Neils Bohr’s concept :- According to this concept, the nucleus is stationary at the centre of atom whereas electrons move around it in circular pathways called shells or energy level.

The nucleus, that dense central core of the atom, contains both protons and neutrons. Electrons are outside the nucleus in energy levels. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge.

The atom consists of a tiny nucleus surrounded by moving electrons. The nucleus contains protons, which have a positive charge equal in magnitude to the electron’s negative charge. The nucleus may also contain neutrons, which have virtually the same mass but no charge.

The empty space between the atomic cloud of an atom and its nucleus is just that: empty space, or vacuum. That’s the simple answer, but there are a few subtleties: 1) Sub-atomic particles such as electrons, protons and neutrons need to be treated as quantum objects.

The nucleus has an overall positive charge as it contains the protons. Every atom has no overall charge (neutral). This is because they contain equal numbers of positive protons and negative electrons. These opposite charges cancel each other out making the atom neutral.

They do not have a predictable, regular motion. The nucleus also has this random positioning. It has a probability density.

In fact, even the most powerful light-focusing microscopes can’t visualise single atoms. What makes an object visible is the way it deflects visible light waves. Atoms are so much smaller than the wavelength of visible light that the two don’t really interact. To put it another way, atoms are invisible to light itself.

Stardust: It’s the Most Important Stuff in the Universe | Time.

Our universe began with an explosion of space itself – the Big Bang. Starting from extremely high density and temperature, space expanded, the universe cooled, and the simplest elements formed. Gravity gradually drew matter together to form the first stars and the first galaxies.