Inside the Atom
Introduction

You've probably heard the words 'proton', 'neutron', and 'electron' before, but do you know what they are? In this unit we will find out about each of these particles and where they are found inside an atom.

Protons, neutrons, and electrons
Inside an
atom
An atom is the smallest particle of an element that can still be defined as that element.
atom
we can find three types of particle. These are called protons, neutrons, and electrons.

The protons and neutrons are crammed into the centre of the atom, called the nucleus. This is where the mass of an atom is concentrated.

The electrons are found orbiting the
nucleus
The nucleus is the centre of an atom, containing protons and neutrons.
nucleus
in specific energy levels or shells.

Figure 1.   The structure of an atom.
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The table below shows the charge on the three sub-atomic particles:


Sub-atomic particle Charge
Proton +1
Neutron 0
Electron −1



Since, under normal conditions, the atom as a whole is electrically neutral, the number of electrons must equal the number of protons in the nucleus.

Match the sub-atomic particle to its charge:
  • Electron
    Proton
    Neutron
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Now let's think of the masses of the sub-atomic particles. Their masses, measured in grams or kilograms, are so small that it is easier to compare the masses with each other. Look at their relative masses below:


Sub-atomic particle Relative mass
Proton 1
Neutron 1
Electron 1/1840 (so small we can usually ignore their mass)



So protons and neutrons have the same mass (approximately), but an
electron
Electrons are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells).
electron
has a mass of only 1/1840 of a proton or neutron.

Where do we find most of the mass of an atom?
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Which two sub-atomic particles make up most of the mass of an atom?
  • Neutrons
    Electrons
    Protons
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Electronic structures
We have mentioned that the electrons don't just orbit the nucleus in a haphazard way. They occupy energy levels or shells at different distances from the centre of the atom. Electrons always occupy the lowest available energy level.

The lowest energy level (the one found nearest to the nucleus) can hold just two electrons. Some people refer to this as the first or innermost shell.

The second energy level can hold eight electrons, as can the third energy level (although this can hold an extra ten in reserve, but you don't need to learn about that unless you study chemistry at a higher level).

This arrangement is summarized in this table:


Energy level Number of electrons it holds
1st 2
2nd 8
3rd 8



The energy levels fill up with electrons from the lowest energy level (innermost shell) and build up outwards. They only start occupying a new energy level when the previous one has been filled.

Look at these examples below:

Figure 2.   Electrons in atoms.
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Now click on the green arrow button in the diagram in Fig.3 below to see how the energy levels are filled for the atoms of the first 20 elements:

Figure 3.   Energy levels filling up.
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Which statement about a sodium atom is true?
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We can represent the arrangement of electrons in an atom using a shorthand called the electronic structure or electronic configuration. This shows the numbers of electrons in each energy level, starting with the lowest level. So, for atoms of the elements below, we have the following electronic structures:

Helium 2
Carbon 2, 4
Sodium 2, 8, 1

The largest atom that you have to know the
electronic structure
The electronic structure is a description of the arrangement of the electrons in an atom, starting from the lowest energy level (shell). For example, the electronic structure of sodium is 2, 8, 1. (Also known as an electronic configuration.)
electronic structure
for is calcium.
It has 20 electrons, arranged 2, 8, 8, 2.

Oxygen has eight electrons in each atom. What is the electronic structure of an atom of oxygen?
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Chlorine atoms have 17 electrons. What is the electronic structure of an atom of chlorine?
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Summary


The mass of an atom is concentrated in its nucleus because this is where we find the heavy particles, called protons and neutrons.

The much lighter electrons orbit the nucleus in energy levels or shells.

The lowest energy level (first or innermost shell) can hold 2 electrons, the second energy level (shell) can hold 8 electrons and the third energy level (shell) holds 8 electrons (with 10 in reserve).

You can see the electronic structures for the first 20 elements by clicking on the green arrow button in the diagram below:

Figure 4.   Electronic structures (configurations) of the first 20 elements.
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Exercises


For the electronic structures of the following atoms, answer the questions below:

Figure 5.  
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1. Which represents a metal?
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2. Which represents lithium?
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3. Which represents oxygen?
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4. Which atom contains eight protons?
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5. Which atom has six electrons in its outer shell?
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6. Which atom could lose one electron and be left with a complete (full) outer shell?
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7. What is the electronic structure of an atom of lithium?
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8. What is the electronic structure of an atom of neon? (Neon atoms have ten electrons.)
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9. What is the electronic structure of an atom of phosphorus (15 electrons)?
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10. What is the electronic structure of an atom of potassium (19 electrons)?
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