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Chapter 4 Atoms

4.1 Atoms

4.2 Structure of atoms

4.3 Atomic number and mass number

4.4 Isotopes

4.5 Relative masses of atoms

4.6 Arrangement of electrons

CONTENTS OF CHAPTER 4

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4.1 ATOMS

WHAT ARE ATOMS?

Everything is made up of particles. There are in fact different type

s of particles that make up matter. The basic type is atom.

An ATOM is the smallest part of an element which has the

properties of that element.

4.1 ATOMS

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a copper wire

copper atoms enlarged many, many times

Figure 4.2

A copper wire consists of trillions of copper atoms.

(1 trillion = 1 000 000 000 000 , i.e. 1 1012)

4.1 ATOMS

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SIZE AND MASS OF AN ATOM

It is difficult to imagine anything as tiny as an atom. You may get a

better idea of its very small size by looking at Figure 4.4.

Figure 4.4

The 10 small spheres as shown in a line would be 5 cm long, but

140 000 000 hydrogen atoms in a line would only be 1 cm long!

4.1 ATOMS

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ELEMENTS AND ATOMS

An element contains only one kind of atoms. For example, silver

consists of silver atoms only; gold consists of gold atoms only.

a silver wire

silver atomsenlarged many, many times

a gold wire

gold atomsenlarged many, many times

Figure 4.5

Silver consists of silver atoms; gold consists of gold atoms.

4.1 ATOMS

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An ELEMENT is a substance that is made up of only one kind of

atoms.

Different elements have different properties because they con

sist of different kinds of atoms.

Symbols for atoms

Chemical symbols of elements are also the atomic symbols for th

eir atoms.

4.1 ATOMS

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A4.1

(a) 112

(b) Br

(c) N

(d) The element copper or a copper atom.

4.1 ATOMS

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4.2 STRUCTURE OF ATOMS

4.2 STRUCTURE OF ATOMS

WHAT ARE ATOMS MADE OF?

Atoms are made up of three fundamental sub-atomic particles —

protons, neutrons and electrons.

ATOMS are made up of protons, neutrons and electrons. The

protons (positively charged) and neutrons (neutral) are

concentrated in the very tiny nucleus. The electrons (negatively

charged) move around the nucleus.

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A simple picture of the structure of an atom.

very tiny positively charged nucleus (containing protons and neutrons)

negatively charged electrons moving around the nucleus

4.2 STRUCTURE OF ATOMS

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MORE ABOUT PROTONS, NEUTRONS AND

ELECTRONS

Table 4.1

Data on the three fundamental sub-atomic particles.

4.2 STRUCTURE OF ATOMS

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BUILDING UP DIFFERENT ATOMS FROM PROTONS,

NEUTRONS AND ELECTRONS

Different atoms have different numbers of protons, neutrons and

electrons.

neutron

electron

proton

nucleus

Figure 4.10

A diagrammatic

representation of a helium

atom. It has 2 protons, 2

neutrons and 2 electrons.

4.2 STRUCTURE OF ATOMS

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4.2 STRUCTURE OF ATOMS

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A4.2

The commonest type of hydrogen atom.

ATOMS ARE ELECTRICALLY NEUTRAL

An atom is electrically neutral. This is because in an atom, the

number of protons is equal to the number of electrons.

4.2 STRUCTURE OF ATOMS

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A4.3

(a) 91 electrons. Number of neutrons cannot be predicted.

(b) It is not an atom. The numbers of protons and electrons are

not equal.

4.2 STRUCTURE OF ATOMS

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4.3 ATOMIC NUMBER AND MASS NUMBER

4.3 ATOMIC NUMBER AND MASS NUMBER

ATOMIC NUMBER

The ATOMIC NUMBER of an atom is the number of protons in the

atom.

A4.4

A magnesium atom would be changed to a chlorine atom.

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MASS NUMBER

The MASS NUMBER of an atom is the sum of the number of

protons and neutrons in the atom.

mass number

atomic number Z

Aatomic symbol Cl17

35mass number

atomic number

EXAMPLE

4.3 ATOMIC NUMBER AND MASS NUMBER

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A4.5

A4.6

silver, silver, silver

(a) Aluminium (b) 13Al

(c) (i) 13 (ii) 13 (iii) 27 13 = 14

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4.3 ATOMIC NUMBER AND MASS NUMBER

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4.4 ISOTOPES

4.4 ISOTOPES

WHAT ARE ISOTOPES?

ISOTOPES are different atoms of the same element, with the

same number of protons (and electrons) but different numbers of

neutrons.

For example, hydrogen has 3 isotopes: 1H, 1H and 1H.1 2 3

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electron

proton

neutron

Figure 4.12

The three isotopes of hydrogen.

RELATIVE ABUNDANCE OF ISOTOPES

Most elements consist of more than one isotope.

4.4 ISOTOPES

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Table 4.4

Isotopes of

some

elements in

Nature.

4.4 ISOTOPES

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A4.7

(a) 3 (b) 8O (or 16O, or oxygen -16)16

COMPARING PROPERTIES OF DIFFERENT ISOTOPES

Isotopes of the same element have the same chemical properties.

4.4 ISOTOPES

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4.5 RELATIVE MASSES OF ATOMS

4.5 RELATIVE MASSES OF ATOMS

RELATIVE ISOTOPIC MASS

The carbon-12 scale

The carbon-12 isotope has been chosen as the reference standar

d for comparing masses of atoms.

On the 12C = 12.000 00 scale,

Relative isotopic mass mass number

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A4.8

(i) 37 (ii) 35 (iii) 4 (iv) 238

(v) We cannot tell from the given data.

(The mass number is not given.)

RELATIVE ATOMIC MASS

The relative atomic mass of an element is determined by:

(1) the relative isotopic masses and

(2) the relative abundance of the natural isotopes present in the

element.

4.5 RELATIVE MASSES OF ATOMS

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The *RELATIVE ATOMIC MASS of an element is the weighted

average of the relative isotopic masses of its natural isotopes on

the 12C = 12.000 00 scale.

A4.9

(a) Relative atomic mass of sodium= mass number of the only type of sodium atom = 23

(b) Relative atomic mass of neon

=

= 20.2100

10229020

4.5 RELATIVE MASSES OF ATOMS

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4.6 ARRANGEMENT OF ELECTRONS

4.6 ARRANGEMENT OF ELECTRONS

ELECTRONIC ARRANGEMENT

Electrons in an atom are arranged in shells. The distribution of

electrons in the various shells is called ELECTRONIC

ARRANGEMENT (or electronic configuration).

Arrangement of electrons in a sodium atom.

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Rules for finding electronic arrangement

(1) The atomic number of the element is first found. This is equal

to the number of protons, and hence the number of electrons

present in an atom of the element.

(2) Electrons go into the shells one by one, starting from the

innermost shell.

WAYS OF REPRESENTING ELECTRONIC

ARRANGEMENT

Electronic arrangement by numbering

Electronic arrangement may be shown by numbering.

4.6 ARRANGEMENT OF ELECTRONS

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2, 8, 1Number of 1st 2nd 3rdelectrons in: shell shell shell

Figure 4.16

Showing the electronic arrangement of a sodium atom by numbering.

Electronic arrangement by diagram

Besides numbering, electronic arrangement can also be

represented by an electron diagram.

4.6 ARRANGEMENT OF ELECTRONS

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Na

Figure 4.17

The electron diagram of a sodium atom.

ELECTRONIC ARRANGEMENTS OF THE FIRST 20

ELEMENTS

4.6 ARRANGEMENT OF ELECTRONS

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4.6 ARRANGEMENT OF ELECTRONS

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A4.10

(a) 17

(b) (i) 2, 8, 7 (ii)

Cl

4.6 ARRANGEMENT OF ELECTRONS

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SUMMARY

1. An atom is the smallest part of an element which has the pro

perties of that element.

2. An element is a substance that is made up of only one kind of

atoms. Different elements have different properties because t

hey consist of different kinds of atoms.

SUMMARY

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3. (a) An atom consists of three types of sub-atomic particl

es – protons, neutrons and electrons.

(b)

(c) An atom has an extremely small centre called nucle

us. The protons and neutrons are in the nucleus.

(d) Electrons move around the nucleus in shells.

(e) An atom is electrically neutral.

SUMMARY

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SUMMARY

4. Atomic number of an atom = number of protons in the atom

Atomic number of an element =

number of protons in an atom of the element

5. Mass number of an atom

= number of protons + number of neutrons in the atom

6. Full atomic symbol

mass number

atomic number Z

Aatomic symbol Cl17

35mass number

atomic number

EXAMPLE

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SUMMARY

7. Isotopes are different atoms of the same element, with the sa

me number of protons (and electrons) but different numbers

of neutrons.

Different isotopes of the same element have the same chemi

cal properties but slightly different physical properties.

8. Relative isotopic mass mass number

9. Relative atomic mass = weighted average of the relative

of an element isotopic masses of its natural

Isotopes on the 12C = 12.

000 00 scale

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SUMMARY

10. The electronic arrangement of an atom is the distribution of el

ectrons in the various shells of the atom.

(See Table 4.6 again.)