This document contains notes about Chapter 2 Atomic Structure for the IB HL Chemistry course. Some topics covered are: atom, nucleus, Dalton's model of the atom, atomic theory, electrons, plum-pudding model, Rutherford's model of the atom, alpha particles, nucleons, Bohr model of the hydrogen atom,...
The mass of an atom is concentrated in its positively charged nucleus.
All things are made of atoms. The 19th century brought scientific evidence to support their
existence.
Quantum theory and adopt a wave description of matter. It is necessary to consider the
wave properties of the electron.
2.1 The Nuclear Atom
Dalton’s model of the atom
‘All matter is built from around 100 elements.’
An atom is the smallest unit of an element. It is indivisible.
There are 92 naturally occurring elements.
Dalton’s Atomic Theory:
1. All matter is composed of tiny invisible particles called atoms.
2. Atoms cannot be created nor destroyed.
3. Atoms of the same element are alike in every way.
4. Atoms of different elements are different.
5. Atoms can combine together in small numbers to form molecules.
Elements combine in fixed proportions to form compounds. E.g.: One water molecule is
formed when 2 hydrogen atoms combine with 1 oxygen atom.
Atoms contain electrons
‘Atoms are indestructible.’ J.J. Thompson discovered that metals produce a stream of
negatively charged particles when a high voltage is applied across 2 electrodes.
The atom had no net charge.
Plum-pudding model: negatively charged electrons scattered in a positively charged
substance.
Rutherford’s model of the atom
He fired alpha particles at a piece of gold foil. Alpha particles are helium nuclei, composed of
2 protons and 2 neutrons.
,Most alpha particles passed straight through, but a few were repelled and bounced back.
Conclusions:
1. The atom is mostly empty space.
2. Some positively charged alpha particles collided with the positively charged, dense
nucleus.
3. The nucleus is very small.
Sub-atomic particles
The nucleus contains nucleons = protons and neutrons. They have a positive charge.
The electron cloud contains the electrons. They have a negative charge.
Relative mass Relative charge
proton 1 +1
electron 0.0005 -1
neutron 1 0
Bohr model of the hydrogen atom
The electron moving in an orbit around the proton like in the ‘solar system’.
The electrostatic force of attraction between the oppositely charged sub-atomic particles
prevents the electron from leaving the atom.
Nuclear radius = 10-15m. Atomic radius = 10-10m.
Neutrons are crucial for the stability of nuclei.
, Atomic number vs mass number
Atomic number (Z) = the number of protons in an atom. It allows scientists to identify the
element. Z = the number of electrons in an uncharged atom.
Mass number (A) = the number/mass of all nucleons in an atom.
Z = p+ = e -
n0 = A - Z
Isotopes
Isotopes: atoms of the same element with different numbers of neutrons (different mass
numbers).
Isotopes show the same chemical properties, but different physical properties such as
boiling/melting points, density (mass per unit volume), and rate of diffusion (the lighter
isotope diffuses slightly faster)
Diffusion = the movement of a substance from an area of high concentration to an area of
low concentration. It happens in liquids and gases. Diffusion is an important process for
living things; it is how substances move in and out of cells.
The average relative mass of multiple isotopes is closest to the most abundant isotope.
total mass
relative averagemass=
number of atoms
Ions
Atoms gain or lose electrons to form ions.
- Cation: positive ion
- Anion: negative ion
The loss/gain of electrons changes the chemical properties.
Relative atomic masses of some elements
Mass spectrometer is a device used to measure the mass of individual atoms.
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