1. Atoms Atoms are the building blocks for the whole Universe.
Physical substances are composed of matter.
Matter is composed of elements, which are made from small particles called atoms.
Atoms are the smallest distinguishable part of an element. Every element, such as hydrogen, carbon or gold, is composed of just one type of atom.
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2. Atoms Atoms are NOT solid spheres of matter as was once thought.
Carbon atom (C)
Although atoms are the smallest distinguishable unit of an element, they are made up of three different sub-atomic particles:
Protons
Neutrons
Electrons
Proton
Neutron
Electron
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3. Question 1
What is the name given to the smallest distinguishable unit of an element?
A) Atom
B) Molecule
C) Particle
D) Matter

4. Nucleus (consisting of protons and neutrons)
Atoms
Protons and neutrons are found in the middle of the atom, the nucleus. They cluster together forming nucleons.
Electron
Nucleus (consisting of protons and neutrons)
Carbon atom (C)
Electrons are much smaller than the nucleons, and orbit (rapidly move around) the nucleus at a considerable distance from it.
The rest of the atom is empty space.
For example, if an atom was a soccer field, then the nucleus would be about the size of a soccer ball in the center of the field.
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5. Question 2
Which of the following is NOT found in the nucleus of any atom?
A) Proton
B) Neutron
C) Nucleon
D) Electron

6. Particle Charges
Protons, neutrons and electrons have different relative electrical charges and masses.
Particle Name
Relative Mass
Relative Charge
Proton 1
Neutron
Electron
1/1836 -1
Nearly all the mass of an atom is in the positively charged nucleus.
The mass of electrons is negligible compared with that of the protons and neutrons.
It is the attractions between the positively charged nucleus and the negative charged electrons that holds the electrons orbiting (moving) around the nucleus.
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7. Atomic Number
The number of protons in an atom is very important because all atoms of the same element have the same number of protons.
One
electron
Direction of electron orbit
One proton in the nucleus
Hydrogen atom (H)
For example, the simplest atom, hydrogen (H) always has one proton and an atom of carbon (C) always has six protons.
The atomic number is equal to the number of protons in an atom's nucleus. The atomic number determines which element an atom is.
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8. Mass Number
The mass of an atom is the total number of protons and neutrons (nucleons) it contains.
Magnesium (Mg)
12 protons and 12 neutrons,
8 protons and 8 neutrons,
Atomic Mass number = 24
Mass number = 16
Oxygen (O)
Electrons are not counted, as they are so light compared to the nucleons that they make very little difference to the overall mass.
The number of neutrons within an atom can therefore be found by subtracting the atomic number from the mass number.
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9. Isotopes
Atoms of an element always have the same number of protons, but may have different numbers of neutrons.
Carbon-12
Mass number = 12
stable
Carbon-13
Mass number = 13
Carbon-14
Mass number = 14
unstable (radioactive)
Atoms of the same element with different numbers of neutrons, and therefore different atomic masses, are called isotopes.
Many of the most common and important elements have several naturally occurring isotopes.
Carbon (C), which is found in all living things, has three isotopes: carbon-12, carbon-13 and carbon-14.
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10. He Relative Atomic Mass Helium
By finding out the proportions of all the isotopes for an element, it is possible to find an average, known as the relative atomic mass (Ar).
Element 2 He
Helium
4.003
Atomic Number
Symbol
Name
Relative
Atomic Mass (Ar)
This is the mean weight for an atom of any given element.
The chemical symbol, atomic number and relative atomic mass of any element can be found on a special chart known as ‘The Periodic Table’.
Each cell on the table shows important information about an element.
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11. Electron Structure
All single atoms of elements have the same number of protons (charge +1) as electrons (charge -1), so their net charge is zero.
Electron
Proton
Neutron
Neon (Ne) atom
The electronic structure, number and arrangement of electrons within an atom, determines the properties of an atom.
All chemical reactions are based around changes to the number and location of the electrons orbiting the nucleus.
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12. Question 3
''Isotopes are atoms of the same element containing the same number of protons and electrons but different numbers of neutrons.'' Is this statement true or false?
Answer True or False.

13. Electron Structure
Electrons move rapidly around the nucleus (orbit) in distinct energy levels (shells).
Nickel (Ni) atom
Energy levels (shells)
Each electron in an atom is in a particular energy level (or shell) and the electrons must occupy the lowest available energy level (or shell) available nearest the nucleus.
Increasing energy of energy levels
When a shell is full the next electron goes into the next lowest available energy level.
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14. Electron Structure
1st shell = 2 electrons
Each energy level is capable of holding only a certain number of electrons.
2nd shell = 8 electrons
The first energy level can hold only two electrons. This energy level is filled first.
The second energy level can hold eight electrons. This energy level is filled after the first energy level is complete (full).
Outer shells
There are further energy levels, each containing larger numbers of electrons. We will refer to these as outer shells.
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15. Question 4
Electrons move rapidly around the nucleus in distinct orbits (energy levels). The second energy level can hold a maximum of....
A) one electron.
B) ten electrons.
C) eight electrons.
D) eighteen electrons.