Atomic Masses

The average weight of an element’s atoms is its atomic mass, which is measured in atomic mass units (amu, also known as daltons, D). The atomic mass is a weighted average of the masses of all of the isotopes of that element. This is done by multiplying the mass of each isotope by how many of them there are. Atomic mass is also called atomic weight, but “mass” is a better word for it.

Experiments have shown that neon has three isotopes: neon-20, which has 10 protons and 10 neutrons in its nucleus and has a mass of 19.992 amu and an abundance of 90.48%; neon-21, which has 10 protons and 11 neutrons in its nucleus and has a mass of 20.994 amu and an abundance of 0.27%; and neon-22, which has 10 protons and 12 neutrons in its nucleus and has a mass The average number of atoms in a piece of neon is:

0.9048	×	19.992 amu	=	18.09 amu
0.0027	×	20.994 amu	=	0.057 amu
0.0925	×	21.991 amu	=	2.03 amu
				20.18 amu

When used with the idea of a mole, the atomic mass is useful in chemistry. The atomic mass of an element, which is measured in amu, is the same as the mass of one mole of an element, which is measured in grammes. So, since the atomic mass of iron is 55.847 amu, 55.847 grammes would be the weight of one mole of iron atoms. You can use the same idea to think about ionic compounds and molecules. One formula unit of sodium chloride (NaCl) would weigh 58.44 amu (22.98977 amu for Na and 35.453 amu for Cl), so a mole of sodium chloride would weigh 58.44 grammes. One molecule of water (H2O) weighs 18.02 amu (21.00797 amu for H + 15.9994 amu for O), and a mole of water molecules weighs 18.02 grammes.

Dimitri Mendeleev published the first periodic table of the elements in 1869. The elements that were known at the time were listed in order of increasing atomic weight, since the nucleus and the inside structure of the atom had not yet been discovered. Instead, the modern periodic table is organised by the number of atoms.

1A

2A

3A

4A

5A

6A

7A

8A

(1)

(2)

(13)

(14)

(15)

(16)

(17)

(18)

3B

4B

5B

6B

7B

—

8B

—

1B

2B

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

1

H

He

2

Li

Be

B

C

N

O

F

Ne

3

Na

Mg

Al

Si

P

S

Cl

Ar

4

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

5

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

6

Cs

Ba

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

7

Fr

Ra

Ac

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Uub

—

Uuq

—

—

—

—

6

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

7

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Elements sorted by Atomic Mass