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2. Atoms are found throughout the universe
Atoms are found throughout the universe. They are minute particles of matter, and the atoms of each element are different from those of any other elements.
Because atoms are so tiny, it is impossible to see an individual atom in sufficient detail to describe it’s structure, but over the last hundred or so years, scientific experiments have led us to a model of the atom which is widely accepted by modern scientists.
Early scientists described an atom as a solid sphere which could not be sub-divided. Our present model describes the atom as a combination of other, even smaller particles called sub-atomic particles.
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3. Atoms are composed of... Electrons Neutrons Protons = Neutron = Proton
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4. Atoms are composed of...
The protons and neutrons are found in the centre or nucleus. The electrons move around the nucleus in zones called orbitals.
= Neutron
= Proton
= Electron
N.B. This diagram represents a Lithium atom. In real atoms, the electrons are very much smaller than the neutrons or protons.
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5. Electrons tend to stay in a particular orbital.
Some orbitals are near the nucleus and some are further away. The structure of large atoms with many protons, neutrons and electrons becomes complicated, as shown in this picture.
The orbitals further from the nucleus tend to be able to hold more electrons than those nearer the nucleus. The smallest orbital near the nucleus can only hold two electrons.
More distant orbitals can hold 8, 18 or even 32 electrons.
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6. The atomic number is the number of protons in an atom.
To describe an atom fully, we need to know the numbers of each type of sub-atomic particle in the atom.
We use these numbers to generate the atomic number and the mass number of the atom.
The atomic number is the number of protons in an atom.
The mass number is the sum of protons and neutrons in the atom.
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7. Li Mass Number 7 Atomic Symbol Atomic Number 3
An atom of Lithium (Li) can be represented as follows:
Mass Number 7 Li
Atomic Symbol
Atomic Number 3
This shows that a Lithium (Li) atom has three protons, and four neutrons.
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8. Li The Lithium Atom Mass Number 7 Atomic Number 3 = Neutrons = Protons
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= Neutrons
= Protons
= Electrons
4 x
3 x
3 x

9. C The Carbon Atom Mass Number 12 Atomic Number 6 = Neutrons = Protons
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= Neutrons
= Protons
= Electrons
6 x
6 x
6 x

10. Protons are positively charged. Neutrons are neutral.
Sub-atomic particles are electrically charged. These charges are important for the structure and stability of the atom.
Protons are positively charged.
Neutrons are neutral.
Electrons are negatively charged.
The charges on the proton and electron are opposite but equal in magnitude, and therefore they cancel one another exactly.
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11. Calculating the Electrical Charge7
Mass Number Li 3
Atomic Number
= 4 Neutrons = 4 x 0 charge = 0
= 3 Protons = 3 x +1 charge = +3
= 3 Electrons = 3 x -1charge = -3
Total Charge = = 0
All atoms have an overall neutral electrical charge.
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12. For each element in turn, calculate the missing entries...
Atom
Symbol
Atomic No.
Mass No.
Protons
Neutrons
Electrons
Hydrogen H 1
Helium 2 4
Carbon 6 12
Chlorine 17 37
Calcium 20
Copper 64 29
Zinc 30 65
Tin 50
Tungsten 74
184
Radon 86
136 1 1 He 2 2 2 C 6 6 6 Cl 17 20 17 Ca 40 20 20 20 Cu 29 35 29 Zn 30 35 30 Sn
119 50 69 50 W 74
110 74 Rn
222 86 86
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13. Atoms are tiny. A speck of dust contains billions of atoms.
The mass of an atom is very small and is not usually expressed in grams. One gram of hydrogen atoms will contain more than 600,000 million, million, million atoms!!!
The following table shows some of the details of sub-atomic particles, which you need to know:-
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14. Some Details of Sub-atomic Particles
Position
Mass
Charge
Proton
Nucleus 1 +1
Neutron
Nucleus 1
Electron
Orbit
1/1840 -1
For each particle in turn, decide what the missing entries should be.
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15. The Atomic Nucleus
So far, the nucleus has been shown as a static body, whose sub-atomic particles fill the space of the nucleus. In fact, the nuclear particles are in constant motion.
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16. The Atomic Nucleus
The nucleus of an atom is tiny compared to the volume occupied by the electrons of the atom.
If an atom could be enlarged to the size of a football stadium, the nucleus of the atom would be about the size of a golf ball, somewhere above the centre spot.
An electron has an incredibly small mass, which is about 1/2000 the mass of a neutron or a proton. Therefore, electrons are not considered to contribute anything to the mass of an atom. Hence, the vast majority of the mass of an atom is found in the nucleus.
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17. Electrons
Electrons possess properties typical of particles and therefore in this presentation, electrons have been depicted as particles orbiting the nucleus of the atom.
However, electrons also possess properties typical of wave energy.
Science is still undecided as to the exact nature of the electron.
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18. All Lithium atoms have three protons.
All of the substances on Earth are built from atoms containing the three sub-atomic particles.
Elements differ from one another in that their atoms contain different numbers of the sub-atomic particles. The number of protons (or the atomic number) of an atom, is the factor which decides the identity (element) of an atom.
All Lithium atoms have three protons.
All Carbon atoms have six protons.
All Uranium atoms have ninety two protons.
An atom is electrically neutral, and therefore any atom always has an equal number of protons and electrons.
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19. The mass of an atom is mainly in the nucleus
The mass of an atom is mainly in the nucleus. Protons and neutrons have an approximately equal mass of one atomic mass unit. Electrons have negligible mass. Therefore, the mass of an atom is equal to the sum of the number of protons and neutrons. This number is called the mass number of the atom.
For example, carbon atoms each have six protons and six neutrons, and therefore the atomic mass is twelve atomic mass units. The mass number of the carbon atom is 12.
N.B. Some atoms of carbon have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called Isotopes of that element.
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20. Summary 1. Atoms are tiny and are in constant motion.
2. Atoms are composed of particles known as protons, neutrons and electrons.
3. Atoms have a small, massive nucleus, which is surrounded by orbiting electrons.
4. The number of particles an atom contains determines the identity of the atom.
5. The atomic number of an atom is equal to the number of protons in its nucleus.
6. The mass number of an atom is equal to the number of protons plus neutrons in its nucleus.
7. Atoms have the same number of protons as electrons, so their overall charge is neutral.
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