Structure of An Atom
What is an atom?
Everything in our universe is made of matter and matter is made of atoms. An atom maybe described as the smallest particle that matter is made with and has the properties of a chemical element. An atom is minute in size and typical sizes are in picometers, a ten billionth of a meter.
What are atoms made of?
An atom is made of three parts – protons, neutrons and electrons.
Each of these parts has an associated charge. The protons carry a positive charge, electrons have a negative charge and neutron possess no charge. Protons and neutrons make up the nucleus of the atom and electrons orbit the nucleus at different energy levels.
What is atomic number?
Atomic number of an atom is defined by the number of protons in the nucleus of an atom.
What is atomic mass?
Atomic mass of an atom is equivalent to the number of protons and neutrons in the atom.
Parts of an atom
What is a proton?
A proton is a positively charged particle found within the atom’s nucleus. Rutherford discovered them in his experiments with cathode ray tubes.
The number of protons in an atom define what the element is. This is what is referred to as the atomic number of that element. The number of protons also determine the chemical behaviour of that element.
What is a neutron?
A neutron is the neutral part of the atom’s nucleus, with no electric charge, and a mass slightly larger than that of a proton. It was discovered by the English physicist, James Chadwick.
Neutrons and protons combined make up the mass of the atom. We can find the number of neutrons if we know the atomic mass and the atomic number of an element, using this simple equation.
Atomic Mass – Atomic Number = Number of Neutrons
Atoms of the same element may have different number of neutrons. Adding neutrons changes the radioactivity of the element, without changing the charge of the atom. This is important in nuclear physics.
What is an electron?
An electron is a negatively charged part of the atom found outside the nucleus in orbits and are attached to the protons in the atom with electromagnetic force. Closer the electron to the nucleus, the stronger the electromagnetic force between them.
Electrons can escape from their orbit in response to an external energy being applied. It can also change its state to a higher energy level by absorbing a photon with sufficient energy to boost it to a new quantum state. It can also drop down to lower energy state emitting the excessive energy as a photon.
Atoms are neutral if the number of protons and electrons are equal. Atoms that have an excess or deficit of electrons are called ions. Electrons have no internal structure, though protons and neutrons on the other hand are made of quarks.
Thomson’s model of an atom
The Thomson Model of an atom was proposed by JJ Thomson, in 1897. He discovered electrons while experimenting with cathode ray tube. The cathode ray tube was negatively charged. He also studied positively charged particles in neon gas. Although his theory explained somethings about atoms and electrons, it failed to provide sufficient information about the positively charged particles and the nucleus of the atom.
Rutherford model of an atom
After the model of an atom, by Thomson, was unable to explain the positively charged particles in an atom, Ernest Rutherford proved the presence of positively charged particles in the nucleus of an atom through the gold foil experiment. This theory proved that the nucleus of an atom contains positively charged particles.
Bohr’s model of an atom
Bohr’s model of an atom was proposed by Neil Bohr in 1915. He specified that electrons move in fixed orbits/shells, which have fixed energy levels.
What is valency?
Valency is a measure of the reactivity of an atom. It is defined by the capacity of the atom to lose or gain valence electrons in the valence shell.
Every atom wants to have 8 electrons in the valence shell and this is known as the octet rule.
What are isotopes?
Isotopes are atoms with the same number of protons but that have a different number of neutrons. Since the atomic number is equal to the number of protons and neutrons, isotopes have the same atomic number, but different mass numbers.
Carbon 14, used in carbon dating to find out the age of really old archeological and biological remains, is an isotope of carbon.
Tritium, an isotope of hydrogen, is used to make glow in the dark faces on clocks and wrist watches.
What are isobars?
Isobars are defined as atoms of different elements that have the same atomic mass number, but different atomic number.
Carbon 14 and Nitrogen have the same mass number, which is 14, hence they are isobars.