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Figure 2.5 The Anatomy of an Atom. The protons and neutrons of an atom are found clustered at the center of the atom in a structure called the nucleus. The electrons orbit the nucleus of the atom within an electron cloud, or the empty space that surrounds the atom’s nucleus. Note that most of the area of an atom is taken up by the empty space of the electron cloud.
Diagram provided by: CNX OpenStax

Fig 2.6 The path of the electron in a hydrogen atom.  Electrons are not in discrete orbits like planets around the sun. Instead there is a probability that an electron may occupy a certain space within the electron cloud (a) The darker the color, the higher the probability that the hydrogen’s one electron will be at that point at any given time. (b) Similarly, the more crowded the dots, the higher the probability that hydrogen’s one electron will be at that point. In both diagrams, the nucleus is in the center of the diagram.
Most of the mass of an atom is in the nucleus, while the orbiting electrons account for an atom’s size. As a result, an atom consists largely of empty space. Atomic particles are so small that it is impractical to measure them in grams, instead we use a relative mass scale which makes the numbers much more manageable. We use Atomic Mass Units (AMU or u) to measure the mass of atomic particles, one AMU is equal to 1/12th the mass of an atom of carbon-12. You may also see Atomic Mass Units referred to as Daltons (Da) after John Dalton, the English Chemist that first proposed the atomic theory. Carbon-12 has 6 protons and 6 neutrons in its nucleus, meaning that one amu is equal to the average of the masses of a proton and a neutron. The mass of an atom in AMUs is equal to the number of protons and neutrons making up the atom. For example the atomic mass of bromine is roughly 80 amu and its proton number is 35, meaning that bromine has 35 protons and 45 neutrons in its nucleus.
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2.6 Atomic Number – Protons Determine the Identity of an Element

When looking at the periodic table you might notice that for each element there are two sets of numbers around the symbol. These symbols correspond to important values that give you important information about each element (Figure 2.7). The most important value corresponding to characteristics of an element is the proton number, which is also called atomic number (represented by the mathematical term, Z). As it turns out, the number of protons that an atom holds in its nucleus is the key determining feature for its chemical properties. In short, an element is defined by the number of protons found in its nucleus. If you refer back to the Periodic Table of Elements shown in figure 2.2, you will see that the periodic table is organized by the number of protons that an element contains. Thus, as you read across each row of the Periodic Table (left to right), each element increases by one proton (or one Atomic Number, Z). When atoms are in their elemental states, their overall charge is zero and the atoms are neutral. Since protons are positively charged and electrons are negatively charged, this means that when atoms are in their elemental form, the number of protons equals the number of electrons. Therefore, if you know the atomic number of an atom, you also know how many electrons are present in that atom when it is in its elemental form. When atoms combine with one another to form compounds, like water (H2O), they will either share or donate/accept electrons from their bonding partners. It is this movement of electrons that facilitates chemical bond formation. Thus, during bond formation the number of electrons around an atom may change, but the atomic number (or number of protons) remains constant and does not change.