1. 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

2. 4.1 ATOMS
WHAT ARE ATOMS?
Everything is made up of particles. There are in fact different types 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.
ATOMS

3. copper atoms enlarged many, many times
a copper wire
copper atoms enlarged many, many times
Figure 4.2
A copper wire consists of trillions of copper atoms.
(1 trillion = , i.e. 1  1012)
ATOMS

4. 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 hydrogen atoms in a line would only be 1 cm long!
ATOMS

5. 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 atoms
enlarged many,
many times
a gold wire
gold atoms
Figure 4.5
Silver consists of silver atoms; gold consists of gold atoms.
ATOMS

6. An ELEMENT is a substance that is made up of only one kind of atoms.
Different elements have different properties because they consist of different kinds of atoms.
Symbols for atoms
Chemical symbols of elements are also the atomic symbols for their atoms.
ATOMS

7. (d) The element copper or a copper atom.
(b) Br
(c) N
(d) The element copper or a copper atom.
ATOMS

8. 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.
STRUCTURE OF ATOMS

9. negatively charged electrons moving around the nucleus
very tiny positively charged nucleus (containing protons and neutrons)
A simple picture of the structure of an atom.
STRUCTURE OF ATOMS

10. MORE ABOUT PROTONS, NEUTRONS AND ELECTRONS
Table 4.1
Data on the three fundamental sub-atomic particles.
STRUCTURE OF ATOMS

11. 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.
STRUCTURE OF ATOMS

12. STRUCTURE OF ATOMS

13. ATOMS ARE ELECTRICALLY NEUTRAL
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.
STRUCTURE OF ATOMS

14. (a) 91 electrons. Number of neutrons cannot be predicted.
(b) It is not an atom. The numbers of protons and electrons are not equal.
STRUCTURE OF ATOMS

15. 4.3 ATOMIC NUMBER AND MASS 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.
ATOMIC NUMBER AND MASS NUMBER

16. MASS NUMBER
The MASS NUMBER of an atom is the sum of the number of protons and neutrons in the atom.
EXAMPLE
mass number Cl 35
mass number A
atomic symbol
atomic number Z
atomic number 17
ATOMIC NUMBER AND MASS NUMBER

17. A4.5 silver, silver, silver A4.6 (a) Aluminium (b) 13Al
(c) (i) 13 (ii) 13 (iii) = 14 27
ATOMIC NUMBER AND MASS NUMBER

18. 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
ISOTOPES

19. RELATIVE ABUNDANCE OF ISOTOPES
electron
proton
neutron
Figure 4.12
The three isotopes of hydrogen.
RELATIVE ABUNDANCE OF ISOTOPES
Most elements consist of more than one isotope.
ISOTOPES

20. Isotopes of some elements in Nature.
Table 4.4
Isotopes of some elements in Nature.
ISOTOPES

21. COMPARING PROPERTIES OF DIFFERENT ISOTOPES
(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.
ISOTOPES

22. Relative isotopic mass  mass number
4.5 RELATIVE MASSES OF ATOMS
RELATIVE ISOTOPIC MASS
The carbon-12 scale
The carbon-12 isotope has been chosen as the reference standard for comparing masses of atoms.
On the 12C = scale,
Relative isotopic mass  mass number
RELATIVE MASSES OF ATOMS

23. RELATIVE ATOMIC MASS 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.
RELATIVE MASSES OF ATOMS

24. (a) Relative atomic mass of sodium = mass number of the only type of
The *RELATIVE ATOMIC MASS of an element is the weighted average of the relative isotopic masses of its natural isotopes on the 12C = 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 =
= 20.2
RELATIVE MASSES OF ATOMS

25. 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.
ARRANGEMENT OF ELECTRONS

26. 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.
ARRANGEMENT OF ELECTRONS

27. 2, 8, 1 Electronic arrangement by diagram
2, 8, 1
Number of 1st 2nd 3rd
electrons 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.
ARRANGEMENT OF ELECTRONS

28. ELECTRONIC ARRANGEMENTS OF THE FIRST 20 ELEMENTSNa
Figure 4.17
The electron diagram of a sodium atom.
ELECTRONIC ARRANGEMENTS OF THE FIRST 20
ELEMENTS
ARRANGEMENT OF ELECTRONS

29. 4.6 ARRANGEMENT OF ELECTRONS

30. A4.10
(a) 17
(b) (i) 2, 8, 7 (ii) Cl
ARRANGEMENT OF ELECTRONS

31. SUMMARY
1. An atom is the smallest part of an element which has the properties 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 they consist of different kinds of atoms.
SUMMARY

32. (d) Electrons move around the nucleus in shells.
3. (a) An atom consists of three types of sub-atomic particles – protons, neutrons and electrons.
(b)
(c) An atom has an extremely small centre called nucleus. The protons and neutrons are in the nucleus.
(d) Electrons move around the nucleus in shells.
(e) An atom is electrically neutral.
SUMMARY

33. 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 A
atomic symbol Cl 17 35
EXAMPLE
SUMMARY

34. 8. Relative isotopic mass  mass number
7. Isotopes are different atoms of the same element, with the same number of protons (and electrons) but different numbers of neutrons.
Different isotopes of the same element have the same chemical 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 = scale
SUMMARY

35. 10. The electronic arrangement of an atom is the distribution of electrons in the various shells of the atom.
(See Table 4.6 again.)
SUMMARY