A neutral atom has an equal number of protons and electrons. The number of electrons in an atom can also vary. The number of neutrons that an atom has determines the isotope of the the element. The electrons surround the nucleus in a diffuse cloud.Īs was described above, the number of protons that an atom contains determines which element it is. In this model, the protons and the neutrons are tightly packed in the center of the atom in region called the nucleus. The images shown above for hydrogen-1, hydrogen-2 and hydrogen-3 are based on a model for the atom that was developed in the early 1900's. The subscript is often omitted, since it is redundant to the chemical symbol hydrogen (H), by definition, has an atomic number of 1. The labels below each model in this image shows another way of representing the isotopes of an element a superscript to the left of the chemical symbol for an element indicates the atomic mass number, while a subscript to the left of the chemical symbol indicates the atomic number. The one proton is represented by the small dark gray sphere, the neutrons are represented by the small blue sphere, and the electon's location is represented by the large diffuse gray sphere that surround the proton and neutrons. The atomic masses that are given for the elements on the periodic table represent a weighted average of the masses for the different isotopes of an element based on the relative abundance of the different isotopes of an element found in nature.īelow is a figure that illustrates this with the three naturally occurring isotopes of hydrogen, hydrogen-1, hydrogen-2 and hydrogen-3:įigure 1: Models of the three isotopes of hydrogen. Other isotopes of carbon that are found in nature include carbon-13, which has 7 neutrons, and carbon-14, which has 8 neutrons. This isotope of carbon is called carbon-12. For example, the most abundant isotope of carbon has 6 neutrons in addition to its 6 protons, which gives it an atomic mass number of 12. The number of neutrons that an atom of a given element has is variable, which gives rise to different isotopes for an element. This number is approximately equal to the mass of an atom in atomic mass units. The sum of the number of protons and neutrons for an atom is called the atomic mass number. The mass of an atom is determined primarily by the number of protons and neutrons it contains. (For further reading, see Raymond, Sections 2.1 to 2.3) A neutral atom has an equal number of protons and electrons, therefore, a neutral atom of carbon also contains 6 electrons. For example, the atomic number for the element carbon (C) is 6, which means that every atom of carbon contains 6 protons.

The atoms for each element is determined by the number of protons it contains, which is indicated by the atomic number for an element. The electrons, on the other hand, have very little mass compared to the protons and neutrons, and a negative charge that is equal to that of the proton. Protons and neutrons have nearly the same mass but the protons are positively charged, whereas the neutrons are neutral. In your introductory chemistry courses you learned that atoms are made up of three types of particles: protons, neutrons and elections.

To fully appreciate the structure and behavior of atoms on must have a working concept of what the atoms that make up molecules look like.

Overall charge of calcium