Atomic Structure
Atoms.
Everything is made of atoms.
An atom is the smallest piece of a substance that can exist.
7 million atoms joined together in a straight line
would be about 1mm long. All atoms have a nucleus (the big bit in the middle).
The nucleus contains protons and neutrons.
All atoms have electrons.
For any neutral atom, the number of protons is the same as the number of electrons.
If an atom loses or gains electrons, it is called an ion.
This is a picture of a sodium atom.
It has 11 protons, 11 electrons and 12 neutrons. Each proton has an electrical charge of
+1.
Each electron has a charge of -1. The neutron has no charge (it is neutral). Because an
atom has the same number of protons and electrons,
the overall charge is zero (it is neutral).
The mass of a neutron and a proton are the same. An electron is very much
smaller, about 1/2000th the size of a proton, although it has an equal and
opposite electrical charge.
The electrons, although tiny, take up most of the space of an atom.
This means that most of the space of an atom contains hardly any mass,
it is empty, with nearly all the mass centered at the nucleus.
Summary
Particle
Relative Mass
Relative Charge
Proton
1
+1
Neutron
1
0
Electron
1/ 1840
-1
The protons, neutrons and electrons
are shown colored to distinguish them from each other.
In reality they have no color.
Atomic Number.
The number of protons in the nucleus is called the atomic number.
For sodium (Na), the atomic number is 11.
The atomic number tells you what the element is (see Periodic Table).
The atomic number of an element never changes.
Mass Number.
The number of protons plus the number of neutrons is called the mass number. Sodium has 11 protons and 12 neutrons,
the mass number is 23. The mass number is the total number of particles in the nucleus. The
mass number and the atomic number are written above and below the chemical symbol for the
element, as shown.
Isotopes.
Atoms of the same element may have different numbers of neutrons.
These are called isotopes.
For example, Chlorine (atomic number 17) may have 18 or 20 neutrons.
The mass number will be either 35 or 37.
Isotopes of the same element will have the same chemical properties,
because the number of protons and electrons will be the same.
Practice Time
Fill in the table below in your notes. Notice the color coding to help you out. The answers are on the last page of the
notes.
Name
Fluorine‒19
Symbol
19
9
Oxygen-16
16
8
Nitrogen-14
Protons
9
Neutrons
10
Electrons
9
F
O
8
7
8
Atomic #
9
Mass #
19
Ions
Atoms can gain or lose electrons. This gives the atom a negative or positive charge. When sodium reacts, it will lose its
outer electron.
The unstable sodium atom loses its outer electron to become a stable sodium ion.
The sodium ion still has 11 protons (11 positive charges), but now only 10 electrons (10 negative charges). The sodium
ion has an extra positive charge, shown by the + sign.
In general, if an atom gains an electron it will become negatively charged (gaining negatively charge electrons causes the
atom to become negatively charged). Losing electrons causes the atom to become positively charged (losing a negative
charge leaves the atom with a positive charge due to more protons).
Practice Time
Fill in the table below in your notes. Notice the color coding to help you out. The answers are on the last page of the
notes.
Name
Nitrogen ‒14
ion
Symbol
14
7
Oxygen-16
ion
16
8
Sodiumion
N
O
3-
2-
Protons
Neutrons
Electrons
Atomic #
Mass #
7
7
10
7
14
8
Charge
of ion
3-
10
11
10
11
1+
Answers to Practice Exercises:
Name
Symbol
Fluorine‒19
19
9
Protons
9
Neutrons
10
Electrons
9
Atomic #
9
Mass #
19
8
8
8
8
16
7
7
7
7
14
F
Oxygen-16
16
8
O
Nitrogen-14
14
N
7
Name
Nitrogen ‒14
ion
Symbol
14
7
Oxygen-16
ion
16
8
Sodium- 22
ion
22
N
3-
2-
O
+
Na
11
Protons
Neutrons
Electrons
Atomic #
Mass #
7
7
10
7
14
Charge
of ion
3-
8
8
10
8
16
2-
11
11
10
11
22
1+