Electron cloud

Nucleus consisting of protons and neutrons

Electron: It carries a negative charge. It has a mass only about 1/1850 of the proton.

Nucleus consisting of protons and neutrons

Electron cloud

It is orbiting the nucleus at high speeds.

Proton: It is found in the nucleus .

It carries a positive charge.

Neutron: It does not carry any charge.

It has about the same mass as a proton.

Nucleus consisting of protons and neutrons

Electron cloud

Atoms of different elements have different number of protons, electrons and neutrons.

Example:

AtomNo. of

electronNo. ofproton

No. ofneutrons

Carbon 6 6 6

Nitrogen 7 7 7

Fluorine 9 9 10

Atom No. ofproton

Atomicnumber

Hydrogen 1 1

Oxygen 8 8

Calcium 20 20

The atomic number is the number of protons in an atom.

Mass number: It is the total number of protons and neutrons in

the nucleus of an atom.

AtomNo. ofproton

No. ofneutron

Mass no.

Carbon 6 6 12

Nitrogen 7 7 14

Fluorine 9 10 19

Atom No. ofproton

No. ofelectrons

Hydrogen 1 1

Oxygen 8 8

Calcium 20 20

For any neutral atom, the number of electrons is equal to the number of protons.

Data for atoms of the first 20 elementsAtom

Atomicnumber

No. ofelectrons

No. ofprotons

No. ofneutrons

Massnumber

H 1 1 1 0 1He 2 2 2 2 4Li 3 3 3 4 7Be 4 4 4 5 9B 5 5 5 6 11C 6 6 6 6 12N 7 7 7 7 4O 8 8 8 8 16F 9 9 9 10 19

Ne 10 10 10 10 20Na 11 11 11 12 23Mg 12 12 12 12 24Al 13 13 13 14 27Si 14 14 14 14 28P 15 15 15 16 31S 16 16 16 16 32Cl 17 17 17 18 35Ar 18 18 18 18 36K 19 19 19 20 39Ca 20 20 20 20 40

Draw a labelled diagram to show the structure of an atom.

Nucleus consisting of protons and neutrons

Electron cloud

What is the charge of electron?

What is the charge of proton?

Compare the mass of electron and that of proton.

Describe the movement of electron in an atom.

Electron: It carries a negative charge.

It carries a positive charge.

It is orbiting the nucleus at high speeds.

Electron has a mass only about 1/1850 of the proton.

What is the charge of a neutron?

Compare the mass of neutron and that of proton.

Neutron: It does not carry any charge.

They have about the same mass.

The atomic number is the number of protons in an atom.

How do you know atomic number of an atom from the number of protons?

1. Complete the following table.

Name ofatom

SymbolAtomic

no.No. ofproton

No. ofelectrons

Potassium 19

Carbon 6

Nitrogen 7

Argon 18

iron 26

Suggested Answers

Name ofatom

SymbolAtomic

no.No. ofproton

No. ofelectrons

Potassium K 19 19 19

Carbon C 6 6 6

Nitrogen N 7 7 7

Argon Ar 18 18 18

iron Fe 26 26 26

2. Complete the following table for three element A, B and C

ElementAtomic

no.Mass no.

No. ofneutrons

No. ofelectrons

A 12 6

B 19 39

C 27 13

Suggested Answers

ElementAtomic

no.Mass no.

No. ofneutrons

No. ofelectrons

A 6 12 6 6

B 19 39 20 19

C 13 27 14 13

3. Complete the following table.

No. ofneutrons

No. ofprotons

No. ofelectrons

Mass no.

26 22

74 127

34 79

82 207

20 39

Suggested answers

No. ofneutrons

No. ofprotons

No. ofelectrons

Mass no.

26 22 22 48

74 53 53 127

45 34 34 79

125 82 82 207

19 20 20 39

It is the distribution of electrons in the various shells of an atom.

1st shell (energy level)

4th shell (energy level)

3rd shell

2nd shell

nucleus

nucleus

Energy of the electron in the first shell is lower

nucleus

Energy of the electron in the third shell is higher

The first shell can hold a maximum of 2 electrons

The second shell can hold a maximum of 8 electrons

The third shell can hold a maximum of 18 electrons

The following rules can be applied to find out the electronic configuration of an atom.

The first 2 electrons go into the first shell and complete it.

The next 8 electrons go into the second shell and complete it.

The next 8 electrons go into the third shell.

The next 2 electrons go into the fourth shell.

The next 10 electrons go into the third shell and complete it.

Electron diagrams also show the distribution of electrons in atoms.

E.C.: 1

For example, hydrogen atom

E.C. : 2, 1

Lithium atom, atomic no. = 3

Sodium atom, atomic no. = 11

E.C. : 2, 8, 1

E.C. :

2, 8, 8, 2

Calcium atom, atomic no. = 20

Beryllium atom

E.C. : 2, 2

Work out electronic configuration and electron diagram for beryllium, atomic no.=4

E.C. : 2, 7

Work out electronic configuration and electron diagram for fluorine atom, atomic no.=9

Work out electronic configuration and electron diagram for aluminium, atomic no.=13

E.C.: 2, 8, 3

Potassium atom, atomic no. = 19

E.C.: 2, 8, 8, 1

He42

atomic number

mass number

Cl3517

atomic number

mass number

XAZatomic number

mass number

3.5 Isotopes

Isotopes are atoms of a particular element with the same number of protons and electrons but a different number of neutrons.

Three isotopes of hydrogen.

1

1H

2

1H

3

1H

3.5 Isotopes

Isotopes of an elements have the same number of electrons, therefore the same electronic configuration.

Two isotopes of chlorine

35

17Cl

37

17Cl

They have different mass numbers.

Which of the following are isotopes of the same element? Explain.

Atoms A B C D E

No. of neutrons 35 45 37 44 35

No. of protons 45 35 45 35 44

No. of electrons 45 35 45 35 44

Mass number 80 80 82 79 79

A, C are isotopes of an element.B, D are isotopes of another element.

Atoms A and atom C have the same number of protons but of different number of neutrons.