were solid, indivisible particles. But, as a result of work done mainly by Lord Rutherford, the idea has been greatly changed in recent years. According to Rutherford, the atom consists of 3 kinds of particles – protons, neutrons and electrons. These are called sub-atomic particles.
The centre of the atom is called nucleus. The nucleus contains a cluster of two sorts of particles, protons and neutrons. Thenucleus is very small, occupying only about 1% of the volume of an atom. The rest of the atom is mostly empty space, withelectrons spread out in it.
Electrons move around the nucleus in special paths calledelectron shells (orbits/or orbitals or energy levels). Protons andelectrons have electric charges. Neutrons have no charges.All the particles in an atom are very light. Their masses aremeasured in atomic mass units rather than grams.
The proton isa positively charged particle. Its mass is about equal to that ofhydrogen atom. The neutron is has no charge, it is neutral. Itsmass is about equal to that of hydrogen atom. The electron isnegatively charged. Its charge is equal but opposite to the chargeon the proton. It has a very small mass, about 1⁄1840 of the massof the proton.
The Properties of each Particle in an Atom
Explain the properties of each particle in an atom
The properties of these particles are summarizedin the table below:
Table 5.1: Properties of sub-atomic particles
A single atom is electrically neutral (it has no electrical charge).This means that in any atom there must be equal numbers of protons and electrons. In this way, the total positive charge on the protons is balanced by the total negative charge on the electrons orbiting the nucleus. So, the charges must cancel.
Electronic arrangement refers to the manner in which electronsare arranged in an atom. An atom contains a central nucleuscontaining protons and neutrons, and a cluster of electronsrevolving in orbits around the nucleus. These electrons aregrouped in shells.
A Maximum Number of Electrons in the Shells
Determine a maximum number of electrons in the shells
Bohr (1913) put forward a theory of electron positioning whichis still generally accepted and used until now for chemicalpurposes. Bohr’s Theory on the arrangement of electrons in anatom can be summarized as follows:
- Electrons are in orbit around the nucleus of the atom.
- The electron orbits are grouped together in shells; a shellis a group of orbits occupied by electrons withapproximately equal energy.
- The electrons in shells distant from the nucleus havehigher energy than those in shells close to the nucleus.
- Electrons fill the shells starting with the first shell, whichis closest to the nucleus. Shells are numbered 1, 2, 3, 4,98etc. outwards from the nucleus. The shells may berepresented by the letters K, L, M and N respectivelystarting from the nucleus.
- The maximum possible number of electrons in a shellnumbered n is 2 2n .
- The first shell can only contain up to 2 electrons. Thesecond shell can contain a maximum of 8 electrons. Thethird shell can contain up to 18 electrons.
- In the outermost shell of any atom, the maximumnumber of electrons possible is 8.
- The outer electrons of some atoms can be removed fairlyeasily to form ions.
- Chemical bonding between atoms to form moleculesinvolves the electrons in the outer shell only.
Electronic arrangement of some typical atoms is shown in figure5.2.
The arrangement of electrons around the nucleus is also knownas electronic configuration. This arrangement depends on themaximum number of electrons that can occupy a shell. An atomwith 13 electrons will have the following electronicconfiguration: 2:8:3. This means that there are 2 electrons in thefirst shell, 8 electrons in the second shell and 3 electrons in the third shell.
The number and arrangement of electrons in the atoms of the first 20 elements are shown in table 5.2.
Table 5.2: The electron arrangements of the first 20 elements
After the first 20 elements, the organization of the electrons becomes increasingly complicated. The third shell (n = 3) can be occupied by a maximum number of 18 electrons
At this stage, you will not be asked to work out electronarrangements beyond element 20 (calcium), but you should beable to understand the electronic structures involving moreelectrons (for example bromine with the arrangement 2:8:18:7).
Energy Shell Diagrams
Draw energy shell diagrams
Energy Shell Diagram
Relationship between Atomic Number and Number of Protons
Relate atomic number with number of protons
All atoms of one element have the same number of protons. Thisis called the atomic number (or proton number) of that element.It is given by the symbol Z.
No two elements can have the same atomic number. Sodiumatoms have 11 protons. This is what makes them different fromall other atoms. Only sodium atoms have 11 protons, and anyatom with 11 protons must be sodium atom.
In the same way, an atom with 6 protons must be carbon atom.Also any atom with 7 protons must be nitrogen atom. So, youidentify an atom by the number of protons in it. There are 109elements altogether. Of these, hydrogen has the smallest atoms,with only 1 proton each. Helium atoms have 2 protons each.Lithium atoms have 3 protons each, and so on up to meitneriumatoms, which have 109 protons each. Table 5.3 shows the first20 elements arranged according to the number of protons theyhave.
Every atom has an equal number of protons and electrons, so theatomic number also tells us the number of electrons in that atom.In any given atom of an element, the number of neutrons has no effect on the identity and properties of that particular element. It is the number of protons and electrons that determine the identity and properties of any given element. The number of neutrons only affects the mass, since each one of them has the same mass as that of a proton.
Mass Number of an Atom from Numbers of Protons and Neutrons
Calculate mass number of an atom from numbers of protons and neutrons
Protons alone do not make up all the mass of an atom. The neutrons in the nucleus also contribute to the total mass. The mass of the electrons can be regarded as so small that it can be ignored. As a proton and a neutron have the same mass, the mass of a particular atom depends on the total number of protons and neutrons present. This is called mass number (or nucleon number). The mass number of an atom is found by adding together the number of protons and neutrons. It is given by the symbol A. Table 5.3 shows the mass number of the first 20 elements, arranged in order of increasing atomic mass (mass number).
If the mass number and atomic number for any given atom are known, then its sub-atomic composition can be worked out.
The mass number = number of protons + neutrons in an atom. Sodium atom has 11 protons and 12 neutrons, so the mass number of sodium is 23. Since the atomic number is the number of protons only, then:
Mass number – atomic number = number of neutrons. So, for sodium atom, the number of neutrons = (23-11) =12. You can also take into account the fact that, because the number of protons is always equal to the number of electrons, then the number of electrons in sodium atom is simply 11. The same rule can be applied to work out the sub-atomic composition of any element.
These two relationships are useful:
- Number of electrons = number of protons = atomic number
- Number of neutrons = mass number (A) – atomic number (Z).
The Concept of Isotope
Explain the concept of isotope
Atoms of the same element may have different numbers of neutrons. In a normal situation, atoms of the same element will have the same number of neutrons. However, many cases occur in which two atoms of the same element contain the same number of protons but different numbers of neutrons. Having equal number of protons, these atoms must also have equal numbers of electrons. However, the differing numbers of neutrons cause the atoms to have different mass numbers. An element showing such properties is said to show isotopy and the varieties of the atom are called isotopes of the element.
Therefore, isotopy can be defined as the tendency of atoms of one element to posses the same atomic number but different mass numbers (atomic masses). Isotopes can be defined as atoms of the same element with the same number of protons but different numbers of neutrons, or as „atoms of the same element with the same atomic number but different atomic masses‟.
The isotopes of an element have the same chemical properties because they contain the same number of electrons. It is the number of electrons in an atom that decides the way in which it forms bonds and reacts with other atoms. However, some physical properties of the isotopes are different. The masses of the atoms differ, and therefore other properties, such as density and rate of diffusion, also vary.
Many isotopes (like tritium) are unstable. The extra neutrons in their nuclei cause them to be unstable so that nuclei break spontaneously (that is, without any extra energy being supplied), emitting certain types of radiation. They are known as radioisotopes.
Notation for isotopes
In order to distinguish between different isotopes of the same element in writing symbols and formulae, a simple system is adopted. The isotope of an element, say X will have the symbol X,AZ , where A is the mass number of the isotope and Z is the atomic number of any atom of X. Thus, for all isotopes of one element, Z is constant, and A varies because there are different numbers of neutrons in the different isotopes of the element. For example, the three isotopes of carbon are expressed as 12C6, 13C6,and 14C6. Chlorine has two isotopes: 35Cl17 and 37Cl17 . Since A represents the total number of neutrons and protons in the nucleus of an atom (mass number/atomic mass), and because Z is the number of protons (atomic number), then the number ofneutrons in the nucleus of a given isotope is given by:Number of neutrons in the nucleus = A – Z
Relative atomic masses
As we have seen, most elements exist naturally as isotopes.Therefore, the value we use for the atomic mass of an element isan average mass. This takes into account the proportions(abundance) of all the naturally occurring isotopes. If aparticular isotope is present in high proportion, it will make a large contribution to the average.
Example
A sample of chlorine gas contains 75% of the isotope 35Cl17 and 25% of the other isotope 37Cl17 . What is the relative atomic mass of chlorine?
Solution
To work out this problem, simply multiply the mass number of each isotope with the abundance and sum up the products thus:
This average value for the masses of atoms of an element is known as the relative atomic mass (Ar).Therefore, the relative atomic mass of chlorine is 35.5 (i.e., Ar =35.5).
