KS4 Chemistry. Atomic Structure. Contents. Atomic Structure. Introducing atoms. Atomic number and mass number. Electron configuration. Isotopes. Summary activities. Discovery of atomic structure. Atoms – the building blocks.
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Atomic number and mass number
All substances are made from very tiny particles called atoms.
John Dalton had ideas about the existence of atoms about 200 years ago but only relatively recently have special microscopes (called electron microscopes) been invented that can ‘see’ atoms.
The yellow blobs in this image are individual gold atoms, as seen through an electron microscope.
Elements are the simplest substances. There are about 100 different elements.
Each element is made up of just one particular type of atom, which is different to the atoms in any other element.
Copper is an
element made up of
copper atoms only.
Carbon is an
element made up of
carbon atoms only.
Atoms are extremely small – they are about 0.00000001cm wide.
To make an atom the size of a football it would have to be enlarged by about 3,000,000,000 times.
If a football was enlarged by the same amount it would stretch from the UK to the USA.
Where are the electrons and nucleus found in an atom?
millions of these atoms join to form each tiny grain of sand
A single grain of sand contains millions of atoms of silicon and oxygen.
Each atom must therefore have an extremely small mass.
Relative atomic mass = 12
Atoms are so small that their mass is not measured in grams but in atomic mass units.
The atoms of each type of element have a relative atomic mass (RAM).
The element carbon is the atom that the mass of all other atoms is compared to. Carbon has a RAM of 12.
12 atoms x 1= 1 atom x 12
1 atom x 24= 2 atoms x 12
The lightest atom is hydrogen. It has 1⁄12 the mass of carbon and so has a RAM of 1.
Magnesium is twice as heavy as carbon. It has a RAM of 24.
For some time people thought that atoms were the smallest particles and could not be broken into anything smaller.
Scientists now know that atoms are actually made from even smaller subatomic particles. There are three types:
Protons, neutrons and electrons are NOT evenly distributed in an atom.
The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus.
The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.
Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons.
The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.
There are two properties of subatomic particles that are especially important:
2. Electrical charge
The number of protons in an atom is known as its atomic numberor proton number.
It is the smaller of the two numbers shown in most periodic tables.
The atoms of any particular element always contain the same number of protons. For example:
What are the atomic numbers of these elements?
Each element has a definite and fixed number of protons. If the number of protons changes, then the atom becomes a different element.
Changes in the number of particles in the nucleus (protons or neutrons) is very rare. It only takes place in nuclear processes such as:
Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus.
The sum of the protons and neutrons in an atom’s nucleus is the mass number. It is the larger of the two numbers shown in most periodic tables.
Mass number = number of protons + number of neutrons
What is the mass number of these atoms?
= mass number - atomic number
Number of neutrons = mass number - number of protons
How many neutrons are there in these atoms?
Atomic number is defined as the number of protons rather than the number of electrons because atoms can lose or gain electrons but do not normally lose or gain protons.
Atoms have no overall electrical charge and are neutral. This means atoms must have an equal number of protons and electrons.
The number of electrons is therefore the same as the atomic number.
What are the missing numbers?
Electrons are not evenly spread but exist in layers called shells.
The arrangement of electrons in these shells is often called the electron configuration.
1st shell holdsa maximum of2 electrons
2nd shell holdsa maximum of8 electrons
3rd shell holdsa maximum of8 electrons
Each shell has a maximum number of electrons that it can hold. Electrons will fill the shells nearest the nucleus first.
The nucleus is:
Elements consist of one type of atom, but sometimes these atoms can be slightly different.
Although atoms of the same element always have the same number of protons, they may have different numbers of neutrons.
Atoms that differ in this way are called isotopes.
mass number is different
atomic number is the same
The isotopes of an element are virtually identical in their chemical reactions.
This is because they have the same number of protons and the same number of electrons.
The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.
Natural samples of elements are often a mixture of isotopes.
Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14.
Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist.
About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37.
What are the particle numbers in each isotope?
Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18.
To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass and add them together.
For example, chlorine exists as two isotopes:chlorine-35 (75%) and chlorine-37 (25%).
RAM of chlorine= (75% x 35) + (25% x 37)
= (0.75 x 35) + (0.25 x 37)
= 26.25 + 9.25
Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this.
RAM of bromine= (50.5% x 79) + (49.5% x 81)
= (0.505 x 79) + (0.495 x 81)
= 39.895 + 40.095
= 80 (the RAM is usually rounded to the nearest whole number)
Bromine contains 50.5% bromine-79 and 49.5% bromine-81.
What is the RAM of naturally-occurring bromine?
atom –The smallest particle that can exist on its own.
atomic number –The number of protons in the nucleus of an element, also known as the proton number.
electron –Negative particle that orbits the nucleus of an atom.
isotopes –Different atoms of the same element. They have the same number of protons and electrons, but a different number of neutrons.
nucleus –The dense positive centre of an atom, made up of protons and neutrons.
neutron –A neutral particle, with a mass of 1. It is found in the nucleus of an atom.
mass number –The number of protons and neutrons in an atom.
proton –A positive particle, with a mass of 1. It is found in the nucleus of an atom.
relative atomic mass (RAM) –The mass of an element compared to the mass of 1⁄12 of the mass of carbon-12.