Helium Atom
In the helium Atom there are 2 protons , 2 neutron and 2 orbiting electrons
Lithium is the next simplest atom after helium. A lithium atom has three protons, four neutrons and three electrons.
The subatomic arrangement of larger atoms gets more complicated with the addition of more protons and electrons.
The number of neutrons required to hold the nucleus together increases as the atomic size increases. An atom of gold consists of 79 protons, 118 neutrons and 79 electrons.
Proton (atomic) number and mass (nucleon) number
Only hydrogen atoms have one proton in their nuclei. Only helium atoms have two protons. Indeed, only gold atoms have 79 protons. The number of protons in the nucleus of an atom determines which element it is.
This important number is known as the proton number or atomic number of an atom. The proton number is given the symbol Z .
We have seen that protons alone do not make up all the mass of an atom. The neutrons in the nucleus also contribute to the total mass. Because a proton and a neutron have the same relative mass, the mass of a particular atom depends on the total number of protons and neutrons present.
Protons and neutrons are known as nucleons and the total number of protons and neutrons present is called the mass number or nucleon number of an atom. The mass number is given the symbol A.
An important way of representing a particular atom of an element is to combine the chemical symbol of the element
The symbol Z representing the proton number and symbol A representing the mass number of an atom can be written alongside the symbol for that element, in the general format XZA.
Helium Model
He is the symbol of element helium
4 is the combined mass of protons and neutrons together
2 is the atomic mass which only includes the protons number inside an atom