Atoms consists of electrons and a central nucleus, which is tiny compared to an atom as a whole. Electrons orbit around the nucleus in energy levels, also known as shells. The nucleus contains protons and neutrons. The nuclear radius is around 1 × 10⁻¹⁴ m whilst the radius of the atom is about 1 × 10⁻¹⁰ m so the nucleus is less than 1/10,000 of the size of the atom. Yet the nucleus has the majority of the mass in an atom.

Neutron is a neutral subatomic particle residing in the nucleus of an atom. Electron is a negatively charged subatomic particle residing in shells surrounding the nucleus of an atom. Proton is positively charged subatomic particle residing in the nucleus of an atom.

Each element has a characteristic number of protons, called the atomic number, which can be found in the periodic table. In atoms the number of electrons is the same as the number of protons. Since they have opposite charges, this ensures that every atom is neutral.

In atoms:
number of protons = atomic number
number of electrons = atomic number
number of neutrons = mass number − atomic number

For charged ions, the number of negatively charged electrons changes. Negative ions will have a higher number of electrons since they have a negative charge that comes from the negative electrons. Thus, positive ions will have the usual number of electrons that belongs to the atoms minus their charge which indicates the number of electrons lost.

Electrons occupy shells surrounding the nucleus of an atom.

electrons

Electron is a negatively charged subatomic particle residing in shells surrounding the nucleus of an atom.

a negatively charged subatomic particle found around the nucleus

Neutron is a neutral subatomic particle residing in the nucleus of an atom.

a neutral subatomic particle found in the nucleus

Proton is a positively charged subatomic particle residing in the nucleus of an atom.

a positively charged subatomic particle found in the nucleus

In atoms the number of electrons, protons and the atomic number are all the same as this is the way atoms stay neutral. Therefore, if an atom has 11 protons, then it has 11 electrons.

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