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## Structure of the atom

As a result of the work done by previous scientists on atomic models, scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom (what the atom looks like inside).

The nucleus

So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by one or more negatively charged electrons. These electrons orbit the nucleus.

Before we look at some useful concepts we first need to understand what electrons, protons and neutrons are.

## The electron

The electron is a very tiny particle. It has a mass of 9,11 × 10−31 kg. The electron carries one unit of negative electric charge (i.e. −1,6 × 10−19 C).

## The nucleus

Unlike the electron, the nucleus can be broken up into smaller building blocks called protons and neutrons. Together, the protons and neutrons are called nucleons.

### Interesting Fact:

Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it cannot be broken down into anything smaller.

### The proton

The electron carries one unit of negative electric charge (i.e. −1,6 × 10−19 C, C is Coulombs).

Each proton carries one unit of positive electric charge (i.e. +1,6 × 10−19 C). Since we know that atoms are electrically neutral, i.e. do not carry any extra charge, then the number of protons in an atom has to be the same as the number of electrons to balance out the positive and negative charge to zero. The total positive charge of a nucleus is equal to the number of protons in the nucleus. The proton is much heavier than the electron (10 000 times heavier!) and has a mass of 1,6726 × 10−27 kg. When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons.

### The neutron

The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is 1,6749 × 10−27 kg (slightly heavier than the proton).

Table 1: Summary of the particles inside the atom.
 proton neutron electron Mass (kg) 1,6726 × 10−27 1,6749 × 10−27 9,11 × 10−31 Units of charge +1 0 −1 Charge (C) 1,6 × 10−19 0 −1,6 × 10−19

## Atomic number and atomic mass number

The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by the letter Z.

Definition 1: Atomic number (Z)

The number of protons in an atom.

You can find the atomic number on the periodic table (see periodic table at front of book). The atomic number is an integer and ranges from 1 to about 118.

The mass of an atom depends on how many nucleons its nucleus contains. The number of nucleons, i.e. the total number of protons plus neutrons, is called the atomic mass number and is denoted by the letter A.

### Interesting Fact:

Currently element 118 is the highest atomic number for an element. Elements of high atomic numbers (from about 93 to 118) do not exist for long as they break apart within seconds of being formed. Scientists believe that after element 118 there may be an “island of stability” in which elements of higher atomic number occur that do not break apart within seconds.

### Interesting Fact:

A nuclide is a distinct kind of atom or nucleus characterised by the number of protons and neutrons in the atom. To be absolutely correct, when we represent atoms like we do here, then we should call them nuclides.

Definition 2: Atomic mass number (A)

The number of protons and neutrons in the nucleus of an atom.

The atomic number (Z) and the mass number (A) are indicated using a standard notation, for example carbon will look like this: $​612C$

Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows:

For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as $​2656Fe$ where the atomic number is $Z=26$ and the mass number $A=56$. The number of neutrons is simply the difference $N=A-Z=30$.

### Tip:

Do not confuse the notation we have used here with the way this information appears on the periodic table. On the periodic table, the atomic number usually appears in the top left-hand corner of the block or immediately above the element's symbol. The number below the element's symbol is its relative atomic mass. This is not exactly the same as the atomic mass number. This will be explained in "Isotopes". The example of iron is shown below.

For a neutral atom the number of electrons is the same as the number of protons, since the charge on the atom must balance. But what happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element? A change in the number of electrons of an atom does not change the type of atom that it is. However, the charge of the atom will change. The neutrality of the atom has changed. If electrons are added, then the atom will become more negative. If electrons are taken away then the atom will become more positive. The atom that is formed in either of these two cases is called an ion. An ion is a charged atom. For example: a neutral sodium atom can lose one electron to become a positively charged sodium atom ($Na +$). A neutral chlorine atom can gain one electron to become a negatively charged chlorine ion ($Cl -$). Another example is $Li +$ which has lost one electron and now has only 2 electrons, instead of 3. Or consider $F -$ which has gained one electron and now has 10 electrons instead of 9.

### Example 1: Standard notation

#### Question

Use standard notation to represent sodium and give the number of protons, neutrons and electrons in the element.

##### Give the element symbol

$Na$

##### Find the number of protons

Sodium has 11 protons, so we have: $​11​Na$

##### Find the number of electrons

Sodium is neutral, so it has the same number of electrons as protons. The number of electrons is 11.

##### Find $A$

From the periodic table we see that $A=23$.

##### Work out the number of neutrons

We know A and Z so we can find $N$: $N=A-Z=23-11=12$.

In standard notation sodium is given by: $​1123Na$. The number of protons is 11, the number of neutrons is 12 and the number of electrons is 11.

## Exercise 1: The structure of the atom

Explain the meaning of each of the following terms:

1. nucleus

2. electron

3. atomic mass

a) Key points: protons, neutrons, nucleons, centre of atom. Learners should have all these words in their definition.

A sample definition would be: The nucleus is the centre of the atom. It consists of neutrons and protons. Together, these neutrons and protons are called nucleons. The nucleus is the most dense part of the atom.

b) Key points: light particles, negatively charged, surround nucleus. Learners should include all these words in their definition.

A sample definition is: The electron is a very light particle. Electrons are found around the nucleus of an atom. Electrons are negatively charged particles.

c) The atomic mass of an atom is the number of protons in the atom.

Complete the following table:

 Element Atomic mass units Atomic number Number of protons Number of electrons Number of neutrons $Mg$ 24 12 $O$ 8 17 $Ni$ 28 40 20 $Zn$ 0 $C$ 12 6 $Al 3+$ 13 $O 2-$ 10

 Element Atomic mass Atomic number Number of protons Number of electrons Number of neutrons Mg 24 12 12 12 12 O 16 8 8 8 8 Cl 35 17 17 17 18 Ni 59 28 28 28 31 Ca 40 20 20 20 20 Zn 65 30 30 30 35 H 1 1 1 1 0 C 12 6 6 6 6 ${\text{Al}}^{3+}$ 27 13 13 10 14 ${\text{O}}^{2-}$ 16 8 8 10 8

Use standard notation to represent the following elements:

1. potassium

2. copper

3. chlorine

1. $​1940K$
2. $​2964Cu$
3. $​1735Cl$

For the element $​1735Cl$, give the number of...

1. protons

2. neutrons

3. electrons

... in the atom.

a) 17

b) 18

c) 17

Which of the following atoms has 7 electrons?

1. $​25He$

2. $​613C$

3. $​37Li$

4. $​715N$

D is correct. There are 7 protons (the subscript number) and since the atom is neutral there will also be 7 electrons.

In each of the following cases, give the number or the element symbol represented by X.

1. $​1840X$

2. $​20XCa$

3. $​X31P$

a) This element has 18 protons and 40 nucleons. So the element must be argon, with symbol Ar since it has 18 protons.

b) Ca has 20 protons and 40 nucleons, so X is 40.

c) Phosphorus has 31 nucleons and 15 protons. So X is 15.

Complete the following table:

Table 2
 A Z N $​92235U$ $​92238U$

In these two different forms of uranium...

1. What is the same?

2. What is different?

AZN
235 92 143
238 92 146

a) Z is the same. The number of protons does not change between these two forms

b) N and A are different. The number of neutrons changes which affects N and A.