Counting Protons, Neutrons, and Electrons in Atoms

Purpose

• Matter is made up of atoms, so it is very important to understand what an atom is and the difference between different atoms. In this lesson, we briefly discuss the composition of an atom and learn how to identify atoms, learn the differences between atoms of the same element and the differences between atoms of different elements. Additionally, we learn how to use the periodic table to determine some of this information.

An atom is made up of 3 particles

• Proton

– Positively charged particle

– In the nucleus

– Symbol: p+

• Neutron

– Uncharged particle

– In the nucleus

– Symbol: n0

• Electron

– Negatively charged particles

– Outside the nucleus

– Much lighter than protons and neutrons – approximately 1/2000th the mass of protons and neutrons

– Symbol: e-

Structure of an atom – protons and neutrons in the nucleus, electrons outside the nucleus

+++

+

-

- -

- +

-

proton

neutron

electronBeryllium atom

Note: This is not to scale – electrons are considerably smaller than protons and neutrons and the nucleus is quite a bit smaller than the overall size of the atom.

The type of atom is determined by the number of protons.

• Any atom with 4 protons is a beryllium atom.

• Any atom with 6 protons is a carbon atom

• The periodic table shows the names and symbols assigned to all known atoms and is organized based on the number of protons each atom contains.

• The number of protons is also called the atomic number (Z)

1 2

3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

55 56 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

87 88

57 58 59 60 61 62 63 64 65 66 67 68 69 70

lanthanides(rare earth metals)

89 90 91 92 93 94 95 96 97 98 99 100 101 102

actinides

Lu Hf Ta

Y Zr

52.00

Mnmanganese

54.94

Crchromium

Vvanadium

157.3

gadoliniumneodymium

138.9 144.2 145 150.4 152.0

266262 261

227

262

americiumactinium

lanthanum

Ac

cerium praseodymium

berkeliumCmTh Pa Pu Am

259247 247 251 252 257 258

fermium mendelevium

237 239 243

curiumNp No

232.0 231.0 238.0

MdCf EsUneptunium plutoniumthorium protactinium uranium nobelium

Bkcalifornium einsteinium

Fm

Ererbiumdysprosium

La Ce Pr Nd Pm Sm Gd

272 285

roentgentiumununbium

promethiumYb

ytterbium

173.0158.9 162.5

DyTbterbium

164.9 167.3

Tmthulium

168.9

Hoholmium

Uublawrenciumrutherfordiumdubnium seaborgium bohrium hassium

samarium europiumEu

meitneriumdarmstadtium

264 277 268 281

Lr Rf Db Sg Bh Hs Mt Ds Rg

iridium platinum gold mercury

175.0 178.5 180.9 183.8 186.2 190.2

lutetium hafnium tantalum tungsten rhenium osmium

192.2 195.1 197.0 200.6

technetium ruthenium rhodium palladium

98 101.1 102.9 106.4

yttrium zirconium

Os Ir Pt Au Hg

88.91 91.22 92.91 95.94 107.9 112.4

W

ununtriumununquadium

Mo Ag CdTc Ru Rh Pdmolybdenum

Tl Pb

106 107 108 109 110 111 112 113 114

silver cadmium

Re

284 289 288 292 294

astatine radon

210 222

ununpentiumununhexiumUup Uuh

ununoctium

204.4 207.2 209.0 209

UuoUut Uuq

Pothallium lead bismuth polonium

At Rn

Sb Te I Xe

126.9 131.3

antimony tellurium iodine xenon

69.72 72.64 74.92 78.96

germanium

79.90 83.80

114.8 118.7 121.8 127.6

arsenic selenium

indium tinIn Sn

30.97

As Kr

39.95

bromine

32.07 35.45

Araluminum silicon phosphorus sulfur chlorine argon

Si

28.09

BrSekrypton

Ga Ge

P S Cl

50.94

Nbniobium

Tititanium

47.87

Cucopper

63.55

Znzinc

65.41

iron

55.85

Cocobalt

58.93

Ninickel

58.69

gallium

Nnitrogen

14.01

Neneon

20.18

Ooxygen

16.00

Ffluorine

19.00

boron

10.81

Ccarbon

12.01

132.9 137.3

sodium

Fe

5

V B

22.99 24.31

Al

26.98

Ca

40.08

Scscandium

44.96

rubidium strontium

85.47 87.62

9.012

K

103 104 105

4.003

Li

Hhydrogen

1.008

13

III A

14

IV A

Mg 6

VI B

7

VII B

8

VIII B

12

II B

9

VIII B

10

VIII B

11

I B

15

V A

16

VI A

Be B

4

5

potassium calcium

39.10

18

VIII A

17

VII AHe

helium

Pe

rio

d

1

2

3

1

I A

lithium

Na

6.941

Rb Sr

2

II A

3

III B

4

IV Bmagnesium

beryllium

6

7

226

140.1 140.9

Fr

Cs Bacesium barium

Rafrancium radium

223

Bi

115 116 118

4

9.012

Beberyllium

Atomic Number gives the number of protons

Symbol

Name

Pause and Practice

• Use the periodic table to complete the table below:

Element Symbol

Element Name

Atomic Number

Number of protons

H Hydrogen 1 1

Mg 12

Iron 26

10

K

Pause and Practice - Answers

• Use the periodic table to complete the table below:

Element Symbol

Element Name

Atomic Number

Number of protons

H Hydrogen 1 1

Mg Magnesium 12 12

Fe Iron 26 26

Ne Neon 10 10

K Potassium 19 19

A note about periodic table symbols.

• Symbols can be one or two letters

– One letter symbols are always capitalized

• Hydrogen = H

• Oxygen = O

– Two letter symbols always have the first letter capitalized and the second letter lowercase

• Helium = He

• Calcium = Ca

Symbols you should know – memorize them!

• The first 20 element symbols

– Hydrogen = H

– Helium = He

– Lithium = Li

– Beryllium = Be

– Boron = B

– Carbon = C

– Nitrogen = N

– Oxygen = O

– Fluorine = F

– Neon = Ne

– Sodium = Na

– Magnesium = Mg

– Aluminum = Al

– Silicon = Si

– Phosphorus = P

– Sulfur = S

– Chlorine = Cl

– Argon = Ar

– Potassium = K

– Calcium = Ca

• Some other common elements to know

– Copper = Cu

– Iron = Fe

– Lead = Pb

– Silver = Ag

– Zinc = Zn

– Tin = Sn

– Mercury = Hg

– Bromine = Br

– Iodine = I

The following are the symbols that are missed most often. Pay particular attention to these.

• Sodium, silicon, and sulfur– Sodium = Na

– Silicon = Si

– Sulfur = S

• Phosphorus and Potassium– Phosphorus = P

– Potassium = K

• Metals with symbols derived from other languages.– Copper = Cu

– Iron = Fe

– Lead = Pb

– Silver = Ag

– Tin = Sn

– Mercury = Hg

Suggestion

• Make a set of flash cards with the name of the element on one side and their symbol on the other for the elements that you have been asked to learn.

The periodic table tells us how many protons are in every atom. What about electrons and neutrons?

• Electrons

– For a neutral atom, the number of electrons is the same as the number of protons so it can be determined by looking at the atomic number on the periodic table.

• Neutrons

– One cannot look at the periodic table and determine the number of neutrons in an atom.

– To know neutrons, you must be given the mass number

Mass number and number of neutrons

• Mass number (Z)

– The mass number is the total number of protons plus neutrons.

Z = #p+ + #n0

– Mass numbers must be given to you, or you must be told the number of neutrons in the atom.

• The reason for this is that while a neutral atom of carbon will always have 6 protons and 6 electrons, it can have 6, 7, or 8 neutrons.

Isotopes are atoms of an element that differ in the number of neutrons they contain.

• All carbon atoms have 6 protons

– Most carbon atoms have 6 neutrons and are named carbon-12

– Some carbon atoms have 7 neutrons and are named carbon-13

– A few carbon atoms have 8 neutrons and are named carbon-14

• The number after the name is the mass number, which represents the sum of the protons and neutrons

Pause and Practice

• An atom has 19 protons and 21 neutrons. What is its mass number? What is its name?

• An atom has a mass number of 37 and contains 17 protons. What is the name of the atom, and how many neutrons does it contain?

Pause and Practice Answers

• An atom has 19 protons and 21 neutrons. What is its mass number? What is its name?

– Mass number: 40

– Name: potassium-40

• An atom has a mass number of 37 and contains 17 protons. What is the name of the atom, and how many neutrons does it contain?

– Name: chlorine-37

– Number of neutrons: 20

Misconception Alert

• You cannot get mass numbers off the periodic table.

• The mass shown on the periodic table will be discussed in a later unit. In short, it is the average mass of all the isotopes of that element – it does not represent any particular atom.

• For example, if we asked how many protons and neutrons are in a chlorine atom, you would look at the periodic table and see that it has an atomic number of 17. It therefore has 17 protons. You cannot answer how many neutrons without being told more information.

We can show atomic number and mass number with the symbols

• Carbon-12 has 6 protons, 6 neutrons, and a mass number of 12. We can represent it with the following symbol:

612𝐶

• When writing symbols this way, the mass number(A) goes on the upper left and the atomic number(Z) goes on the lower left of the symbol(X).

𝑍𝐴𝑋

Pause and Practice - Fill in the table

Complete Symbol

Name Number of protons

Number of neutrons

Number of electrons

Atomic Number

Mass Number

612𝐶 Carbon-12 6 6 6 6 12

Hydrogen-1

16 20 16

7 15

2046𝐶𝑎

Pause and Practice Answers

Complete Symbol

Name Number of protons

Number of neutrons

Number of electrons

Atomic Number

Mass Number

612𝐶 Carbon-12 6 6 6 6 12

11𝐻 Hydrogen-1 1 0 1 1 1

1636𝑆 Sulfur-36 16 20 16 16 36

715𝑁 Nitrogen-15 7 8 7 7 15

2046𝐶𝑎 Calcium-46 20 26 20 20 46

Try the exercises!