A Level Chemistry – Atomic Structure

THE STRUCTURE OF THE ATOM

 a) Protons, neutrons and electrons

 

Atoms are made up of three fundamental particles: protons, neutrons and electrons.

 

Protons and neutrons are found in the nucleus and are collectively called nucleons. Electrons orbit the nucleus in a similar way to that in which planets orbit a sun. In between the electrons and nucleus there is nothing (empty space).

The nucleus is very small; if an atom were the size of a football pitch, the nucleus would be the size of a drawing pin.

 

The basic properties of these three particles can be summarized in the following table:

 

Particle

 

ChargeMass
Proton+1 unitApprox 1 unit
NeutronNo chargeApprox 1 unit
Electron-1 unitApprox 1/1840 units (very small)

 

1 unit of charge is 1.602 x 10-19 coulombs. A proton is given a charge of +1 and an electron a charge of -1. All charges are measured in these units.

 

1 unit of mass is 1.661 x 10-27 kg. This is also not a convenient number, so we use “atomic mass units”.

Since the mass of protons and neutrons varies slightly depending on the nucleus, then in order to define an “atomic mass unit” we need to choose one nucleus as a standard. For this purpose 126C , or “carbon-12”, was chosen because its mass per nucleon

(1.661 x 10 –27 kg) is around average, which means all the other nuclei have masses close to whole numbers. An atomic mass unit is thus defined as 1/12th of the mass of one atom of carbon-12. Everything else is measured relative to this quantity.

 

b) Atomic numbers, mass numbers and isotopes

 

An atom is named after the number of protons in its nucleus. If the nucleus of an atom has 1 proton, it is hydrogen; if it has two protons, it is helium; if it has 3, it is lithium etc. The number of protons in the nucleus of an atom is called the atomic number. It has the symbol Z.

 

The atomic number is the number of protons in the nucleus of an atom

 

 

 

Not all atoms of the same element have equal numbers of neutrons; this may vary slightly. The sum of the number of protons and neutrons in the nucleus of an atom is called its mass number. It is represented by the symbol A.

 

The mass number is the sum of the number of protons and neutrons in the nucleus of an atom

 

 

The nucleus of an atom can thus be completely described by its mass number and its atomic number. It is generally represented as follows:

 

AZE

 

Eg. 94Be, 126C, 2412Mg

 

Atoms with the same atomic number but with different mass numbers (ie different numbers of neutrons) are called isotopes.

 

Isotopes are atoms with the same atomic number but with different mass numbers

 

Eg magnesium (atomic number 12) has 3 naturally occurring isotopes:

 

2412Mg: 12 protons, 12 neutrons

2512Mg: 12 protons, 13 neutrons

2612Mg: 12 protons, 14 neutrons

 

In a neutral atom, the number of protons and electrons are the same. However, many elements do not exist as neutral atoms, but exist as ions. Ions are species in which the proton and electron numbers are not the same, and hence have an overall positive or negative charge. The number of electrons in a species can be deduced from its charge:

 

Eg

2412Mg2+: 12p, 12n, 10e

2412Mg+: 12p, 12n, 11e

2412Mg 12p, 12n, 12e

2412Mg: 12p, 12n, 13e

 

Ions with a positive charge are called CATIONS

Ions with a negative charge are called ANIONS.

 

c) Relative atomic mass

 

The mass of an atom is measured in atomic mass units, where one unit is 12th of the mass of one atom of carbon-12.

 

The relative isotopic mass of an isotope is the ratio of the mass of one atom of that isotope to 1/12th of the mass of one atom of carbon-12.

 

It is usually very close to a whole number ratio:

 

Isotope

 

Mass numberRelative isotopic mass
11H11.006
42He44.003
94Be99.012
2713Al2726.919
5927Co5958.933

 

The masses of protons and neutrons vary slightly from isotope to isotope, so the relative isotopic mass is not exactly a whole number.

 

The relative atomic mass of an atom is the ratio of the average mass of one atom of that element to 1/12th of the mass of one atom of carbon-12.

 

The RAM is the average mass of all the isotopes, and is often not close to a whole number:

 

Element

 

Common mass numbersRelative atomic mass
Mg24, 25, 2624.32
Cl35, 3735.45
Br79, 8179.91
Ba134, 135, 136, 137, 138137.33

 

Some elements and compounds exist as molecules; these also have a characteristic mass:

 

The relative molecular mass of a molecule is the ratio of the average mass of that molecule to 1/12th of the mass of an atom of carbon-12.

 

The relative molecular mass of a molecule is the sum of the relative atomic masses of its constituent atoms.

 

Eg The relative molecular mass of CO2 is 12.0 + 16.0 + 16.0 = 44.0