Atomic and Mass Number

Section 1 of 9Structure of the atom

Particle	Relative mass	Relative charge	Location
Proton	1	+1	In the nucleus
Neutron	1	0	In the nucleus
Electron	Very small (about 1/2000)	−1	In shells around the nucleus

Section 2 of 9Size and make-up

Tiny radius

• Tiny radius An atom has a radius of about 1 × 10⁻¹⁰ m, and the nucleus is about 10 000 times smaller.

Mass in the nucleus

• Mass in the nucleus Nearly all the mass of the atom is concentrated in the nucleus, since electrons are so light.

Neutral overall

• Neutral overall An atom has equal numbers of protons and electrons, so the charges cancel and it has no net charge.

Section 3 of 9Reading nuclear notation

1

The atomic number (Z), written at the bottom, is the number of protons in the nucleus; it identifies the element.

2

The mass number (A), written at the top, is the total number of protons and neutrons.

3

Number of neutrons = mass number − atomic number.

4

In a neutral atom, number of electrons = number of protons. For carbon-14 (A = 14, Z = 6): 6 protons, 8 neutrons, 6 electrons.

Section 4 of 9Worked examples

Atom	Atomic number	Mass number	Protons	Neutrons	Electrons
Lithium-7	3	7	3	4	3
Oxygen-16	8	16	8	8	8
Sodium-23	11	23	11	12	11
Chlorine-35	17	35	17	18	17
Chlorine-37	17	37	17	20	17
Uranium-238	92	238	92	146	92

Section 5 of 9Isotopes

Same element, different mass

Definition

• Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Example

• Example Carbon-12 has 6 neutrons; carbon-14 has 8 neutrons. Both have 6 protons.

Chemical properties

• Chemical properties Isotopes react identically because they have the same electron arrangement.

Stability

• Stability Some isotopes have unstable nuclei and are radioactive.

Section 6 of 9Where the numbers are found

Periodic table

• Periodic table Elements are arranged in order of increasing atomic number, so the position of an element tells you its number of protons.

Relative atomic mass

• Relative atomic mass The mass shown for most elements is an average of all of its isotopes,
  • which is why it is often not a whole number.
    • (e.g. chlorine, 35.5)

Electron arrangement

• Electron arrangement The electrons fill shells from the inside outwards (2, 8, 8),
  • and this arrangement controls the chemical behaviour of the element.

Section 7 of 9Using the periodic table

Finding protons

• Finding protons Look up the element and read its atomic number;
  • this equals the number of protons and, in a neutral atom, the number of electrons.

Finding neutrons

• Finding neutrons Subtract the atomic number from the mass number of the particular isotope you are given,
  • because different isotopes of one element have different neutron numbers.

Section 8 of 9Key terms

Term	Meaning
Atomic number	The number of protons in the nucleus of an atom.
Mass number	The total number of protons and neutrons in the nucleus.
Isotope	An atom of an element with the same number of protons but a different number of neutrons.
Nucleon	A particle in the nucleus, either a proton or a neutron.
Nucleus	The small, dense, positively charged centre of an atom.
Relative atomic mass	The average mass of the atoms of an element, taking account of its isotopes, compared with carbon-12.

Section 9 of 9Exam technique

1

Swapping atomic and mass numbersthe larger number at the top is the mass number.

2

Subtracting from the wrong number to find neutrons — use mass number minus atomic number.

3

Saying isotopes have different numbers of electrons or protons; only neutrons differ.

4

Giving the electron a relative mass of 1 rather than a very small value.