Atomic Number and Protons – MCAT General Chemistry | MedSchoolCoach

Atomic Structure and Protons

MCAT General Chemistry - Chapter 2: Atomic Nucleus - Section 2.3: Atomic Structure

Sample MCAT Question - Atomic Number and Protons

C-13 is an isotope of carbon and contains how many protons?

a) 6

b) 7

c) 12

d) 13

A is correct. 6.

The identity of an element is determine by the number of protons in an atom. As carbon has 6 protons, all isotopes of carbon have six carbons.

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Chemical Symbol

In nuclear notation,’X’ stands for chemical symbol, which represents an element of the periodic table. The chemical symbol may consist of a single letter, such as ‘C’ for carbon, or two letters, such as the ‘He’ for helium. Most elements have fairly intuitive chemical symbols. Some, however, do not. For instance, the chemical symbol for lead is ‘Pb’, which derived from ‘plumbum’, the Latin name for lead. Because obscure elements do not generally appear on the MCAT, memorizing the name and chemical symbol of every element is neither necessary nor advised. Test-takers should, however, know the chemical symbols of elements commonly discussed in general and organic chemistry courses, as these will certainly appear on the exam.

Atomic Number and Protons

The ‘Z’ in nuclear notation stands for atomic number, or the number of protons in the nucleus of the atom. Each proton has a mass of 1 atomic mass unit (amu) and a positive charge of 1.6 x 10-19 coulombs. The charge of a proton is known as the elementary charge and is written as ‘1 e‘ or, more commonly, simply as ‘+1’. Also, because the proton is the only charged subatomic particle located in the nucleus (neutrons have no charge), atomic number is equal to nuclear charge (the charge carried by the entire nucleus).


It is important to note that an element’s identity is defined in terms of its atomic number. Carbon atoms may have six, seven, or eight neutrons, but they always have six protons. Further, all atoms with six protons are carbon atoms. So, if you know an atom’s identity, you can determine its atomic number, and vice-versa.

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