1.1 PARTICLES IN THE ATOMS AND ATOMIC RADIUS
Learning outcomes
Candidates should be able to:
1. Understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and
neutrons; electrons are found in shells in the empty space around the nucleus
2. Identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
3. Understand the terms atomic and proton number, mass, and nucleon number
4. Describe the distribution of mass and charge within an atom
5. Describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field
6. Determine the numbers of protons, neutrons and electrons present in both atoms and ions given the atomic or
proton number, mass or nucleon number and charge
7. State and explain qualitatively the variations in atomic radius and ionic radius across a period and down a group
Introduction
 Atoms are mostly empty space surrounding a very small, dense nucleus containing protons and
neutrons.
 The electrons are found in shells in the empty space around the nucleus
 When we use a simple model of the atom, we talk about shells (n = 1, n = 2, etc) and sub-shells 2s, 2p, etc. In this
model, the electrons are at a fixed distance from the nucleus. This model is useful when we discuss ionisation
energies
Protons, neutrons and electrons
Fill up the following table.
Particles
Electron (e)
Relative charges
Relative mass
Proton (p)
Neutron (n)
Protons, neutrons and electrons in the electric
field
 The nucleus of atoms is dense and positively charged.
 Most of the mass of an atom is condensed into a tiny nucleus.
 Electrons revolving around the nucleus are negatively charged.
 Like charges repel each other and unlike charges attract each other.
1. Beams of protons, neutrons and electrons behave differently in an electric field due to their differing properties.
The diagram shows the path of a beam of electrons in an electric field.
Add and label lines to represent the paths of beams of protons and neutrons in the same field.
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[3]
2. The following figure shows the behaviour of a beam of protons in an electric field.
Complete the figure to show the behaviour of separate beams of neutrons and electrons in the same electric field.
Label your diagram clearly. Assume that the beams of each particle are moving at the same velocity.
[3]
3. Atoms contain the subatomic particles electrons, protons and neutrons. Protons and electrons were discovered by
observations of their behaviours in electric fields.
(a) The diagram shows the behaviour of separate beams of electrons and protons in an electric field.
(i) Complete the diagram with the relative charge of each of the electrically charged plates.
[1]
(ii) On the diagram, draw a line to show how a separate beam of neutrons from the same source behaves in the
same electric field.
[1]
1.
Beams of charged particles are deflected by an electrical field. The angle of deflection of a particle is proportional to its
charge/mass ratio. In an experiment protons are deflected by an angle of +15°. In another experiment under identical
conditions 2H– ions are deflected by an angle of Y°. What is the value of Y?
A –30.0
2.
B –7.5
C +7.5
D +30.0
Beams of charged particles are deflected by an electric field. In identical conditions the angle of deflection of a particle
is proportional to its charge/mass ratio.
In an experiment, protons are deflected by an angle of +15°. In another experiment under identical conditions, particle
Y is deflected by an angle of –5°.
What could be the composition of particle Y?
A
B
C
D
protons
1
3
4
4
neutrons
2
3
5
5
Atomic and proton Number; Mass and nucleon number
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Electrons
1
5
1
3
 The number of protons in the nucleus of an atom is called the atomic number (proton number) (Z).
Every atom of the same element has the same number of protons in its nucleus. It is the atomic number
that makes an atom what it is.
 The Periodic Table of elements is arranged in order of the atomic numbers of the individual elements
 The mass number (nucleon number) (A) is the number of protons plus neutrons in the nucleus of an
atom.
number of neutrons = A − Z
3.
Technetium (Tc) is a second-row transition element that does not occur naturally on Earth. One of its isotopes has 56
neutrons. What is the nucleon number of this isotope?
A 43
B 56
C 99
D 112
Determination of the number of protons, neutrons and electrons
Play some simulation games at the following link to have a clearer visual concept of the simple structure of atoms.
Build an Atom (colorado.edu)
 Ions: charged particles formed by the loss or gain of electrons from an atom or group of covalently
bonded atoms. Remember that positive ions are formed when one or more electrons are lost by an atom,
and that negative ions are formed when one or more electrons are gained by an atom.
4.
Which atom has the same number of electrons as the hydroxide ion, OH –?
AF
5.
B Ne
B Na
DO
B NH
C NO
D PN
Sodium azide, NaN3 is an explosive used to inflate airbags in cars when they crash. It consists of positive
sodium ions and negative azide ions.
What are the numbers of electrons in the sodium ion and the azide ion?
A
B
C
D
8.
C Ne
HCN has been detected in interstellar gas. The molecules below have also been detected in interstellar gas.
Which molecule contains the same total number of valence shell (outer shell) electrons as HCN?
A HNO
7.
D Mg
Which atom has the same number of electrons as an ammonium ion?
A Mg
6.
C Na
sodium ion
azide ion
10
10
12
12
20
22
20
22
The ion Y3– contains 18 electrons and has a mass number of 31. How many protons and neutrons does Y 3– contain?
A
B
C
D
protons
15
15
18
21
neutrons
16
18
13
10
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9.
An ion with a charge of 2– contains 10 electrons and 14 neutrons.
What is its nucleon number?
A 14
B 22
C 24
D 26
4. Complete the following table using relevant information from the Periodic Table.
[2]
5. Complete Table 1.1 to show the total numbers of protons and electrons in the molecular ions NH4+, CO32– and
PO43–.
[3]
molecular
total number
total number
ion
of protons
of electrons
NH4+
CO32–
PO43–
10. Which species contains a different number of electrons from the other three?
A ClO4–
B H2SO4
C SO42–
D Te2–
 The full symbol for an atom shows its mass number as a superscript and its atomic (proton) number as a
subscript. For example,
shows a carbon atom with mass number 12 and proton number 6.
6. All the Group 2 elements except beryllium have more than one stable isotope.
(i)
Beryllium exists as the single isotope 49𝐵𝑒. Describe the distribution of mass within an atom of 49𝐵𝑒.
………………………………………...........................................................................................................................................
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7. Tellurium is an element in Group 16. The most common isotope of tellurium is 130Te. Its electronic configuration
is [Kr] 4d10 5s2 5p4.
Complete the following table.
[3]
nucleon number
number of neutrons
number of electrons
130
Te
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8.
25
Mg is an isotope of magnesium.
Determine the number of protons and neutrons in an atom of 2512Mg.
12
[1]
9. The composition of atoms and ions can be determined from knowledge of atomic number, nucleon number and
charge.
(a) Complete the table.
[2]
11. Which statement about 131
53I is correct?
A A negative ion of 131
53I contains 53 neutrons and 52 electrons.
B A negative ion of 131
53I contains 53 neutrons and 54 electrons.
C A negative ion of 131
53I contains 78 neutrons and 52 electrons.
D A negative ion of 131
53I contains 78 neutrons and 54 electrons.
12. Which particle contains 8 protons, 9 neutrons and 10 electrons?
13. What number of protons, neutrons and electrons are present in the ion 54Fe3+?
A
B
C
D
protons
26
26
29
29
neutrons
28
28
25
25
electrons
23
29
23
26
14. Which atom contains four times as many neutrons as the
atom?
15. Which species contains the same number of neutrons as
and the same number of electrons as
16. In which species are the numbers of protons, neutrons and electrons all different?
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?
17. What is the total number of protons, neutrons and electrons present in an ammonium ion with a relative formula mass
of 21?
A
B
C
D
Number
protons
11
10
10
11
of Number
neutrons
10
11
11
10
of
Number
electrons
10
11
10
11
of
Variations in atomic radius and ionic radius
Atomic radius: The covalent atomic radius is half the distance between the nuclei of two covalently bonded
atoms of the same type. This is not the only type of atomic radius, but it gives us the best data when
comparing the elements across a period.
two major influences on the size of an atom:
1. The number of shells: the more shells an atom has, the bigger it should be. We should see this effect as
we go down a group.
2. The effective nuclear charge: the larger the charge, the more the orbitals are pulled in towards the
nucleus, and so the smaller the atom should be. We should see this effect as we go across a period.
These two factors combine to produce a predictable pattern in the plot of atomic radius against proton
number, which is borne out by experimental observations.
 Stable metal ions are smaller than metal atoms because the atoms have lost their outer shell electrons, so
the attractive forces between the nucleus and outer electrons are larger.
 Stable non-metal ions are larger than metal atoms because the atoms have gained electrons to complete
their outer shells, so the attractive forces between the nucleus and outer electrons is smaller.
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10. The elements silicon, phosphorus and sulfur are in Period 3 of the Periodic Table.
Describe the variation in atomic radius from silicon to sulfur.
[1]
11. State the trend in ionic radius shown by P3–, S2– and Cl –.
Explain your answer.
[2]
12. Explain the variation in anionic radius for the elements with atomic numbers 14 to 17.
[2]
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13. The table shows information about some of the elements in the third period.
(i)
Explain why the atomic radius of elements in the third period decreases from Na to Cl.
(ii)
The radius of the most common ion of Mg is much smaller than the radius of the most common ion of S.
Identify both ions and explain the difference in their radii.
[2]
14. State & explain the difference in the ionic radius of Al3+ compared to Mg2+.
[3]
18. Which ion has the smallest radius?
A Al3+
B Ba2+
C Mg2+
D Na+
19. Why is the ionic radius of a sulfide ion larger than the ionic radius of a potassium ion?
A Ionic radius always decreases with increasing atomic number.
B Positive ions always have smaller radii than negative ions.
C The potassium ion has more protons in its nucleus than the sulfide ion.
D The sulfide ion is doubly charged; the potassium ion is singly charged.
20. Which row describes the relative sizes of the ionic radii of Na+, Mg2+ and S2–?
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[3]
A
B
C
D
smallest
Na+
Mg2+
S2–
S2–
→
largest
Mg2+
Na+
Na+
Mg2+
S2–
S2–
Mg2+
Na+
21. This question refers to isolated gaseous species.
The species F–, Ne and Na+ are isoelectronic. This means they have the same number of electrons.
In which order do their radii increase?
A
B
C
D
smallest
Na+
F–
Na+
Ne
largest
F–
Ne
Ne
F–
Ne
Na+
F–
Na+
22. Element X has six more protons than element Y.
Which statement must be correct?
A Atoms of element Y are smaller than atoms of element X.
B Element X has a full shell of electrons.
C Element X and element Y are in the same group.
D Element X and element Y are in the same period.
***This is not a complete note. It is just to guide you. It is recommended to study the prescribed textbooks along with this
material. ***
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