Physics – Key Topic 4
4.4 Atomic Structure
4.4.1 Atoms and Isotopes
4.4.1.1 The structure of an atom
Atoms are very small, having a radius of about 1 x 10-10 metres
The basic structure of an atom is a positively charged nucleus composed of both
protons and neutrons surrounded by negatively charged electrons
The radius of a nucleus is less than 1/10000 of the radius of an atom. Most of the
mass of an atom is concentrated in the nucleus
The electrons are arranged at different distances from the nucleus (different energy
levels). The electron arrangements may change with the absorption of
electromagnetic radiation (move further away from the nucleus) or by the emission
of electromagnetic radiation (move closer to the nucleus)
4.4.1.2 Mass number, atomic number and isotopes
In an atom the number of electrons is equal to the number of protons in the nucleus.
Atoms have no overall electric charge
All atoms of a particular element have the same number of protons. The number of
protons in an atom of an element is called its atomic number
The total number of protons and neutrons in an atom is called its mass number
Atoms of the same element can have different numbers of neutrons; these atoms are
called isotopes of that element
- All elements have different isotopes, but there are usually only one or two stable
ones
- The other unstable isotopes tend to decay into other elements and give out
radiation as they try to become more stable – radioactive decay
Atoms turn into positive ions if they lose one of more outer electrons
4.4.1.3 The development of the model of the atom
New experimental evidence may lead to a scientific model being changed or replaced
Before the discovery of the electron, atoms were thought to be tiny spheres that
could not be divided
, Niel Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at
specific distances
1. Dalton’s model (1803)
He proposed the ideas that all matter is made from atoms, atoms cannot be
divided into smaller pieces, all the atoms of an element are exactly alike, different
elements are made of different atoms and combinations of atoms for
compounds.
He pictured the atom to be a hard sphere that was the same throughout,
like a tiny marble
2. Thomson’s plum pudding model (1904)
Thomson showed that there were
negatively charged particles smaller than an atom, that were later called
electrons
He pictured a sphere of positive charge with negatively charged electrons
embedded in it. Therefore it had an overall neutral charge
3. Rutherford’s nuclear model (1911)
4.4 Atomic Structure
4.4.1 Atoms and Isotopes
4.4.1.1 The structure of an atom
Atoms are very small, having a radius of about 1 x 10-10 metres
The basic structure of an atom is a positively charged nucleus composed of both
protons and neutrons surrounded by negatively charged electrons
The radius of a nucleus is less than 1/10000 of the radius of an atom. Most of the
mass of an atom is concentrated in the nucleus
The electrons are arranged at different distances from the nucleus (different energy
levels). The electron arrangements may change with the absorption of
electromagnetic radiation (move further away from the nucleus) or by the emission
of electromagnetic radiation (move closer to the nucleus)
4.4.1.2 Mass number, atomic number and isotopes
In an atom the number of electrons is equal to the number of protons in the nucleus.
Atoms have no overall electric charge
All atoms of a particular element have the same number of protons. The number of
protons in an atom of an element is called its atomic number
The total number of protons and neutrons in an atom is called its mass number
Atoms of the same element can have different numbers of neutrons; these atoms are
called isotopes of that element
- All elements have different isotopes, but there are usually only one or two stable
ones
- The other unstable isotopes tend to decay into other elements and give out
radiation as they try to become more stable – radioactive decay
Atoms turn into positive ions if they lose one of more outer electrons
4.4.1.3 The development of the model of the atom
New experimental evidence may lead to a scientific model being changed or replaced
Before the discovery of the electron, atoms were thought to be tiny spheres that
could not be divided
, Niel Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at
specific distances
1. Dalton’s model (1803)
He proposed the ideas that all matter is made from atoms, atoms cannot be
divided into smaller pieces, all the atoms of an element are exactly alike, different
elements are made of different atoms and combinations of atoms for
compounds.
He pictured the atom to be a hard sphere that was the same throughout,
like a tiny marble
2. Thomson’s plum pudding model (1904)
Thomson showed that there were
negatively charged particles smaller than an atom, that were later called
electrons
He pictured a sphere of positive charge with negatively charged electrons
embedded in it. Therefore it had an overall neutral charge
3. Rutherford’s nuclear model (1911)
