Chapter 1.1: Atomic Structure| Page 1 of 22

Chapter 1.1: Atomic Structure

Today’s Lesson:

1) Atomic Structure

2) Atomic Model

3) Atomic Number and Mass Number

4) Isotopes

5) Electronic Structure and Electronic Configuration

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1) Atomic Structure

Refer to Page 1 of your notes:

What is an atom?

An atom is the smallest unit of an element

, having the properties of that element.

Are there particles smaller than the atom?

Atoms are not like solid balls (Figure 1) as proposed by Dalton in 1803.

Figure 1: Dalton's model of the atom

For your information:

Atoms cannot be seen with the naked eye and even microscope, as they are too small. An atom has an average size of about 0.0000000001 m or 1×10-10 m.

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For your information:

Ways to Represent an Atom of an Element

(1) Using Circles

A simple way to represent the atom of an element is to use circles. Atoms vary in size, so a smaller circle is used to a smaller atom than that of a bigger atom.

(2) Using Symbols for Atoms

Each atom has its own symbol. Symbol of atom = Symbol for the element

Usually the first letter or the first two letters of the element’s English name or its name in another language

hydrogen oxygen carbon iron gold potassium

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Example:

Hydrogen: H Iron (Latin name: Ferrum): Fe

Note: The first letter must be in capital and the second letter in small letter.

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To summarise the structure of atom:

Atoms are made up of 3 smaller kinds of sub-atomic particles:

(1) Protons

(2) Neutrons

(3) Electrons

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Refer to Page 2 of your notes:

The table summaries the properties of protons, neutrons and electrons

Sub-atomic Particle

Symbol Actual Mass Relative Mass

Relative Charge

Proton p 1.6726 x 10-27 kg 1

+1

Electron e 9.1094 x 10-31 kg

11840

-1

Neutron n 1.6749 × 10−27 kg

1 0 (no electric

charge)

Examples

Element Hydrogen Helium Model

Number of

Protons 1 2

Number of Electrons

1 2

Number of Neutrons

0 2

electron

proton

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Thinking Time !

Why is the term "relative mass" used in the Table 1 rather than just mass?

Mass of proton, neutron and electron are too small and inconvenient to work with.

By using relative mass, we do not have to remember the exact value of the various masses.

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2) Atomic Model

Refer to Page 2 of your notes:

With reference to the diagram on the left:

(a) The centre of an atom is called the nucleus which contains the protons and neutrons.

(b) The electrons in an atom are arranged in shells (orbits) at different distances from the nucleus. The shell nearest to the nucleus is numbered 1, the second nearest is numbered 2 and so on.

Note: Shells are also called energy levels

For the 1st 20 elements, the maximum number of electrons that can go into the third shell is 8.

.

(c) Each shell can hold a certain maximum number of electrons.

(i) 1st shell - 2 electrons (ii) 2nd shell - 8 electrons

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Refer to Page 3 of your notes:

Advanced: For elements after calcium in the 4th period, their third shell can hold up to 18 electrons.

Some interesting facts... • The nucleus takes up less than 1% of the volume of a

nucleus. • More than 99% of an atom is empty space occupied by rapid

moving electrons.

Protons, neutrons and electrons are the building blocks for all atoms.

Hydrogen atoms are the simplest atoms. Each hydrogen atom has only one proton and one electron.

The next simplest atoms are those of helium with two protons, two electrons and two neutrons.

After helium comes lithium, with three protons, three electrons and four neutrons.

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Notice in all these examples, that an atom always has the same number of protons and electrons. This ensures that the positive charges on the protons balance the negative charges on the electrons

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3) Atomic Number and Mass Number

Refer to Page 4 of your notes:

atomic number, Z = number of protons

mass number, A = number of protons + number of neutrons

Atomic number and mass number

Besides the chemical symbol of an element in the Periodic Table, you will also find its nucleon number and proton number. For example, a gold atom is represented by:

Practice time

The mass number and the atomic number for gold (Au).

How many protons, neutrons and electrons are there in a gold atom?

79 protons, 118 neutons and 79 electrons

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Thinking Time !

An atom can be described as an electrically neutral entity made up of a positively charged nucleus at its centre with negatively charged electrons moving around the nucleus.

(a) Why is the atom electrically neutral?

Number of electrons = number of protons.

Equal positive and negative charges

(b) Why is the nucleus positively charged?

The nucleus contains protons and neutrons. Protons are positively charged while neutrons are electrically neutral.

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4) Isotopes

Refer to Page 5 of your notes:

Isotopes and Relative Atomic Mass

Several elements have relative atomic masses which are whole numbers.

For example, the relative atomic mass of carbon is 12.0, that of fluorine is 19.0 and that of sodium is 23.0.

This is not surprising, as the mass of an atom depends on the mass of its protons and neutrons, both of which have a relative mass of 1.0.

For example, we could calculate the relative mass of fluorine as follows.

199F atoms have:

9 protons — relative mass = 9.0

9 electrons — relative mass = 0.0

10 neutrons — relative mass = 10.0

Therefore, relative atomic mass of 199F = 19.0

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Unlike fluorine, carbon and sodium, some elements have relative atomic masses that are nowhere near whole numbers. For example, the relative atomic mass of chlorine is 35.5 and that of copper is 63.5.

These unexpected results were explained in 1919 when W.F. Aston built the first mass spectrometer.

Using his mass spectrometer, Aston found that one element could have atoms with different masses.

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For your information:

What is a mass spectrometer?

Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of charged particles. It is used for determining masses of particles.

How it works?

The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.

Refer to this website for more details: http://www.chemguide.co.uk/analysis/masspec/howitworks.html

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Each isotope has a relative atomic mass which is a whole number, but the average relative atomic mass for the mixture of isotopes is not always a whole number.

For your information:

Atomic mass is the mass of a specific isotope

Refer to Page 5 of your notes (bottom):

To summarise:

Chlorine is a good example of an element with isotopes. Naturally occurring chlorine contains two isotopes

3517

C1 3717

C1

No of

protons 17 17

No of electrons

17 17

No of neutrons

18 20

Both isotopes have the same atomic number and the same chemical properties because these are determined by the number of electrons.

Both isotopes have different mass numbers, different masses and hence different physical properties because these depend on the

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masses of atoms and molecules.

Refer to Page 6 of your notes:

The similarities and differences between isotopes of the same element are summarised in the table below.

Isotopes can be divided into two types.

One type is radioactive; the other is non-radioactive.

Radioactive isotopes give out radiation. This radiation is invisible but it can be detected with special instruments. Radiation is harmful to life, and in large amounts, can kill people.

Radioactive isotopes have important uses. The cobalt isotope, 60Co, is used in hospitals to treat cancer patients. The intense radiation from this isotope destroys the cancer cells. Cobalt-60 is also used to sterilise surgical instruments used in hospital operations. The powerful radiation kills germs. Radioactive isotopes produce heat which can be used to produce electrical energy. Small amounts of radioactive isotopes are used to supply energy in remote places. Radioactive isotopes provide energy for spacecraft exploring the outer planets, such as Jupiter and Saturn. Some people have irregular heartbeats. They need to have a heart pacemaker implanted inside their chest. This instrument provides a tiny electrical shock to ensure a steady heartbeat. Pacemakers can

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be powered by radioactive isotopes. Such a pacemaker can work reliably for over 20 years. An ordinary battery would have to be replaced every 10 years.

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Refer to Page 8 of your notes:

A mass spectrometer trace for chlorine.

What are the relative amounts of isotopes 35 17

C1 and 37 17

C1?

If chlorine contained 100% 35 17 C1 , its relative atomic mass would be 35.

If it contained 100% 37 17 C1 its relative atomic mass would be 37.

Calculating relative atomic masses (Optional)

The relative atomic mass of an element is the average mass of one atom. This can be calculated from the relative masses of its isotopes and their relative proportions.

Look closely at the figure below which shows a mass spectrometer trace for chlorine. The trace shows that chlorine contains two isotopes, with mass numbers of 35 and 37.

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If it contained 50% 35 17 C1 and 50% 37 17 C1 , the relative atomic mass would be:

Now, the previous graphical figure shows that chlorine contains three times as much 35

17 C1 as

3717

C1 i.e. the percentages of the two isotopes are 75% to 25%. Therefore the atomic mass of chlorine

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5) Electronic Structure and Electronic Configuration

For your information:

Arranging Electrons in Atoms The atom consists of a positively charge nucleus surrounded by negatively charged electrons. The way in which the electrons are arranged determines the chemical properties of the atom.

The electrons in an atom move around the nucleus in regions known as electron shells. Each shell can only hold a certain number of electrons.

Refer to Page 9 of your notes

Electronic Structure (Full Electronic Configuration) of Nitrogen

Electronic Structure (Full Electronic Configuration) of Argon

No of electrons

7 18

Electronic configuration

2.5

2.8.8

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(b) Valence Shell (Outer Shell)

The shell which is farthest from the nucleus and occupied by electrons is called the valence shell (outer shell). The electrons in the valence shell are known as valence electrons (outer electrons). In a chemical reaction, only these valence electrons are involved in chemical bonding between atoms.

** For the 1st 20 elements, the maximum number of electrons that can go into the third shell is 8.

Advanced: For elements after calcium in the 4th period, their third shell can hold up to 18