2 ATOMIC STRUCTURE
CHEMISTRY IS THE STUDY OF HOW ATOMS OF THE VARIOUS ELEMENTS ARE JOINED TOGETHER TO MAKE
EVERYTHING WE SEE AROUND US
ATOMS
In the simplest picture of the atom the negatively charged electrons orbit around the central,
positively charged nucleus (Figure 2.1). The nucleus is made up of protons and neutrons
Protons and neutrons make up the nucleus and are therefore sometimes called nucleons
The atomic number (Z) defines an element
Atomic number (Z) is the number of protons in the nucleus of an atom.
The overall charge on an atom is zero and therefore:
Number of protons in an atom = number of electrons
Number of neutrons in an atom =mass number−atomic number
IONS
Ions are charged particles that are formed when an atom loses or gains (an) electron(s).
A positive ion (cation) is formed when an atom loses (an) electron(s)
A negative ion (anion) is formed when an atom gains (an) electron(s)
ISOTOPES
Isotopes are different atoms of the same element with different mass numbers: i.e. different
numbers of neutrons in the nucleus.
Isotopes have the same chemical properties (they react in exactly the same way) but different
physical properties (e.g. different melting points and boiling points).
Because of the different isotopes present, it is most convenient to quote an average mass for an
atom – this is the relative atomic mass (Ar).
, The relative atomic mass (Ar) of an element is the average of the masses of the isotopes in a
naturally occurring sample of the element relative to the mass of 1 of an atom of carbon-12.
𝐴𝑟 − 𝑚𝑎𝑠𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡𝑒𝑟 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
× 100 = % 𝑜𝑓 ℎ𝑒𝑎𝑣𝑖𝑒𝑟 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑚𝑎𝑠𝑠 𝑓 𝑡𝑤𝑜 𝑖𝑠𝑜𝑡𝑜𝑝𝑒𝑠
MASS SPECTRUM OF AN ELEMENT AND RELATIVE ATOMIC MASS
The proportion of each isotope present in a sample of an element can be measured using an
instrument called a mass spectrometer
In a mass spectrum of an element, we get one peak for each individual isotope.
ELECTRON CONFIGURATION
The shells increase in energy as they get further from the nucleus
principal quantum number written as 2n2.
The general rule for filling these energy levels is that the electrons fill them from the lowest energy
to the highest (from the nucleus out).The first two energy levels must be completely filled before an
electron goes into the next energy level.The third main energy level is, however, only filled to 8
before electrons are put into the fourth main energy level.This scheme works for elements with
atomic number up to 20.
ELECTROMAGNETIC SPECTRUM
Light is a form of energy. Visible light is just one part of the electromagnetic spectrum
1
Frequency ∝
wavelength
frequency ∝ energy
The various forms of electromagnetic radiation are usually regarded as waves that travel at the
speed of light in a vacuum (3.0×108 ms−1) but vary in their frequency/energy/wavelength.
particles of electromagnetic radiation called photons.
White light is visible light
increasing energy :
red < orange < yellow < green < blue < indigo < violet.
CHEMISTRY IS THE STUDY OF HOW ATOMS OF THE VARIOUS ELEMENTS ARE JOINED TOGETHER TO MAKE
EVERYTHING WE SEE AROUND US
ATOMS
In the simplest picture of the atom the negatively charged electrons orbit around the central,
positively charged nucleus (Figure 2.1). The nucleus is made up of protons and neutrons
Protons and neutrons make up the nucleus and are therefore sometimes called nucleons
The atomic number (Z) defines an element
Atomic number (Z) is the number of protons in the nucleus of an atom.
The overall charge on an atom is zero and therefore:
Number of protons in an atom = number of electrons
Number of neutrons in an atom =mass number−atomic number
IONS
Ions are charged particles that are formed when an atom loses or gains (an) electron(s).
A positive ion (cation) is formed when an atom loses (an) electron(s)
A negative ion (anion) is formed when an atom gains (an) electron(s)
ISOTOPES
Isotopes are different atoms of the same element with different mass numbers: i.e. different
numbers of neutrons in the nucleus.
Isotopes have the same chemical properties (they react in exactly the same way) but different
physical properties (e.g. different melting points and boiling points).
Because of the different isotopes present, it is most convenient to quote an average mass for an
atom – this is the relative atomic mass (Ar).
, The relative atomic mass (Ar) of an element is the average of the masses of the isotopes in a
naturally occurring sample of the element relative to the mass of 1 of an atom of carbon-12.
𝐴𝑟 − 𝑚𝑎𝑠𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡𝑒𝑟 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
× 100 = % 𝑜𝑓 ℎ𝑒𝑎𝑣𝑖𝑒𝑟 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑚𝑎𝑠𝑠 𝑓 𝑡𝑤𝑜 𝑖𝑠𝑜𝑡𝑜𝑝𝑒𝑠
MASS SPECTRUM OF AN ELEMENT AND RELATIVE ATOMIC MASS
The proportion of each isotope present in a sample of an element can be measured using an
instrument called a mass spectrometer
In a mass spectrum of an element, we get one peak for each individual isotope.
ELECTRON CONFIGURATION
The shells increase in energy as they get further from the nucleus
principal quantum number written as 2n2.
The general rule for filling these energy levels is that the electrons fill them from the lowest energy
to the highest (from the nucleus out).The first two energy levels must be completely filled before an
electron goes into the next energy level.The third main energy level is, however, only filled to 8
before electrons are put into the fourth main energy level.This scheme works for elements with
atomic number up to 20.
ELECTROMAGNETIC SPECTRUM
Light is a form of energy. Visible light is just one part of the electromagnetic spectrum
1
Frequency ∝
wavelength
frequency ∝ energy
The various forms of electromagnetic radiation are usually regarded as waves that travel at the
speed of light in a vacuum (3.0×108 ms−1) but vary in their frequency/energy/wavelength.
particles of electromagnetic radiation called photons.
White light is visible light
increasing energy :
red < orange < yellow < green < blue < indigo < violet.
